ORCID Profile
0000-0002-4879-3519
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Astronomical and Space Sciences | Stellar Astronomy and Planetary Systems | Astronomical and Space Instrumentation | Astrobiology | Galactic Astronomy | Astronomy And Astrophysics |
Expanding Knowledge in the Physical Sciences | Physical sciences
Publisher: Oxford University Press (OUP)
Date: 19-05-2020
Abstract: The presence of mixed modes makes subgiants excellent targets for asteroseismology, providing a probe for the internal structure of stars. Here we study 36 Kepler subgiants with solar-like oscillations and report their oscillation mode parameters. We performed a so-called peakbagging exercise, i.e. estimating oscillation mode frequencies, linewidths, and litudes with a power spectrum model, fitted in the Bayesian framework and s led with a Markov chain Monte Carlo algorithm. The uncertainties of the mode frequencies have a median value of 0.180 μHz. We obtained seismic parameters from the peakbagging, analysed their correlation with stellar parameters, and examined against scaling relations. The behaviour of seismic parameters (e.g. Δν, νmax, ϵp) is in general consistent with theoretical predictions. We presented the observational p–g diagrams, namely γ1–Δν for early subgiants and ΔΠ1–Δν for late subgiants, and demonstrate their capability to estimate stellar mass. We also found a log g dependence on the linewidths and a mass dependence on the oscillation litudes and the widths of oscillation excess. This s le will be valuable constraints for modelling stars and studying mode physics such as excitation and d ing.
Publisher: American Astronomical Society
Date: 19-07-2011
Publisher: Oxford University Press (OUP)
Date: 22-07-2022
Abstract: K2 was a community-driven NASA mission where all targets were proposed through guest observer programmes. Here we provide an overview of one of the largest of these endeavours, the K2 Galactic Archaeology Programme (K2GAP), with about 25 per cent of the observed targets being allocated to this programme. K2GAP provides asteroseismic parameters for about 23 000 giant stars across the Galaxy, which together with spectroscopic stellar parameters can give age and masses of stars. We discuss in detail the target selection procedure and provide a python program that implements the selection function (anjibs/k2gap). Broadly speaking, the targets were selected on 2MASS colour J − Ks & 0.5, with finely tuned adjustments for each c aign. We discuss the detection completeness of the asteroseismic parameters νmax and Δν. About 14 per cent of giants were found to miss νmax detections and it was difficult to detect Δν for RC stars. Making use of the selection function, we compare the observed distribution of asteroseismic masses to theoretical predictions. The median asteroseismic mass is higher by about 4 per cent compared to predictions. We provide a selection-function-matched mock catalogue of stars based on a synthetic model of the Galaxy for the community to use in subsequent analyses of the K2GAP data set (physics.usyd.edu.au/k2gap/download/).
Publisher: American Astronomical Society
Date: 16-01-2012
Publisher: Oxford University Press (OUP)
Date: 26-07-2011
Publisher: American Astronomical Society
Date: 28-04-2017
Publisher: American Astronomical Society
Date: 23-08-2011
Publisher: Oxford University Press (OUP)
Date: 06-09-2017
Publisher: Oxford University Press (OUP)
Date: 05-09-2019
Abstract: Gaia DR2 has revealed new small-scale and large-scale patterns in the phase-space distribution of stars in the Milky Way. In cylindrical Galactic coordinates $(R,\phi ,z)$, ridge-like structures can be seen in the $(R,V_\phi)$ plane and asymmetric arch-like structures in the $(V_R,V_\phi)$ plane. We show that the ridges are also clearly present when the third dimension of the $(R,V_\phi)$ plane is represented by $\langle z \rangle$, $\langle V_z \rangle$, $\langle V_R \rangle$, $\langle$[Fe/H]$\rangle$, and $\langle [\alpha /{\rm Fe}]\rangle$. The maps suggest that stars along the ridges lie preferentially close to the Galactic mid-plane ($|z|\lt 0.2$ kpc), and have metallicity and $\alpha$ elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the arches. It also generates discrete groupings in orbital energy – the ridges and arches are simply surfaces of constant energy. We identify eight distinct ridges in the Gaia DR2 data: six of them have constant energy while two have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in $\langle z \rangle$ and $\langle V_z \rangle$ suggests a coupling between planar and vertical directions. We demonstrate, using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.
Publisher: Oxford University Press (OUP)
Date: 30-05-2013
DOI: 10.1093/MNRAS/STT802
Publisher: American Astronomical Society
Date: 30-08-2011
Publisher: Oxford University Press (OUP)
Date: 07-11-2015
Publisher: American Astronomical Society
Date: 25-06-2020
Abstract: This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library “MaStar”). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).
Publisher: Oxford University Press (OUP)
Date: 05-09-2018
Publisher: American Astronomical Society
Date: 29-03-2011
Publisher: American Association for the Advancement of Science (AAAS)
Date: 18-10-2013
Abstract: Stars with multiple coplanar planets have not been seen to show misalignments between the equatorial plane of the star and the orbital plane of the planets—a diagnostic of the dynamical history of planetary systems. Huber et al. (p. 331 ) analyzed the Kepler 56 planetary system, which contains a giant-sized and an intermediate-sized planet. The planets have orbits that are close to coplanar, but the planetary orbits are misaligned with the stellar equator. A third companion in a wide orbit, which could be another star or a planet, could explain the misaligned configuration.
Publisher: IOP Publishing
Date: 02-2010
DOI: 10.1086/650399
Publisher: American Astronomical Society
Date: 27-06-2014
Publisher: American Astronomical Society
Date: 06-01-2015
Publisher: EDP Sciences
Date: 2008
DOI: 10.1051/EAS:0833004
Publisher: American Astronomical Society
Date: 06-04-2011
Publisher: EDP Sciences
Date: 04-2019
DOI: 10.1051/0004-6361/201833218
Abstract: The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and Gaia provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho- Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this s le. Stellar parameters and elemental abundances are derived from the GALAH spectra with the spectral synthesis code S PECTROSCOPY M ADE E ASY . We determine kinematics and dynamics, including action angles, from the Gaia astrometry and GALAH radial velocities. Stellar masses and ages are determined with Bayesian isochrone matching, using our derived stellar parameters and absolute magnitudes. We report measurements of Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, as well as Ba and we note that we have employed non-LTE calculations for Li, O, Al, and Fe. We show that the use of astrometric and photometric data improves the accuracy of the derived spectroscopic parameters, especially log g . Focusing our investigation on the correlations between stellar age, iron abundance [Fe/H], and mean alpha-enhancement [ α /Fe] of the magnitude-selected s le, we recover the result that stars of the high- α sequence are typically older than stars in the low- α sequence, the latter spanning iron abundances of −0.7 [Fe/H] +0.5. While these two sequences become indistinguishable in [ α /Fe] vs. [Fe/H] at the metal-rich regime, we find that age can be used to separate stars from the extended high- α and the low- α sequence even in this regime. When dissecting the s le by stellar age, we find that the old stars ( Gyr) have lower angular momenta L z than the Sun, which implies that they are on eccentric orbits and originate from the inner disc. Contrary to some previous smaller scale studies we find a continuous evolution in the high- α -sequence up to super-solar [Fe/H] rather than a gap, which has been interpreted as a separate “high- α metal-rich” population. Stars in our s le that are younger than 10 Gyr, are mainly found on the low α -sequence and show a gradient in L z from low [Fe/H] ( L z L z , ⊙ ) towards higher [Fe/H] ( L z L z , ⊙ ), which implies that the stars at the ends of this sequence are likely not originating from the close solar vicinity.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 08-04-2011
Abstract: The Kepler satellite reveals details of the oscillations patterns of an evolved star in an exotic triple-star system.
Publisher: American Astronomical Society
Date: 04-04-2013
Publisher: American Astronomical Society
Date: 12-2020
Abstract: Studies of Galactic structure and evolution have benefited enormously from Gaia kinematic information, though additional, intrinsic stellar parameters like age are required to best constrain Galactic models. Asteroseismology is the most precise method of providing such information for field star populations en masse, but existing s les for the most part have been limited to a few narrow fields of view by the CoRoT and Kepler missions. In an effort to provide well-characterized stellar parameters across a wide range in Galactic position, we present the second data release of red giant asteroseismic parameters for the K2 Galactic Archaeology Program (GAP). We provide ν max and Δ ν based on six independent pipeline analyses first-ascent red giant branch (RGB) and red clump (RC) evolutionary state classifications from machine learning and ready-to-use radius and mass coefficients, κ R and κ M , which, when appropriately multiplied by a solar-scaled effective temperature factor, yield physical stellar radii and masses. In total, we report 4395 radius and mass coefficients, with typical uncertainties of 3.3% (stat.) ± 1% (syst.) for κ R and 7.7% (stat.) ± 2% (syst.) for κ M among RGB stars, and 5.0% (stat.) ± 1% (syst.) for κ R and 10.5% (stat.) ± 2% (syst.) for κ M among RC stars. We verify that the s le is nearly complete—except for a dearth of stars with ν max ≲ 10 – 20 μ Hz —by comparing to Galactic models and visual inspection. Our asteroseismic radii agree with radii derived from Gaia Data Release 2 parallaxes to within 2.2% ± 0.3% for RGB stars and 2.0% ± 0.6% for RC stars.
Publisher: American Astronomical Society
Date: 08-01-2013
Publisher: Oxford University Press (OUP)
Date: 15-05-2023
Abstract: In asteroseismology, the surface effect refers to a disparity between the observed and the modelled frequencies in stars with solar-like oscillations. It originates from improper modelling of the surface layers. Correcting the surface effect usually requires using functions with free parameters, which are conventionally fitted to the observed frequencies. On the basis that the correction should vary smoothly across the H–R diagram, we parameterize it as a simple function of surface gravity, effective temperature, and metallicity. We determine this function by fitting a wide range of stars. The absolute amount of the surface correction decreases with luminosity, but the ratio between it and νmax increases, suggesting the surface effect is more important for red giants than dwarfs. Applying the prescription can eliminate unrealistic surface correction, which improves parameter estimations with stellar modelling. Using two open clusters, we found a reduction of scatter in the model-derived ages for each star in the same cluster. As an important application, we provide a new revision for the Δν scaling relation that, for the first time, accounts for the surface correction. The values of the correction factor, fΔν, are up to 2 per cent smaller than those determined without the surface effect considered, suggesting decreases of up to 4 per cent in radii and up to 8 per cent in masses when using the asteroseismic scaling relations. This revision brings the asteroseismic properties into an agreement with those determined from eclipsing binaries. The new correction factor and the stellar models with the corrected frequencies are available at arallelpro/surface.
Publisher: Wiley
Date: 09-2016
Abstract: Stellar population synthesis based models of the Milky Way play a crucial role in understanding and interpreting observational data from large surveys of the Milky Way. We describe the basic theoretical framework for modelling the Milky Way and discuss algorithms for generating a synthetic s le of stars out of such models. Next, we discuss how asteroseismology can be used to test and possibly refine theoretical models of the Milky Way. As an application we test the ability of Galaxia to reproduce the properties of stars observed by the NASA Kepler mission. We present some preliminary results. We find that Galaxia can reproduce the photometric properties of the s le. Additionally, it can also reproduce the distribution of average asteroseismic parameters and radius. In future, comparing the mass distributions will allow us to test the Galactic models more rigorously.
Publisher: American Astronomical Society
Date: 17-09-2012
Publisher: Oxford University Press (OUP)
Date: 15-05-2020
Abstract: Detailed modelling of stellar oscillations is able to give precise estimates for stellar ages, but the inferred results typically depend on the adopted model parameters used for the age inference. High-quality asteroseismic data with precise measurements of mixed modes are available for 36 Kepler subgiants. To obtain a handle on the robustness of the ages for these stars, we first study the dependencies of seismic ages on three model input parameters. We find that inferred ages do not change systematically with the helium fraction (Y) or the mixing-length parameter (αMLT) but depend strongly on the metallicity ([M/H]) of the model. The results indicate that age estimates of subgiants have less model dependence and hence are more reliable than those of main-sequence stars or red giants. We then model in idual oscillation frequencies of the same 36 Kepler subgiants, using observed metallicities, and obtain their ages with an average precision of ${\\sim}15{{\\ \\rm per\\ cent}}$. The comparison with previous age estimates with different stellar codes or input physics shows good agreement (mostly within 2σ). We hence suggest that seismology-determined ages of subgiants are not greatly model dependent.
Publisher: American Astronomical Society
Date: 05-2021
Abstract: The results from the ESA Gaia astrometric mission and deep photometric surveys have revolutionized our knowledge of the Milky Way. There are many ongoing efforts to search these data for stellar substructure to find evidence of in idual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk. Using a combination of elemental abundances and stellar parameters from the GALAH and Apache Point Observatory Galactic Evolution Experiment (APOGEE) surveys, we find that the abundances of the highest likelihood Nyx members are entirely consistent with membership of the thick disk, and inconsistent with a dwarf galaxy origin. We conclude that the postulated Nyx stream is most probably a high-velocity component of the Milky Way’s thick disk. With the growing availability of large data sets including kinematics, stellar parameters, and detailed abundances, the probability of detecting chance associations increases, and hence new searches for substructure require confirmation across as many data dimensions as possible.
Publisher: Oxford University Press (OUP)
Date: 30-12-2015
Publisher: American Astronomical Society
Date: 21-10-2016
Publisher: American Astronomical Society
Date: 06-2018
Publisher: American Astronomical Society
Date: 16-10-2019
Publisher: American Astronomical Society
Date: 03-01-0003
Publisher: American Astronomical Society
Date: 2022
Publisher: American Astronomical Society
Date: 08-04-2013
Publisher: American Astronomical Society
Date: 09-05-2014
Publisher: SPIE
Date: 27-08-2008
DOI: 10.1117/12.789783
Publisher: American Astronomical Society
Date: 30-05-2019
Publisher: EDP Sciences
Date: 04-2012
Publisher: Oxford University Press (OUP)
Date: 22-02-2018
DOI: 10.1093/MNRAS/STY483
Publisher: EDP Sciences
Date: 07-2010
Publisher: American Astronomical Society
Date: 26-02-2013
Publisher: Oxford University Press (OUP)
Date: 15-06-2011
Publisher: American Astronomical Society
Date: 06-09-2012
Publisher: American Astronomical Society
Date: 10-2021
Publisher: Springer Science and Business Media LLC
Date: 11-04-2016
DOI: 10.1038/NCOMMS11201
Abstract: Simulations predict that hot super-Earth sized exoplanets can have their envelopes stripped by photoevaporation, which would present itself as a lack of these exoplanets. However, this absence in the exoplanet population has escaped a firm detection. Here we demonstrate, using asteroseismology on a s le of exoplanets and exoplanet candidates observed during the Kepler mission that, while there is an abundance of super-Earth sized exoplanets with low incident fluxes, none are found with high incident fluxes. We do not find any exoplanets with radii between 2.2 and 3.8 Earth radii with incident flux above 650 times the incident flux on Earth. This gap in the population of exoplanets is explained by evaporation of volatile elements and thus supports the predictions. The confirmation of a hot-super-Earth desert caused by evaporation will add an important constraint on simulations of planetary systems, since they must be able to reproduce the dearth of close-in super-Earths.
Publisher: EDP Sciences
Date: 08-2019
DOI: 10.1051/0004-6361/201935834
Abstract: We present further evidence of acoustic oscillations in the slowly rotating overactive G8 sub-giant EK Eri. This star was observed with the 1m Hertzsprung SONG telescope at the Observatorio del Teide for two different runs of 8 and 13 nights, respectively, that were separated by about a year. We determined a significant excess of power around ν max = 253 ± 3 μ Hz in the first observing run and were able to determine a large separation, Δ ν = 16.43 ± 0.22 μ Hz. No significant excess of power was instead detected in a subsequent SONG observing season, as also supported by our analysis of the simultaneous TESS photometric observations. We propose a new litude-luminosity relation in order to account for the missing power in the power spectrum. Based on the evolutionary stage of this object, we argue that a standard α 2 Ω dynamo cannot be excluded as the possible origin for the observed magnetic field.
Publisher: Oxford University Press (OUP)
Date: 13-07-2001
Publisher: EDP Sciences
Date: 23-12-2012
Publisher: Oxford University Press (OUP)
Date: 27-04-2023
Abstract: Data-driven analysis methods can help to infer physical properties of red giant stars where ‘gold-standard’ asteroseismic data are not available. The study of optical and infrared spectra of red giant stars with data-driven analyses has revealed that differences in oscillation frequencies and their separations are imprinted in said spectra. This makes it possible to confidently differentiate core helium burning red clump (RC) stars from those that are still on their first ascent of the red giant branch (RGB). We extend these studies to a tenfold larger wavelength range of 0.33–2.5 µm with the moderate-resolution VLT/X-shooter spectrograph. Our analysis of 49 stars with asteroseismic data from the K2 mission confirms that CN, CO, and CH features are indeed the primary carriers of spectroscopic information on the evolutionary stages of red giant stars. We report 215 informative features for differentiating the RC from the RGB within the range of 0.33–2.5 µm. This makes it possible for existing and future spectroscopic surveys to optimize their wavelength regions to deliver both a large variety of elemental abundances and reliable age estimates of luminous red giant stars.
Publisher: Cambridge University Press (CUP)
Date: 2016
DOI: 10.1017/PASA.2016.9
Abstract: An exciting new theoretical result shows that observing suppression of dipole oscillation modes in red giant stars can be used to detect strong magnetic fields in the stellar cores. A fundamental facet of the theory is that nearly all the mode energy leaking into the core is trapped by the magnetic greenhouse effect. This results in clear predictions for how the mode visibility changes as a star evolves up the red giant branch, and how that depends on stellar mass, spherical degree, and mode lifetime. Here, we investigate the validity of these predictions with a focus on the visibility of different spherical degrees. We find that mode suppression weakens for higher degree modes with a reduction in the quadrupole mode visibility of up to 49%, and no detectable suppression of octupole modes, in agreement with theory. We find evidence for the influence of increasing mode lifetimes on the visibilities along the red giant branch, in agreement with previous independent observations. These results support the theory that strong internal magnetic fields cause suppression of non-radial modes in red giants. We also find preliminary evidence that stars with suppressed dipole modes on average have slightly lower metallicity than normal stars.
Publisher: IOP Publishing
Date: 26-09-2019
Publisher: American Astronomical Society
Date: 22-10-2021
Publisher: Oxford University Press (OUP)
Date: 28-02-2022
Abstract: Red giant asteroseismology can provide valuable information for studying the Galaxy as demonstrated by space missions like CoRoT and Kepler. However, previous observations have been limited to small data sets and fields of view. The TESS mission provides far larger s les and, for the first time, the opportunity to perform asteroseimic inference from full-frame images full-sky, instead of narrow fields and pre-selected targets. Here, we seek to detect oscillations in TESS data of the red giants in the Kepler field using the 4-yr Kepler results as a benchmark. Because we use 1–2 sectors of observation, our results are representative of the typical scenario from TESS data. We detect clear oscillations in ∼3000 stars with another ∼1000 borderline (low S/N) cases. In comparison, best-case predictions suggest ∼4500 detectable oscillating giants. Of the clear detections, we measure Δν in 570 stars, meaning a ∼20 per cent Δν yield (14 per cent for one sector and 26 per cent for two sectors). These yields imply that typical (1–2 sector) TESS data will result in significant detection biases. Hence, to boost the number of stars, one might need to use only νmax as the seismic input for stellar property estimation. However, we find little bias in the seismic measurements and typical scatter is about 5–6 per cent in νmax and 2–3 per cent in Δν. These values, coupled with typical uncertainties in parallax, Teff, and [Fe/H] in a grid-based approach, would provide internal uncertainties of 3 per cent in inferred stellar radius, 6 per cent in mass, and 20 per cent in age for low-luminosity giant stars. Finally, we find red giant seismology is not significantly affected by seismic signal confusion from blending for stars with Tmag ≲ 12.5.
Publisher: Oxford University Press (OUP)
Date: 08-06-2202
Abstract: Lithium depletion and enrichment in the cosmos is not yet well understood. To help tighten constraints on stellar and Galactic evolution models, we present the largest high-resolution analysis of Li abundances A(Li) to date, with results for over $100\\, 000$ GALAH (Galactic Archeology with HERMES) field stars spanning effective temperatures $5900\\, \\mathrm{K} \\lesssim T_{\\mathrm{eff}}\\lesssim 7000\\, \\mathrm{K}$ and metallicities −3 ≲ [Fe/H] ≲ +0.5. We separated these stars into two groups, on the warm and cool sides of the so-called Li dip, a localized region of the Kiel diagram wherein lithium is severely depleted. We discovered that stars in these two groups show similar trends in the A(Li)–[Fe/H] plane, but with a roughly constant offset in A(Li) of $0.4\\, \\mathrm{dex}$, the warm group having higher Li abundances. At $\\rm [Fe/H]\\gtrsim -0.5$, a significant increase in Li abundance with increasing metallicity is evident in both groups, signalling the onset of significant Galactic production. At lower metallicity, stars in the cool group sit on the Spite plateau, showing a reduced lithium of around $0.4\\, \\mathrm{dex}$ relative to the primordial value predicted from big bang nucleosynthesis (BBN). However, stars in the warm group between [Fe/H] = −1.0 and −0.5 form an elevated plateau that is largely consistent with the BBN prediction. This may indicate that these stars in fact preserve the primordial Li produced in the early Universe.
Publisher: American Astronomical Society
Date: 21-12-2016
Publisher: American Astronomical Society
Date: 31-01-2012
Publisher: American Astronomical Society
Date: 24-05-2018
Publisher: American Astronomical Society
Date: 24-02-2014
Publisher: EDP Sciences
Date: 03-2020
DOI: 10.1051/0004-6361/201935843
Abstract: Context. With the advent of space-based asteroseismology, determining accurate properties of red-giant stars using their observed oscillations has become the focus of many investigations due to their implications in a variety of fields in astrophysics. Stellar models are fundamental in predicting quantities such as stellar age, and their reliability critically depends on the numerical implementation of the physics at play in this evolutionary phase. Aims. We introduce the Aarhus red giants challenge, a series of detailed comparisons between widely used stellar evolution and oscillation codes that aim to establish the minimum level of uncertainties in properties of red giants arising solely from numerical implementations. We present the first set of results focusing on stellar evolution tracks and structures in the red-giant-branch (RGB) phase. Methods. Using nine state-of-the-art stellar evolution codes, we defined a set of input physics and physical constants for our calculations and calibrated the convective efficiency to a specific point on the main sequence. We produced evolutionary tracks and stellar structure models at a fixed radius along the red-giant branch for masses of 1.0 M ⊙ , 1.5 M ⊙ , 2.0 M ⊙ , and 2.5 M ⊙ , and compared the predicted stellar properties. Results. Once models have been calibrated on the main sequence, we find a residual spread in the predicted effective temperatures across all codes of ∼20 K at solar radius and ∼30–40 K in the RGB regardless of the considered stellar mass. The predicted ages show variations of 2–5% (increasing with stellar mass), which we attribute to differences in the numerical implementation of energy generation. The luminosity of the RGB-bump shows a spread of about 10% for the considered codes, which translates into magnitude differences of ∼0.1 mag in the optical V -band. We also compare the predicted [C/N] abundance ratio and find a spread of 0.1 dex or more for all considered masses. Conclusions. Our comparisons show that differences at the level of a few percent still remain in evolutionary calculations of red giants branch stars despite the use of the same input physics. These are mostly due to differences in the energy generation routines and interpolation across opacities, and they call for further investigation on these matters in the context of using properties of red giants as benchmarks for astrophysical studies.
Publisher: Oxford University Press (OUP)
Date: 20-09-2021
Abstract: GALAH+ is a magnitude-limited survey of high resolution stellar spectra obtained by the HERMES spectrograph at the Australian Astronomical Observatory. Its third data release provides reduced spectra with new derivations of stellar parameters and abundances of 30 chemical elements for 584,015 dwarfs and giants, 88 per cent of them in the Gaia magnitude range 11 & G & 14. Here we use these improved values of stellar parameters to build a library of observed spectra which is useful to study variations of in idual spectral lines with stellar parameters. This and other improvements are used to derive radial velocities with uncertainties which are generally within 0.1 km s−1 or ∼25 per cent smaller than in the previous release. Median differences in radial velocities measured here and by the Gaia DR2 or APOGEE DR16 surveys are smaller than 30 m s−1, a larger offset is present only for Gaia measurements of giant stars. We identify 4483 stars with intrinsically variable velocities and 225 stars for which the velocity stays constant over ≥3 visits spanning more than a year. The combination of radial velocities from GALAH+ with distances and sky plane motions from Gaia enables studies of dynamics within streams and clusters. For ex le, we estimate that the open cluster M67 has a total mass of ∼3300 M⊙ and its outer parts seem to be expanding, though astrometry with a larger time-span than currently available from Gaia eDR3 is needed to judge if the latter result is real.
Publisher: Oxford University Press (OUP)
Date: 13-02-2019
DOI: 10.1093/MNRAS/STZ432
Publisher: American Astronomical Society
Date: 15-09-2011
Publisher: Oxford University Press (OUP)
Date: 04-05-2017
Publisher: Oxford University Press (OUP)
Date: 07-06-2022
Abstract: The Milky Way was shaped by the mergers with several galaxies in the past. We search for remnant stars that were born in these foreign galaxies and assess their ages in an effort to put upper limits on the merger times and thereby better understand the evolutionary history of our Galaxy. Using 5D-phase space information from Gaia eDR3, radial velocities from Gaia DR2 and chemical information from apogee DR16, we kinematically and chemically select 21 red giant stars belonging to former dwarf galaxies that merged with the Milky Way. With added asteroseismology from Kepler and K2 , we determine the ages of the 21 ex situ stars and 49 in situ stars with an average σage/age of ∼31 per cent. We find that all the ex situ stars are consistent with being older than 8 Gyr. While it is not possible to associate all the stars with a specific dwarf galaxy, we classify eight of them as Gaia-Enceladus/Sausage stars, which is one of the most massive mergers in our Galaxy’s history. We determine their mean age to be 9.5 ± 1.3 Gyr consistent with a merger time of 8–10 Gyr ago. The rest of the stars are possibly associated with Kraken, Thamnos, Sequoia, or another extragalactic progenitor. The age determination of ex situ stars paves the way to more accurately pinning down when the merger events occurred and hence provide tight constraints useful for simulating how these events unfolded.
Publisher: American Astronomical Society
Date: 19-09-2014
Publisher: Oxford University Press (OUP)
Date: 15-05-2017
Publisher: American Astronomical Society
Date: 31-03-2010
Publisher: EDP Sciences
Date: 02-10-2007
Publisher: American Astronomical Society
Date: 03-2023
Abstract: The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, allowing for detailed asteroseismic studies of in idual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet ( e = 0.878) with an orbital period of 415 days, using five sectors of TESS light curve that cover around 140 days of data. Solar-like oscillations in HD 76920 are detected around 52 μ Hz by TESS for the first time. By utilizing asteroseismic modeling that takes classical observational parameters and stellar oscillation frequencies as constraints, we determine improved measurements of the stellar mass (1.22 ± 0.11 M ⊙ ), radius (8.68 ± 0.34 R ☉ ), and age (5.2 ± 1.4 Gyr). With the updated parameters of the host star, we update the semimajor axis and mass of the planet as a = 1.165 ± 0.035 au and M p sin i = 3.57 ± 0.22 M Jup . With an orbital pericenter of 0.142 ± 0.005 au, we confirm that the planet is currently far away enough from the star to experience negligible tidal decay until being engulfed in the stellar envelope. We also confirm that this event will occur within about 100 Myr, depending on the stellar model used.
Publisher: EDP Sciences
Date: 26-11-2014
Publisher: American Astronomical Society
Date: 08-12-2017
Publisher: Oxford University Press (OUP)
Date: 03-07-2018
Publisher: American Astronomical Society
Date: 10-12-2018
Publisher: American Astronomical Society
Date: 09-02-2011
Publisher: Springer Science and Business Media LLC
Date: 14-04-2022
Publisher: Springer Science and Business Media LLC
Date: 2016
DOI: 10.1038/NATURE16171
Abstract: Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a s le of 3,600 red giant stars. About 20 per cent of our s le show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.
Publisher: Oxford University Press (OUP)
Date: 22-11-2019
Publisher: American Astronomical Society
Date: 29-10-2019
Publisher: American Astronomical Society
Date: 12-11-2019
Publisher: American Astronomical Society
Date: 03-2016
Publisher: Oxford University Press (OUP)
Date: 10-12-2021
Abstract: We present a neural network autoencoder structure that is able to extract essential latent spectral features from observed spectra and then reconstruct a spectrum from those features. Because of the training with a set of unpeculiar spectra, the network is able to reproduce a spectrum of high signal-to-noise ratio that does not show any spectral peculiarities, even if they are present in an observed spectrum. Spectra generated in this manner were used to identify various emission features among spectra acquired by multiple surveys using the HERMES spectrograph at the Anglo-Australian telescope. Emission features were identified by a direct comparison of the observed and generated spectra. Using the described comparison procedure, we discovered 10 364 candidate spectra with varying intensities (from partially filled-in to well above the continuum) of the Hα/Hβ emission component, produced by different physical mechanisms. A fraction of these spectra belong to the repeated observation that shows temporal variability in their emission profile. Among the emission spectra, we find objects that feature contributions from a nearby rarefied gas (identified through the emission of [N ii] and [S ii] lines) that was identified in 4004 spectra, which were not all identified as having Hα emission. The positions of identified emission-line objects coincide with multiple known regions that harbour young stars. Similarly, detected nebular emission spectra coincide with visually prominent nebular clouds observable in the red all-sky photographic composites.
Publisher: American Astronomical Society
Date: 14-04-2015
Publisher: EDP Sciences
Date: 2023
DOI: 10.1051/0004-6361/202244579
Abstract: Context. The Transiting Exoplanet Survey Satellite (TESS) mission has provided photometric light curves for stars across nearly the entire sky. This allows for the application of asteroseismology to a pool of potential solar-like oscillators that is unprecedented in size. Aims. We aim to produce a catalogue of solar-like oscillators observed by TESS in the 120-s and 20-s cadence modes. The catalogue is intended to highlight stars oscillating at frequencies above the TESS 30-min cadence Nyquist frequency with the purpose of encompassing the main-sequence and subgiant evolutionary phases. We aim to provide estimates for the global asteroseismic parameters v max and ∆ v . Methods. We applied a new probabilistic detection algorithm to the 120-s and 20-s light curves of over 250 000 stars. This algorithm flags targets that show characteristic signatures of solar-like oscillations. We manually vetted the resulting list of targets to confirm the presence of solar-like oscillations. Using the probability densities computed by the algorithm, we measured the global asteroseismic parameters v max and ∆ v . Results. We produce a catalogue of 4177 solar-like oscillators, reporting ∆ v and v max for 98% of the total star count. The asteroseismic data reveal a vast coverage of the Hertzsprung-Russell diagram, populating the red giant branch, the subgiant regime, and extending towards the main sequence. Conclusions. A crossmatch with external catalogues shows that 25 of the detected solar-like oscillators are a component of a spectroscopic binary, and 28 are confirmed planet host stars. These results provide the potential for precise, independent asteroseismic constraints on these and any additional TESS targets of interest.
Publisher: Oxford University Press (OUP)
Date: 18-02-2021
Abstract: Open clusters are unique tracers of the history of our own Galaxy’s disc. According to our membership analysis based on Gaia astrometry, out of the 226 potential clusters falling in the footprint of the GALactic Archaeology with HERMES (GALAH) survey or the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we find that 205 have secure members that were observed by at least one of the surveys. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disc of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is −0.076 ± 0.009 dex kpc−1, which is in agreement with previous works based on smaller s les. Furthermore, the gradient in the [Fe/H]–guiding radius (rguid) plane is −0.073 ± 0.008 dex kpc−1. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disc differently than field stars. In particular, at the given radius, open clusters show an age–metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]–rguid–age space, which are important to understand production rates of different elements as a function of space and time.
Publisher: Oxford University Press (OUP)
Date: 24-12-2021
Abstract: Mass-loss by red giants is an important process to understand the final stages of stellar evolution and the chemical enrichment of the interstellar medium. Mass-loss rates are thought to be controlled by pulsation-enhanced dust-driven outflows. Here, we investigate the relationships between mass-loss, pulsations, and radiation, using 3213 luminous Kepler red giants and 13 5000 ASAS–SN semiregulars and Miras. Mass-loss rates are traced by infrared colours using 2MASS and Wide-field Infrared Survey Explorer(WISE) and by observed-to-model WISE fluxes, and are also estimated using dust mass-loss rates from literature assuming a typical gas-to-dust mass ratio of 400. To specify the pulsations, we extract the period and height of the highest peak in the power spectrum of oscillation. Absolute magnitudes are obtained from the 2MASS Ks band and the Gaia DR2 parallaxes. Our results follow. (i) Substantial mass-loss sets in at pulsation periods above ∼60 and ∼100 d, corresponding to Asymptotic-Giant-Branch stars at the base of the period-luminosity sequences C′ and C. (ii) The mass-loss rate starts to rapidly increase in semiregulars for which the luminosity is just above the red-giant-branch tip and gradually plateaus to a level similar to that of Miras. (iii) The mass-loss rates in Miras do not depend on luminosity, consistent with pulsation-enhanced dust-driven winds. (iv) The accumulated mass-loss on the red giant branch consistent with asteroseismic predictions reduces the masses of red-clump stars by 6.3 per cent, less than the typical uncertainty on their asteroseismic masses. Thus mass-loss is currently not a limitation of stellar age estimates for galactic archaeology studies.
Publisher: Oxford University Press (OUP)
Date: 22-06-2020
Abstract: The study of planet occurrence as a function of stellar mass is important for a better understanding of planet formation. Estimating stellar mass, especially in the red giant regime, is difficult. In particular, stellar masses of a s le of evolved planet-hosting stars based on spectroscopy and grid-based modelling have been put to question over the past decade with claims they were overestimated. Although efforts have been made in the past to reconcile this dispute using asteroseismology, results were inconclusive. In an attempt to resolve this controversy, we study four more evolved planet-hosting stars in this paper using asteroseismology, and we revisit previous results to make an informed study of the whole ensemble in a self-consistent way. For the four new stars, we measure their masses by locating their characteristic oscillation frequency, νmax, from their radial velocity time series observed by SONG. For two stars, we are also able to measure the large frequency separation, Δν, helped by extended SONG single-site and dual-site observations and new Transiting Exoplanet Survey Satellite observations. We establish the robustness of the νmax-only-based results by determining the stellar mass from Δν, and from both Δν and νmax. We then compare the seismic masses of the full ensemble of 16 stars with the spectroscopic masses from three different literature sources. We find an offset between the seismic and spectroscopic mass scales that is mass dependent, suggesting that the previously claimed overestimation of spectroscopic masses only affects stars more massive than about 1.6 M⊙.
Publisher: Oxford University Press (OUP)
Date: 08-03-2022
Abstract: Precise asteroseismic parameters can be used to quickly estimate radius and mass distributions for large s les of stars. A number of automated methods are available to calculate the frequency of maximum acoustic power (νmax) and the frequency separation between overtone modes (Δν) from the power spectra of red giants. However, filtering through the results requires manual vetting, elaborate averaging across multiple methods or sharp cuts in certain parameters to ensure robust s les of stars free of outliers. Given the importance of ensemble studies for Galactic archaeology and the surge in data availability, faster methods for obtaining reliable asteroseismic parameters are desirable. We present a neural network classifier that vets Δν by combining multiple features from the visual Δν vetting process. Our classifier is able to analyse large numbers of stars, determining whether their measured Δν are reliable and thus delivering clean s les of oscillating stars with minimal effort. Our classifier is independent of the method used to obtain νmax and Δν, and therefore can be applied as a final step to any such method. Tests of our classifier’s performance on manually vetted Δν measurements reach an accuracy of 95 per cent. We apply the method to giants observed by the K2 Galactic Archaeology Program and find that our results retain stars with astrophysical oscillation parameters consistent with the parameter distributions already defined by well-characterized Kepler red giants.
Publisher: American Astronomical Society
Date: 20-01-2017
Publisher: American Astronomical Society
Date: 25-07-2012
Publisher: Wiley
Date: 09-2016
Abstract: Stellar models provide a vital basis for many aspects of astronomy and astro‐physics. Recent advances in observational astronomy – through asteroseismology, precision photometry, high‐resolution spectroscopy, and large‐scale surveys – are placing stellar models under greater quantitative scrutiny than ever. The model limitations are being exposed and the next generation of stellar models is needed as soon as possible. The current uncertainties in the models propagate to the later phases of stellar evolution, hindering our understanding of stellar populations and chemical evolution. Here we give a brief overview of the evolution, importance, and substantial uncertainties of core helium burning stars in particular and then briefly discuss a range of methods, both theoretical and observational, that we are using to advance the modelling.
Publisher: American Astronomical Society
Date: 03-04-2014
Publisher: Springer Science and Business Media LLC
Date: 13-01-2020
Publisher: Oxford University Press (OUP)
Date: 05-11-2018
Publisher: Springer Science and Business Media LLC
Date: 07-12-2011
DOI: 10.1038/NATURE10612
Abstract: When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star's radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes indirect evidence supports this. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected 'mixed modes'. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior.
Publisher: Oxford University Press (OUP)
Date: 13-08-2018
Publisher: Oxford University Press (OUP)
Date: 23-08-2018
Publisher: Oxford University Press (OUP)
Date: 04-2011
Publisher: American Astronomical Society
Date: 03-08-2017
Publisher: Oxford University Press (OUP)
Date: 13-11-2006
Publisher: Oxford University Press (OUP)
Date: 23-06-2018
Publisher: EDP Sciences
Date: 05-2017
Publisher: American Astronomical Society
Date: 02-2022
Abstract: We present the third and final data release of the K2 Galactic Archaeology Program (K2 GAP) for C aigns C1–C8 and C10–C18. We provide asteroseismic radius and mass coefficients, κ R and κ M , for ∼19,000 red giant stars, which translate directly to radius and mass given a temperature. As such, K2 GAP DR3 represents the largest asteroseismic s le in the literature to date. K2 GAP DR3 stellar parameters are calibrated to be on an absolute parallactic scale based on Gaia DR2, with red giant branch and red clump evolutionary state classifications provided via a machine-learning approach. Combining these stellar parameters with GALAH DR3 spectroscopy, we determine asteroseismic ages with precisions of ∼20%–30% and compare age-abundance relations to Galactic chemical evolution models among both low- and high- α populations for α , light, iron-peak, and neutron-capture elements. We confirm recent indications in the literature of both increased Ba production at late Galactic times as well as significant contributions to r -process enrichment from prompt sources associated with, e.g., core-collapse supernovae. With an eye toward other Galactic archeology applications, we characterize K2 GAP DR3 uncertainties and completeness using injection tests, suggesting that K2 GAP DR3 is largely unbiased in mass/age, with uncertainties of 2.9% (stat.) ± 0.1% (syst.) and 6.7% (stat.) ± 0.3% (syst.) in κ R and κ M for red giant branch stars and 4.7% (stat.) ± 0.3% (syst.) and 11% (stat.) ± 0.9% (syst.) for red clump stars. We also identify percent-level asteroseismic systematics, which are likely related to the time baseline of the underlying data, and which therefore should be considered in TESS asteroseismic analysis.
Publisher: EDP Sciences
Date: 02-2019
DOI: 10.1051/0004-6361/201834690
Abstract: Context . Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Aims . Using asteroseismic techniques combined with other methods, we aim to derive physical parameters and the evolutionary stage for the planet hosting star ϵ Tau, which is one of the four red giants located in the Hyades. Methods . We analysed time-series data from both ground and space to perform the asteroseismic analysis. By combining high signal-to-noise radial-velocity data from the ground-based SONG network with continuous space-based data from the revised Kepler mission K2, we derive and characterize 27 in idual oscillation modes for ϵ Tau, along with global oscillation parameters such as the large frequency separation Δ ν and the ratio between the litude of the oscillations measured in radial velocity and intensity as a function of frequency. The latter has been measured previously for only two stars, the Sun and Procyon. Combining the seismic analysis with interferometric and spectroscopic measurements, we derive physical parameters for ϵ Tau, and discuss its evolutionary status. Results . Along with other physical parameters, we derive an asteroseismic mass for ϵ Tau of M = 2.458 ± 0.073 M ⊙ , which is slightly lower than previous estimates, and which leads to a revised minimum mass of the planetary companion. Noting that the SONG and K2 data are non-simultaneous, we estimate the litude ratio between intensity and radial velocity to be 42.2 ± 2.3 ppm m −1 s, which is higher than expected from scaling relations.
Publisher: American Astronomical Society
Date: 03-04-2012
Publisher: EDP Sciences
Date: 20-09-2011
Publisher: EDP Sciences
Date: 2012
Publisher: Springer Science and Business Media LLC
Date: 09-2011
DOI: 10.1038/NATURE10389
Abstract: Delta Scuti (δSct) stars are opacity-driven pulsators with masses of 1.5-2.5 M⊙, their pulsations resulting from the varying ionization of helium. In less massive stars such as the Sun, convection transports mass and energy through the outer 30 per cent of the star and excites a rich spectrum of resonant acoustic modes. Based on the solar ex le, with no firm theoretical basis, models predict that the convective envelope in δSct stars extends only about 1 per cent of the radius, but with sufficient energy to excite solar-like oscillations. This was not observed before the Kepler mission, so the presence of a convective envelope in the models has been questioned. Here we report the detection of solar-like oscillations in the δSct star HD187547, implying that surface convection operates efficiently in stars about twice as massive as the Sun, as the ad hoc models predicted.
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 12-10-2019
Abstract: Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the Kepler satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We use an importance-s ling framework, which takes the selection function into account, to fit for the metallicities of a population synthesis model using spectroscopic data. We show that spectroscopic measurements of [Fe/H] and [α/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of log (Z/Z⊙) = −0.16 for the thick disc. Here Z is the effective solar-scaled metallicity, which is a function of [Fe/H] and [α/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This indirectly verifies that the asteroseismic mass scaling relation is good to within five per cent. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old α-enhanced thick disc.
Publisher: American Association for the Advancement of Science (AAAS)
Date: 03-08-2012
Abstract: In our solar system, the rocky planets have very distinct orbits from those of the gas giants. Carter et al. (p. 556 , published online 21 June) report on a planetary system where this pattern does not apply, posing a challenge to theories of planet formation. Data from the Kepler space telescope reveal two planets with radically different densities orbiting the same star with very similar orbital periods. One planet has a rocky Earth-like composition and the other is akin to Neptune.
Publisher: Oxford University Press (OUP)
Date: 04-03-2019
DOI: 10.1093/MNRAS/STZ622
Abstract: The recently published Kepler mission Data Release 25 (DR25) reported on ∼197 000 targets observed during the mission. Despite this, no wide search for red giants showing solar-like oscillations have been made across all stars observed in Kepler’s long-cadence mode. In this work, we perform this task using custom apertures on the Kepler pixel files and detect oscillations in 21 914 stars, representing the largest s le of solar-like oscillating stars to date. We measure their frequency at maximum power, νmax, down to $\\nu _{\\mathrm{max}}\\simeq 4\\, \\mu$Hz and obtain log (g) estimates with a typical uncertainty below 0.05 dex, which is superior to typical measurements from spectroscopy. Additionally, the νmax distribution of our detections show good agreement with results from a simulated model of the Milky Way, with a ratio of observed to predicted stars of 0.992 for stars with $10 \\lt \\nu _{\\mathrm{max}}\\lt 270\\, \\mu$Hz. Among our red giant detections, we find 909 to be dwarf/subgiant stars whose flux signal is polluted by a neighbouring giant as a result of using larger photometric apertures than those used by the NASA Kepler science processing pipeline. We further find that only 293 of the polluting giants are known Kepler targets. The remainder comprises over 600 newly identified oscillating red giants, with many expected to belong to the Galactic halo, serendipitously falling within the Kepler pixel files of targeted stars.
Publisher: American Astronomical Society
Date: 02-12-2011
Publisher: American Astronomical Society
Date: 27-04-2016
Publisher: American Astronomical Society
Date: 31-10-2012
Publisher: American Astronomical Society
Date: 10-2014
Publisher: Oxford University Press (OUP)
Date: 11-05-2012
Publisher: American Astronomical Society
Date: 15-05-2013
Publisher: Oxford University Press (OUP)
Date: 31-03-2022
Abstract: APOGEE and GALAH are two high resolution multi-object spectroscopic surveys that provide fundamental stellar parameters and multiple elemental abundance estimates for about half a million stars in the Milky Way. Both surveys observe in different wavelength regimes and use different data reduction pipelines leading to significant offsets and trends in stellar parameters and abundances for the common stars observed in both surveys. Such systematic differences/offsets in stellar parameters and abundances make it difficult to effectively utilize them to investigate Galactic abundance trends in spite of the unique advantage provided by their complementary sky coverage and different Milky Way components they observe. Hence, we use the Cannon data-driven method selecting a training set of 4418 common stars observed by both surveys. This enables the construction of two catalogues, one with the APOGEE-scaled and the other with the GALAH-scaled stellar parameters. Using repeat observations in APOGEE and GALAH, we find high precision in metallicity (∼0.02–0.4 dex) and alpha abundances (∼0.02–0.03 dex) for spectra with good signal-to-noise ratio (SNR & 80 for APOGEE and SNR & 40 for GALAH). We use open and globular clusters to validate our parameter estimates and find small scatter in metallicity (0.06 dex) and alpha abundances (0.03 dex) in APOGEE-scaled case. The final catalogues have been cross-matched with the Gaia EDR3 catalogue to enable their use to carry out detailed chemo-dynamic studies of the Milky Way from perspectives of APOGEE and GALAH.
Publisher: Oxford University Press (OUP)
Date: 12-10-2022
Abstract: We present the first large-scale study that demonstrates how ages can be determined for large s les of stars through Galactic chemical evolution. Previous studies found that the elemental abundances of a star correlate directly with its age and metallicity. Using this knowledge, we derive ages for 214 577 stars in GALAH DR3 using only overall metallicities and chemical abundances. Stellar ages are estimated via the machine learning algorithm XGBoost for stars belonging to the Milky Way disc with metallicities in the range −1 & [Fe/H] & 0.5, using main-sequence turn-off stars as our training set. We find that stellar ages for the bulk of GALAH DR3 are precise to 1–2 Gyr using this method. With these ages, we replicate many recent results on the age-kinematic trends of the nearby disc, including the solar neighbourhood’s age–velocity dispersion relationship and the larger global velocity dispersion relations of the disc found using Gaia and GALAH. These results show that chemical abundance variations at a given birth radius are small, and that strong chemical tagging of stars directly to birth clusters may prove difficult with our current elemental abundance precision. Our results highlight the need to measure abundances for as many nucleosynthetic production sites as possible in order to estimate reliable ages from chemistry. Our methods open a new door into studies of the kinematic structure and evolution of the disc, as ages may potentially be estimated to a precision of 1–2 Gyr for a large fraction of stars in existing spectroscopic surveys.
Publisher: Oxford University Press (OUP)
Date: 27-10-2017
Publisher: American Astronomical Society
Date: 21-10-2021
Abstract: The NASA Transiting Exoplanet Survey Satellite (TESS) is observing tens of millions of stars with time spans ranging from ∼27 days to about 1 yr of continuous observations. This vast amount of data contains a wealth of information for variability, exoplanet, and stellar astrophysics studies but requires a number of processing steps before it can be fully utilized. In order to efficiently process all the TESS data and make it available to the wider scientific community, the TESS Data for Asteroseismology working group, as part of the TESS Asteroseismic Science Consortium, has created an automated open-source processing pipeline to produce light curves corrected for systematics from the short- and long-cadence raw photometry data and to classify these according to stellar variability type. We will process all stars down to a TESS magnitude of 15. This paper is the next in a series detailing how the pipeline works. Here, we present our methodology for the automatic variability classification of TESS photometry using an ensemble of supervised learners that are combined into a metaclassifier. We successfully validate our method using a carefully constructed labeled s le of Kepler Q9 light curves with a 27.4 days time span mimicking single-sector TESS observations, on which we obtain an overall accuracy of 94.9%. We demonstrate that our methodology can successfully classify stars outside of our labeled s le by applying it to all ∼167,000 stars observed in Q9 of the Kepler space mission.
Publisher: American Astronomical Society
Date: 23-11-2016
Publisher: EDP Sciences
Date: 26-10-2010
Publisher: American Astronomical Society
Date: 27-01-2017
Publisher: Springer Science and Business Media LLC
Date: 06-12-2021
Publisher: American Astronomical Society
Date: 13-11-2019
Publisher: American Astronomical Society
Date: 12-2011
Publisher: Oxford University Press (OUP)
Date: 21-12-2021
Abstract: Asteroseismic scaling relations are often used to derive stellar masses and radii, particularly for stellar, exoplanet, and Galactic studies. It is therefore important that their precisions are known. Here we measure the intrinsic scatter of the underlying seismic scaling relations for Δν and νmax, using two sharp features that are formed in the H–R diagram (or related diagrams) by the red giant populations. These features are the edge near the zero-age core-helium-burning phase, and the strong clustering of stars at the so-called red giant branch bump. The broadening of those features is determined by factors including the intrinsic scatter of the scaling relations themselves, and therefore it is capable of imposing constraints on them. We modelled Kepler stars with a Galaxia synthetic population, upon which we applied the intrinsic scatter of the scaling relations to match the degree of sharpness seen in the observation. We found that the random errors from measuring Δν and νmax provide the dominating scatter that blurs the features. As a consequence, we conclude that the scaling relations have intrinsic scatter of $\\sim 0.5$ (Δν), $\\sim 1.1$ (νmax), $\\sim 1.7$ (M), and $\\sim 0.4{{\\ \\rm per\\ cent}}$ (R), for the SYD pipeline measured Δν and νmax. This confirms that the scaling relations are very powerful tools. In addition, we show that standard evolution models fail to predict some of the structures in the observed population of both the HeB and RGB stars. Further stellar model improvements are needed to reproduce the exact distributions.
Publisher: Cambridge University Press (CUP)
Date: 2009
DOI: 10.1071/AS08048
Abstract: PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5-m optical/infrared telescope to be located at Dome C on the Antarctic plateau. Conditions at Dome C are known to be exceptional for astronomy. The seeing (above ∼30 m height), coherence time, and isoplanatic angle are all twice as good as at typical mid-latitude sites, while the water-vapour column, and the atmosphere and telescope thermal emission are all an order of magnitude better. These conditions enable a unique scientific capability for PILOT, which is addressed in this series of papers. The current paper presents an overview of the optical and instrumentation suite for PILOT and its expected performance, a summary of the key science goals and observational approach for the facility, a discussion of the synergies between the science goals for PILOT and other telescopes, and a discussion of the future of Antarctic astronomy. Paper II and Paper III present details of the science projects ided, respectively, between the distant Universe (i.e. studies of first light, and the assembly and evolution of structure) and the nearby Universe (i.e. studies of Local Group galaxies, the Milky Way, and the Solar System).
Publisher: Springer Science and Business Media LLC
Date: 03-2011
DOI: 10.1038/NATURE09935
Abstract: Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is h ered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly ∼ 50 seconds) and those that are also burning helium (period spacing ∼ 100 to 300 seconds).
Publisher: American Astronomical Society
Date: 26-05-2010
Publisher: American Astronomical Society
Date: 29-01-2020
Publisher: Oxford University Press (OUP)
Date: 24-07-2012
Publisher: Springer Science and Business Media LLC
Date: 13-05-2020
Publisher: American Association for the Advancement of Science (AAAS)
Date: 08-04-2011
Abstract: Using asteroseismology—the study of stellar oscillations, it is possible to probe the interior of stars and to derive stellar parameters, such as mass and radius (see the Perspective by Montgomery ). Based on asteroseismic data from the NASA Kepler mission, Chaplin et al. (p. 213 ) detected solarlike oscillations in 500 solartype stars in our Galaxy. The distribution of the radii of these stars matches that expected from stellar evolution theory, but the distribution in mass does not, which challenges our knowledge of star formation rates, the mass of forming stars, and the models of the stars themselves. Derekas et al. (p. 216 ) report the detection of a triple-star system comprising a red giant star and two red dwarfs. The red giant star, instead of the expected solarlike oscillations, shows evidence for tidally induced oscillations driven by the orbital motion of the red dwarf pair. Finally, Beck et al. (p. 205 ) describe unusual oscillations from a red giant star that may elucidate characteristics of its core.
Publisher: American Astronomical Society
Date: 06-01-2012
Publisher: Oxford University Press (OUP)
Date: 25-07-2022
Abstract: Mass-loss remains a major uncertainty in stellar modelling. In low-mass stars, mass-loss is most significant on the red giant branch (RGB), and will impact the star’s evolutionary path and final stellar remnant. Directly measuring the mass difference of stars in various phases of evolution represents one of the best ways to quantify integrated mass-loss. Globular clusters (GCs) are ideal objects for this. M4 is currently the only GC for which asteroseismic data exist for stars in multiple phases of evolution. Using K2 photometry, we report asteroseismic masses for 75 red giants in M4, the largest seismic s le in a GC to date. We find an integrated RGB mass-loss of $\\Delta \\overline{M} = 0.17 \\pm 0.01 ~\\mathrm{M}_{\\odot }$, equivalent to a Reimers’ mass-loss coefficient of ηR = 0.39. Our results for initial mass, horizontal branch mass, ηR, and integrated RGB mass-loss show remarkable agreement with previous studies, but with higher precision using asteroseismology. We also report the first detections of solar-like oscillations in early asymptotic giant branch (EAGB) stars in GCs. We find an average mass of $\\overline{M}_{\\text{EAGB}}=0.54 \\pm 0.01 ~\\mathrm{M}_{\\odot }$, significantly lower than predicted by models. This suggests larger-than-expected mass-loss on the horizontal branch. Alternatively, it could indicate unknown systematics in the scaling relations for the EAGB. We discover a tentative mass bimodality in the RGB s le, possibly due to the multiple populations. In our red horizontal branch s le, we find a mass distribution consistent with a single value. We emphasize the importance of seismic studies of GCs since they could potentially resolve major uncertainties in stellar theory.
Publisher: American Astronomical Society
Date: 23-05-2014
Publisher: Oxford University Press (OUP)
Date: 11-05-2007
Publisher: American Astronomical Society
Date: 29-02-2012
Publisher: American Astronomical Society
Date: 23-08-2022
Abstract: The asteroseismic scaling relation, Δ ν ≈ ρ 0.5 , linking a star’s large frequency separation, Δ ν , and its mean density, ρ , is not exact. Yet, it provides a very useful way to obtain fundamental stellar properties. Common ways to make the relation more accurate is to apply correction factors to it. Because the corrections depend on stellar properties, such as mass, T eff , and metallicity, it is customary to interpolate these properties over stellar model grids that include both Δ ν , measured from adiabatic frequencies of the models, and the models’ stellar density hence linking both sides of the scaling relation. A grid and interpolation tool widely used for this purpose, known as Asfgrid, was published by Sharma & Stello. Here, we present a significant extension of Asfgrid to cover higher- and lower-mass stars and to increase the density of grid points, especially in the low-metallicity regime.
Publisher: Oxford University Press (OUP)
Date: 22-09-2020
Abstract: With the observations of an unprecedented number of oscillating subgiant stars expected from NASA’s TESS mission, the asteroseismic characterization of subgiant stars will be a vital task for stellar population studies and for testing our theories of stellar evolution. To determine the fundamental properties of a large s le of subgiant stars efficiently, we developed a deep learning method that estimates distributions of fundamental parameters like age and mass over a wide range of input physics by learning from a grid of stellar models varied in eight physical parameters. We applied our method to four Kepler subgiant stars and compare our results with previously determined estimates. Our results show good agreement with previous estimates for three of them (KIC 11026764, KIC 10920273, KIC 11395018). With the ability to explore a vast range of stellar parameters, we determine that the remaining star, KIC 10005473, is likely to have an age 1 Gyr younger than its previously determined estimate. Our method also estimates the efficiency of overshooting, undershooting, and microscopic diffusion processes, from which we determined that the parameters governing such processes are generally poorly constrained in subgiant models. We further demonstrate our method’s utility for ensemble asteroseismology by characterizing a s le of 30 Kepler subgiant stars, where we find a majority of our age, mass, and radius estimates agree within uncertainties from more computationally expensive grid-based modelling techniques.
Publisher: Oxford University Press (OUP)
Date: 16-05-2018
Publisher: Oxford University Press (OUP)
Date: 11-07-2007
Publisher: Oxford University Press (OUP)
Date: 03-06-2020
Abstract: Accurate and precise radius estimates of transiting exoplanets are critical for understanding their compositions and formation mechanisms. To know the planet, we must know the host star in as much detail as possible. We present complete results for planet-candidate hosts from the K2-HERMES survey, which uses the HERMES multi-object spectrograph on the Anglo-Australian Telescope to obtain $R\\sim 28\\, 000$ spectra for more than 30 000 K2 stars. We present complete host-star parameters and planet-candidate radii for 224 K2 candidate planets from C1–C13. Our results cast severe doubt on 30 K2 candidates, as we derive unphysically large radii, larger than 2RJup. This work highlights the importance of obtaining accurate, precise, and self-consistent stellar parameters for ongoing large planet search programs – something that will only become more important in the coming years, as TESS begins to deliver its own harvest of exoplanets.
Publisher: American Astronomical Society
Date: 11-06-2020
Publisher: American Association for the Advancement of Science (AAAS)
Date: 08-04-2011
Abstract: Measurements of 500 Sun-like stars show that their properties differ from those predicted by stellar population models.
Publisher: American Astronomical Society
Date: 27-07-2017
Publisher: EDP Sciences
Date: 2015
Publisher: EDP Sciences
Date: 03-2020
DOI: 10.1051/0004-6361/201936766
Abstract: Contact. The large quantity of high-quality asteroseismic data that have been obtained from space-based photometric missions and the accuracy of the resulting frequencies motivate a careful consideration of the accuracy of computed oscillation frequencies of stellar models, when applied as diagnostics of the model properties. Aims. Based on models of red-giant stars that have been independently calculated using different stellar evolution codes, we investigate the extent to which the differences in the model calculation affect the model oscillation frequencies and other asteroseismic diagnostics. Methods. For each of the models, which cover four different masses and different evolution stages on the red-giant branch, we computed full sets of low-degree oscillation frequencies using a single pulsation code and, from these frequencies, typical asteroseismic diagnostics. In addition, we carried out preliminary analyses to relate differences in the oscillation properties to the corresponding model differences. Results. In general, the differences in asteroseismic properties between the different models greatly exceed the observational precision of these properties. This is particularly true for the nonradial modes whose mixed acoustic and gravity-wave character makes them sensitive to the structure of the deep stellar interior and, hence, to details of their evolution. In some cases, identifying these differences led to improvements in the final models presented here and in Paper I here we illustrate particular ex les of this. Conclusions. Further improvements in stellar modelling are required in order fully to utilise the observational accuracy to probe intrinsic limitations in the modelling and improve our understanding of stellar internal physics. However, our analysis of the frequency differences and their relation to stellar internal properties provides a striking illustration of the potential, in particular, of the mixed modes of red-giant stars for the diagnostics of stellar interiors.
Publisher: Cambridge University Press (CUP)
Date: 2009
DOI: 10.1071/AS08051
Abstract: PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5-m optical/infrared telescope to be located at Dome C on the Antarctic plateau. The atmospheric conditions at Dome C deliver a high sensitivity, high photometric precision, wide-field, high spatial resolution, and high-cadence imaging capability to the PILOT telescope. These capabilities enable a unique scientific potential for PILOT, which is addressed in this series of papers. The current paper presents a series of projects dealing with the nearby Universe that have been identified as key science drivers for the PILOT facility. Several projects are proposed that examine stellar populations in nearby galaxies and stellar clusters in order to gain insight into the formation and evolution processes of galaxies and stars. A series of projects will investigate the molecular phase of the Galaxy and explore the ecology of star formation, and investigate the formation processes of stellar and planetary systems. Three projects in the field of exoplanet science are proposed: a search for free-floating low-mass planets and dwarfs, a program of follow-up observations of gravitational microlensing events, and a study of infrared light-curves for previously discovered exoplanets. Three projects are also proposed in the field of planetary and space science: optical and near-infrared studies aimed at characterising planetary atmospheres, a study of coronal mass ejections from the Sun, and a monitoring program searching for small-scale Low Earth Orbit satellite debris items.
Publisher: American Astronomical Society
Date: 04-02-2014
Publisher: Oxford University Press (OUP)
Date: 27-02-2018
DOI: 10.1093/MNRAS/STY525
Publisher: American Astronomical Society
Date: 28-10-2011
Publisher: Springer Science and Business Media LLC
Date: 28-06-2023
Publisher: Oxford University Press (OUP)
Date: 10-03-2011
Publisher: American Astronomical Society
Date: 04-08-2015
Publisher: EDP Sciences
Date: 26-04-1020
Publisher: American Astronomical Society
Date: 15-05-2012
Publisher: Oxford University Press (OUP)
Date: 09-06-2016
Publisher: Oxford University Press (OUP)
Date: 17-08-2017
Publisher: American Astronomical Society
Date: 14-03-2019
Publisher: American Astronomical Society
Date: 29-01-2021
Publisher: American Astronomical Society
Date: 02-05-2016
Publisher: American Astronomical Society
Date: 24-01-2018
Publisher: Oxford University Press (OUP)
Date: 11-02-2009
Publisher: American Astronomical Society
Date: 25-10-2010
Publisher: EDP Sciences
Date: 18-05-2011
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 08-12-2018
Publisher: American Astronomical Society
Date: 10-01-2014
Publisher: Oxford University Press (OUP)
Date: 07-2015
Publisher: Oxford University Press (OUP)
Date: 24-07-2015
Publisher: Oxford University Press (OUP)
Date: 08-05-2023
Abstract: In this study we combine asteroseismic, spectroscopic, and kinematic information to perform a detailed analysis of a s le of 16 stars from the Kepler field. Our selection focuses on stars that appear to contradict Galactic chemical evolution models: young and α-rich, old and metal-rich, as well as other targets with unclear classification in past surveys. Kinematics are derived from Gaia DR3 parallaxes and proper motions, and high-resolution spectra from HIRES/Keck are used to calculate chemical abundances for over 20 elements. This information is used to perform careful checks on asteroseismic masses and ages derived via grid-based modelling. Among the seven stars previously classified as young and α-rich, only one seems to be an unambiguously older object masking its true age. We confirm the existence of two very old (≥11 Gyr), super metal-rich (≥0.1 dex) giants. These two stars have regular thin disc chemistry and in-plane solar circle orbits that fit well in the picture of radial migration via the churning mechanism. The alternative explanation that these stars have younger ages would require mass-loss rates that strongly increase with increasing metallicity. Finally, we suggest further investigations to explore the suitability of Zn as a chemical clock in red giants.
Publisher: American Astronomical Society
Date: 25-10-2011
Publisher: EDP Sciences
Date: 2015
Publisher: American Astronomical Society
Date: 13-09-2022
Abstract: We present HD-TESS, a catalog of 1709 bright ( V ∼ 3–10) red giants from the Henry Draper (HD) Catalog with asteroseismic measurements based on photometry from NASA’s Transiting Exoplanet Survey Satellite (TESS). Using light curves spanning at least 6 months across a single TESS observing cycle, we provide measurements of global asteroseismic parameters ( ν max and Δ ν ) and the evolutionary state for each star in the catalog. We adopt literature values of atmospheric stellar parameters to estimate the masses and radii of the giants in our catalog using asteroseismic scaling relations, and observe that HD-TESS giants on average have larger masses compared to Kepler red giants. Additionally, we present the discovery of oscillations in 99 red giants in astrometric binary systems, including those with subdwarf or white dwarf companions. Finally, we benchmark radii from asteroseismic scaling relations against those measured using long-baseline interferometry for 18 red giants and find that correction factors to the scaling relations improve the agreement between asteroseismic and interferometric radii to approximately 3%.
Publisher: Oxford University Press (OUP)
Date: 19-08-2016
Publisher: Oxford University Press (OUP)
Date: 07-02-2013
DOI: 10.1093/MNRAS/STT059
Publisher: American Astronomical Society
Date: 11-12-2013
Publisher: Oxford University Press (OUP)
Date: 17-10-2017
Publisher: Oxford University Press (OUP)
Date: 18-01-2018
DOI: 10.1093/MNRAS/STY150
Publisher: American Astronomical Society
Date: 27-04-2023
Abstract: Asteroseismology is playing an increasingly important role in the characterization of red giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved red giant branch (RGB) hosts KOI-3886 and ι Draconis, making use of end-of-mission Kepler (KOI-3886) and multisector TESS ( ι Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of ℓ = 1 and 2 modes improves the precision of the stellar parameters, albeit marginally, compared to adopting radial modes alone, with 1.9%–3.0% (radius), 5%–9% (mass), and 19%–25% (age) reached when using all p -dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed g -dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multiyear time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find the mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance, Y i . Attempts to measure Y i from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics.
Publisher: Oxford University Press (OUP)
Date: 03-11-2016
Publisher: EDP Sciences
Date: 24-02-2006
Publisher: IOP Publishing
Date: 10-2015
DOI: 10.1086/683103
Publisher: Oxford University Press (OUP)
Date: 21-11-2009
Publisher: Oxford University Press (OUP)
Date: 31-01-2020
Abstract: While long-period variables (LPVs) have been extensively investigated, especially with MACHO and OGLE data for the Magellanic Clouds, there still exist open questions in their pulsations regarding the excitation mechanisms, radial order, and angular degree assignment. Here, we perform asteroseismic analyses on LPVs observed by the 4-year Kepler mission. Using a cross-correlation method, we detect unambiguous pulsation ridges associated with radial fundamental modes (n = 1) and overtones (n ≥ 2), where the radial order assignment is made using theoretical frequencies and observed frequencies. Our results confirm that the litude variability seen in semiregulars is consistent with oscillations being solar-like. We identify that the dipole modes, l = 1, are dominant in the radial orders of 3 ≤ n ≤ 6, and that quadrupole modes, l = 2, are dominant in the first overtone n = 2. A test of seismic scaling relations using Gaia DR2 parallaxes reveals the possibility that the relations break down when νmax ≲ 3 $\\mu {\\rm Hz}$ (R ≳ 40 R⊙, or log $\\rm L/L_{\\odot }$ ≳ 2.6). Our homogeneous measurements of pulsation litude and period for 3213 LPVs will be valuable for probing effects of pulsation on mass-loss, in particular in those stars with periods around 60 d, which has been argued as a threshold of substantial pulsation-triggered mass-loss.
Publisher: Wiley
Date: 22-11-2010
Abstract: We present a brief overview of the history of attempts to obtain a clear detection of solar‐like oscillations in cluster stars, and discuss the results on the first clear detection, which was made by the Kepler Asteroseismic Science Consortium (KASC) Working Group 2 (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Publisher: American Astronomical Society
Date: 14-03-2013
Publisher: Wiley
Date: 22-11-2010
Abstract: We report on the ground‐based follow‐up program of spectroscopic and photometric observations of solar‐like asteroseismic targets for the Kepler space mission. These stars constitute a large group of more than a thousand objects which are the subject of an intensive study by the Kepler Asteroseismic Science Consortium Working Group 1 (KASC WG‐1). In the current work we will discuss the methods we use to determine the fundamental stellar atmospheric parameters using high‐quality stellar spectra. These provide essential constraints for the asteroseismic modelling and make it possible to verify the parameters in the Kepler Input Catalogue (KIC) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Publisher: American Astronomical Society
Date: 2007
DOI: 10.1086/508766
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 03-10-2013
Publisher: Springer Science and Business Media LLC
Date: 04-2013
DOI: 10.1038/NATURE12067
Publisher: Springer Science and Business Media LLC
Date: 04-2004
Publisher: American Astronomical Society
Date: 21-06-2019
Publisher: Oxford University Press (OUP)
Date: 08-2002
Publisher: Oxford University Press (OUP)
Date: 29-08-2019
Abstract: The internal working of low-mass stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal structure of stars and allows for the determination of the evolutionary state – i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from red-giant stars in a combination (known as APOKASC) of asteroseismology (from the Kepler mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of erse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these stars based on comparison of observables with stellar evolutionary models (‘grid-based modelling’) and as a training set for machine-learning and other data-driven methods of evolutionary state determination.
Publisher: American Astronomical Society
Date: 09-09-2019
Publisher: American Astronomical Society
Date: 05-12-1111
Publisher: EDP Sciences
Date: 2017
Publisher: American Astronomical Society
Date: 14-04-2011
Publisher: American Astronomical Society
Date: 05-08-2016
Publisher: EDP Sciences
Date: 06-2020
DOI: 10.1051/0004-6361/202037484
Abstract: Context. Binary stellar systems form a large fraction of the Galaxy’s stars. They are useful as laboratories for studying the physical processes taking place within stars, and must be correctly taken into account when observations of stars are used to study the structure and evolution of the Galaxy. The advent of large-scale spectroscopic and photometric surveys allows us to obtain large s les of binaries that permit characterising their populations. Aims. We aim to obtain a large s le of double-lined spectroscopic binaries (SB2s) by analysis of spectra from the GALAH survey in combination with photometric and astrometric data. A combined analysis will provide stellar parameters of thousands of binary stars that can be combined to form statistical observables of a given population. We aim to produce a catalogue of well-characterised systems, which can in turn be compared to models of populations of binary stars, or to follow-up in idual systems of interest. Methods. We obtained a list of candidate SB2 systems from a t-distributed stochastic neighbour embedding (t-SNE) classification and a cross-correlation analysis of GALAH spectra. To compute parameters of the primary and secondary star, we used a Bayesian approach that includes a parallax prior from Gaia DR2, spectra from GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. We used a Markov chain Monte Carlo approach to s le the posterior distributions of the following model parameters for the two stars: T eff[1,2] , log g [1,2] , [Fe/H], V r [1,2] , v mic[1,2] , v broad[1,2] , R [1,2] , and E ( B − V ). Results. We present results for 12 760 binary stars detected as SB2s. We construct the statistical observables T 1 ∕ T 2 , Δ V r , and R 1 ∕ R 2 , which demonstrate that our s le mostly consists of dwarfs, with a significant fraction of evolved stars and several dozen members of the giant branch. The majority of these binary stars is concentrated at the lower boundary of the Δ V r distribution, and the R 1 ∕ R 2 ratio is mostly close to unity. The derived metallicity of our binary stars is statistically lower than that of single dwarf stars from the same magnitude-limited s le. Conclusions. Our s le of binary stars represents a large population of well-characterised double-lined spectroscopic binaries that are appropriate for statistical studies of the binary populations. The derived stellar properties and their distributions show trends that are expected for a population of close binary stars ( a 10 AU) detected through double lines in their spectra. Our detection technique allows us to probe binary systems with mass ratios 0.5 ≤ q ≤ 1.
Publisher: Cambridge University Press (CUP)
Date: 08-2014
DOI: 10.1017/S174392131301497X
Abstract: The Kepler space telescope has proven to be a gold mine for the study of variable stars. Unfortunately, Kepler only returns a handful of pixels surrounding each star on the target list, which omits a large number of stars in the Kepler field. For the open clusters NGC 6791 and NGC 6819, Kepler also reads out larger superst s which contain complete images of the central region of each cluster. These cluster images can potentially be used to study additional stars in the open clusters. We present preliminary results from using traditional photometric techniques to identify and analyze additional variable stars from these images.
Publisher: EDP Sciences
Date: 2015
Publisher: American Astronomical Society
Date: 02-12-2014
Publisher: Oxford University Press (OUP)
Date: 26-05-2011
Publisher: American Astronomical Society
Date: 29-04-2022
No related organisations have been discovered for Dennis Stello.
Start Date: 06-2019
End Date: 04-2025
Amount: $416,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2012
End Date: 12-2015
Amount: $375,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2019
End Date: 12-2020
Amount: $159,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2015
End Date: 04-2019
Amount: $706,790.00
Funder: Australian Research Council
View Funded ActivityStart Date: 02-2007
End Date: 12-2010
Amount: $251,092.00
Funder: Australian Research Council
View Funded ActivityStart Date: 06-2016
End Date: 12-2018
Amount: $800,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2018
End Date: 07-2023
Amount: $792,859.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2016
End Date: 10-2017
Amount: $550,000.00
Funder: Australian Research Council
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