ORCID Profile
0000-0003-0174-0564
Current Organisations
Monash University
,
University of Cambridge
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Astronomical and Space Sciences | Stellar Astronomy and Planetary Systems | Astronomy And Astrophysics | Knowledge Representation and Machine Learning |
Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Information and Computing Sciences | Physical sciences
Publisher: Oxford University Press (OUP)
Date: 11-01-2017
DOI: 10.1093/MNRAS/STX047
Publisher: American Astronomical Society
Date: 08-2018
Publisher: Zenodo
Date: 2016
Publisher: EDP Sciences
Date: 24-01-2017
Publisher: American Astronomical Society
Date: 18-07-2017
Publisher: EDP Sciences
Date: 13-06-2016
Publisher: EDP Sciences
Date: 05-2017
Publisher: American Astronomical Society
Date: 02-2022
Abstract: Stellar abundances and ages afford the means to link chemical enrichment to galactic formation. In the Milky Way, in idual element abundances show tight correlations with age, which vary in slope across ([Fe/H]–[ α /Fe]). Here, we step from characterizing abundances as measures of age, to understanding how abundances trace properties of stellar birth environment in the disk over time. Using measurements from ∼27,000 APOGEE stars ( R = 22,500, signal-to-noise ratio 200), we build simple local linear models to predict a s le of elements (X = Si, O, Ca, Ti, Ni, Al, Mn, Cr) using (Fe, Mg) abundances alone, as fiducial tracers of supernovae production channels. Given [Fe/H] and [Mg/H], we predict these elements, [X/H], to about double the uncertainty of their measurements. The intrinsic dispersion, after subtracting measurement errors in quadrature is ≈0.015–0.04 dex. The residuals of the prediction (measurement − model) for each element demonstrate that each element has an in idual link to birth properties at fixed (Fe, Mg). Residuals from primarily massive-star supernovae (i.e., Si, O, Al) partially correlate with guiding radius. Residuals from primarily supernovae Ia (i.e., Mn, Ni) partially correlate with age. A fraction of the intrinsic scatter that persists at fixed (Fe, Mg), however, after accounting for correlations, does not appear to further discriminate between birth properties that can be traced with present-day measurements. Presumably, this is because the residuals are also, in part, a measure of the typical (in)-homogeneity of the disk’s stellar birth environments, previously inferred only using open cluster systems. Our study implies at fixed birth radius and time that there is a median scatter of ≈0.01–0.015 dex in elements generated in supernovae sources.
Publisher: American Astronomical Society
Date: 23-03-2016
Publisher: Oxford University Press (OUP)
Date: 11-05-2018
Publisher: American Astronomical Society
Date: 22-08-2018
Publisher: American Astronomical Society
Date: 04-05-2017
Publisher: Oxford University Press (OUP)
Date: 03-10-2019
Abstract: We introduce the southern stellar stream spectroscopy survey (S5), an on-going program to map the kinematics and chemistry of stellar streams in the southern hemisphere. The initial focus of S5 has been spectroscopic observations of recently identified streams within the footprint of the dark energy survey (DES), with the eventual goal of surveying streams across the entire southern sky. Stellar streams are composed of material that has been tidally striped from dwarf galaxies and globular clusters and hence are excellent dynamical probes of the gravitational potential of the Milky Way, as well as providing a detailed snapshot of its accretion history. Observing with the 3.9 m Anglo-Australian Telescope’s 2-degree-Field fibre positioner and AAOmega spectrograph, and combining the precise photometry of DES DR1 with the superb proper motions from Gaia DR2, allows us to conduct an efficient spectroscopic survey to map these stellar streams. So far S5 has mapped nine DES streams and three streams outside of DES the former are the first spectroscopic observations of these recently discovered streams. In addition to the stream survey, we use spare fibres to undertake a Milky Way halo survey and a low-redshift galaxy survey. This paper presents an overview of the S5 program, describing the scientific motivation for the survey, target selection, observation strategy, data reduction, and survey validation. Finally, we describe early science results on stellar streams and Milky Way halo stars drawn from the survey. Updates on S5, including future public data releases, can be found at s5collab.github.io.
Publisher: Oxford University Press (OUP)
Date: 20-05-2016
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: 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: 09-08-2019
Abstract: Observations of stellar remnants linked to Type Ia and Type Iax supernovae are necessary to fully understand their progenitors. Multiple progenitor scenarios predict a population of kicked donor remnants and partially burnt primary remnants, both moving with relatively high velocity. But only a handful of ex les consistent with these two predicted populations have been observed. Here we report the likely first known ex le of an unbound white dwarf that is consistent with being the fully cooled primary remnant to a Type Iax supernova. The candidate, LP 93-21, is travelling with a galactocentric velocity of $v_{\\textrm {gal}} \\simeq 605\\, {\\rm km}\\, {\\rm s}^{-1}$, and is gravitationally unbound to the Milky Way. We rule out an extragalactic origin. The Type Iax supernova ejection scenario is consistent with its peculiar unbound trajectory, given anomalous elemental abundances are detected in its photosphere via spectroscopic follow-up. This discovery reflects recent models that suggest stellar ejections likely occur often. Unfortunately the intrinsic faintness of white dwarfs, and the uncertainty associated with their direct progenitor systems, makes it difficult to detect and confirm such donors.
Publisher: Oxford University Press (OUP)
Date: 27-10-2017
Publisher: American Physical Society (APS)
Date: 07-08-2019
Publisher: American Astronomical Society
Date: 05-02-2018
Publisher: EDP Sciences
Date: 10-2018
DOI: 10.1051/0004-6361/201833224
Abstract: Context . The abundance ratio N/O is a useful tool to study the interplay of galactic processes, for ex le star formation efficiency, timescale of infall, and outflow loading factor. Aims . We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to compare this ratio with a set of chemical evolution models to understand the role of infall, outflow, and star formation efficiency in the building up of the Galactic disc. Methods . We used the abundances from IDR2-3, IDR4, IDR5 data releases of the Gaia -ESO Survey both for Galactic field and open cluster stars. We determined membership and average composition of open clusters and we separated thin and thick disc field stars. We considered the effect of mixing in the abundance of N in giant stars. We computed a grid of chemical evolution models, suited to reproduce the main features of our Galaxy, exploring the effects of the star formation efficiency, infall timescale, and differential outflow. Results . With our s les, we map the metallicity range −0.6 ≤ [Fe/H] ≤ 0.3 with a corresponding −1.2 ≤ log(N/O) ≤ −0.2, where the secondary production of N dominates. Thanks to the wide range of Galactocentric distances covered by our s les, we can distinguish the behaviour of log(N/O) in different parts of the Galaxy. Conclusions . Our spatially resolved results allow us to distinguish differences in the evolution of N/O with Galactocentric radius. Comparing the data with our models, we can characterise the radial regions of our Galaxy. A shorter infall timescale is needed in the inner regions, while the outer regions need a longer infall timescale, coupled with a higher star formation efficiency. We compare our results with nebular abundances obtained in MaNGA galaxies, finding in our Galaxy a much wider range of log(N/O) than in integrated observations of external galaxies of similar stellar mass, but similar to the ranges found in studies of in idual H II regions.
Publisher: American Astronomical Society
Date: 20-12-2016
Publisher: Oxford University Press (OUP)
Date: 16-07-2014
Publisher: American Astronomical Society
Date: 02-2022
Abstract: Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ∼4000 metal-poor stars ([Fe/H] ≤ −0.8) compiled from the Best and Brightest survey. Selected as metal-poor candidates based on broadband near- and far-IR photometry, 43% of these stars had medium-resolution (1200 ≲ R ≲ 2000) validation spectra obtained over a 7 yr c aign from 2014 to 2020 with a variety of telescopes. The remaining stars were chosen based on photometric metallicity determinations from the Huang et al. recalibration of the Sky Mapper Southern Survey. Dynamical clusters of these stars are obtained from the orbital energy and cylindrical actions using the HDBSCAN unsupervised learning algorithm. We identify 52 dynamically tagged groups (DTGs) with between five and 21 members 18 DTGs have at least 10 member stars. Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the Metal-Weak Thick-Disk, Thamnos, the Splashed Disk, and the Helmi Stream are identified. Associations with MW globular clusters are determined for eight DTGs no recognized MW dwarf galaxies were associated with any of our DTGs. Previously identified dynamical groups are also associated with our DTGs, with emphasis placed on their structural determination and possible new identifications. Chemically peculiar stars are identified as members of several DTGs, with six DTGs that are associated with r -process-enhanced stars. We demonstrate that the mean carbon and α -element abundances of our DTGs are correlated with their mean metallicity in an understandable manner. Similarly, we find that the mean metallicity, carbon, and α -element abundances are separable into different regions of the mean rotational-velocity space.
Publisher: Oxford University Press (OUP)
Date: 04-11-2020
Abstract: In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a s le of 475 stars with a metallicity range of $-6.5 \le \rm [Fe/H] \le -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of $\rm [Fe/H]=-3.31\, \mathrm{ and }\, -3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that ∼21 per cent of the s le have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Zmax| ≤ 3 kpc. Of particular interest is a subs le (∼11 per cent of the total) of low |Zmax| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = –4.30 and the subs le is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Zmax|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Zmax|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our s le (∼4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.
Publisher: Oxford University Press (OUP)
Date: 24-10-2014
Publisher: Oxford University Press (OUP)
Date: 11-09-2017
Publisher: EDP Sciences
Date: 05-2017
Publisher: Oxford University Press (OUP)
Date: 26-04-2021
Abstract: Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as in idual events into the binary population synthesis code binary_c. This treatment of novae represents a significant improvement on the ‘averaging’ treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae, respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave-driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately 41 ± 4 for H novae, underpredicting the current observational estimate of $65^{+15}_{-16}$, and 0.14 ± 0.015 for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year.
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: EDP Sciences
Date: 06-2019
DOI: 10.1051/0004-6361/201834636
Abstract: Context . Pismis 18 is a moderately populated, intermediate-age open cluster located within the solar circle at a Galactocentric distance of about seven kpc. Few open clusters have been studied in detail in the inner disc region before the Gaia -ESO Survey. Aims . New data from the Gaia -ESO Survey allowed us to conduct an extended radial velocity membership study as well as spectroscopic metallicity and detailed chemical abundance measurements for this cluster. Methods . Gaia -ESO Survey data for 142 potential members, lying on the upper main sequence and on the red clump, yielded radial velocity measurements, which, together with proper motion measurements from the Gaia Second Data Release ( Gaia DR2), were used to determine the systemic velocity of the cluster and membership of in idual stars. Photometry from Gaia DR2 was used to re-determine cluster parameters based on high confidence member stars only. Cluster abundance measurements of six radial-velocity member stars with UVES high-resolution spectroscopy are presented for 23 elements. Results . The average radial velocity of 26 high confidence members is −27.5 ± 2.5 (std) km s −1 with an average proper motion of pmra = −5.65 ± 0.08 (std) mas yr −1 and pmdec = −2.29 ± 0.11 (std) mas yr −1 . According to the new estimates, based on high confidence members, Pismis 18 has an age of τ = 700 +40 −50 Myr, interstellar reddening of E ( B − V ) = 0.562 +0.012 −0.026 mag and a de-reddened distance modulus of DM 0 = 11.96 +0.10 −0.24 mag. The median metallicity of the cluster (using the six UVES stars) is [Fe/H] = +0.23 ± 0.05 dex, with [ α /Fe] = 0.07 ± 0.13 and a slight enhancement of s - and r -neutron-capture elements. Conclusions . With the present work, we fully characterized the open cluster Pismis 18. We confirmed its present location in the inner disc. We estimated a younger age than the previous literature values and we gave, for the first time, its metallicity and its detailed abundances. Its [ α /Fe] and [ s -process/Fe], both slightly super-solar, are in agreement with other inner-disc open clusters observed by the Gaia -ESO survey.
Publisher: American Astronomical Society
Date: 05-12-2016
Publisher: American Astronomical Society
Date: 29-08-2018
Publisher: Oxford University Press (OUP)
Date: 03-12-2021
Abstract: Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus (GSE), appears to be an early ‘building block’ given its virial mass $\\gt 10^{10}\\, \\mathrm{M_\\odot }$ at infall (z ∼ 1−3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-α abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via [Na/Fe] versus [Mg/Mn] in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 \\lt \\sqrt{J_R / \\, \\mathrm{kpc\\, km\\, s^{-1}}} \\lt 55$, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for ex le mean $\\sqrt{J_R / \\, \\mathrm{kpc\\, km\\, s^{-1}}} =$$26_{-14}^{+9}$. We find only $(29\\pm 1){{\\ \\rm per\\ cent}}$ of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
Publisher: American Astronomical Society
Date: 14-03-2012
Publisher: American Astronomical Society
Date: 09-07-2015
Publisher: American Astronomical Society
Date: 26-05-2020
Publisher: Oxford University Press (OUP)
Date: 15-07-2021
Abstract: The European Space Agency (ESA) Gaia mission has enabled the remarkable discovery that a large fraction of the stars near the solar neighbourhood are debris from a single in-falling system, the so-called Gaia-Sausage-Enceladus (GSE). This discovery provides astronomers for the first time with a large cohort of easily observable, unevolved stars that formed in a single extragalactic environment. Here we use these stars to investigate the ‘Spite plateau’ – the near-constant lithium abundance observed in unevolved metal-poor stars across a wide range of metallicities (−3 & [Fe/H] & −1). Our aim is to test whether in idual galaxies could have different Spite plateaus – e.g. the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We identified 93 GSE dwarf stars observed and analysed by the GALactic Archaeology with HERMES (GALAH) survey as part of its Data Release 3 (DR3). Orbital actions were used to select s les of GSE stars, and comparison s les of halo and disc stars. We find that the GSE stars show the same lithium abundance as other likely accreted stars and in situ Milky Way stars. Formation environment leaves no imprint on lithium abundances. This result fits within the growing consensus that the Spite plateau, and more generally the ‘cosmological lithium problem’ – the observed discrepancy between the amount of lithium in warm, metal-poor dwarf stars in our Galaxy, and the amount of lithium predicted to have been produced by big bang nucleosynthesis – is the result of lithium depletion processes within stars.
Publisher: Oxford University Press (OUP)
Date: 10-11-2018
Publisher: Oxford University Press (OUP)
Date: 28-10-2021
Abstract: The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from Z = 10−4 to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anticorrelation between metallicity and the number of novae produced, with the number of novae at Z = 0.03 roughly half that at Z = 10−4. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of $65^{+15}_{-16}$. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution.
Publisher: EDP Sciences
Date: 19-12-2016
Publisher: EDP Sciences
Date: 2023
DOI: 10.1051/0004-6361/202244784
Abstract: Interpreting and modelling astronomical catalogues requires an understanding of the catalogues’ completeness or selection function: what properties determine an object’s probability of being including in the catalogue? Here we set out to empirically quantify the completeness of the overall catalogue of Gaia ’s third data release (DR3). This task is not straightforward because Gaia is the all-sky optical survey with the highest angular resolution to date and no consistent ground truth exists to allow direct comparisons. However, well-characterised deeper imaging enables an empirical assessment of Gaia ’s G -band completeness across parts of the sky. On this basis, we devised a simple analytical completeness model of Gaia as a function of the observed G magnitude and position over the sky, which accounts for both the effects of crowding and the complex Gaia scanning law. Our model only depends on a single quantity: the median magnitude M 10 in a patch of the sky of catalogued sources with astrometric_matched_transits ≤10. We note that M 10 reflects elementary completeness decisions in the Gaia pipeline and is computable from the Gaia DR3 catalogue itself and therefore applicable across the whole sky. We calibrated our model using the Dark Energy Camera Plane Survey (DECaPS) and tested its predictions against Hubble Space Telescope observations of globular clusters. We found that our model predicts Gaia ’s completeness values to a few per cent (RMS) across the sky. We make the model available as a part of the gaiaunlimited Python package built and maintained by the GaiaUnlimited project.
Publisher: Springer Science and Business Media LLC
Date: 30-08-2021
Publisher: American Astronomical Society
Date: 05-2022
Abstract: Extremely metal-poor (EMP) stars provide a valuable probe of early chemical enrichment in the Milky Way. Here we leverage a large s le of ∼600,000 high-resolution stellar spectra from the GALAH survey plus a machine-learning algorithm to find 54 candidates with estimated [Fe/H] ≤−3.0, six of which have [Fe/H] ≤−3.5. Our s le includes ∼20% main-sequence EMP candidates, unusually high for EMP star surveys. We find the magnitude-limited metallicity distribution function of our s le is consistent with previous work that used more complex selection criteria. The method we present has significant potential for application to the next generation of massive stellar spectroscopic surveys, which will expand the available spectroscopic data well into the millions of stars.
Publisher: Oxford University Press (OUP)
Date: 16-05-2018
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: Oxford University Press (OUP)
Date: 12-09-2019
Abstract: We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have [Fe/H] ≤ −2.75 and only approximately seven per cent have [Fe/H] −2.0 dex. The most metal-poor candidate in the s le has [Fe/H] −4.75 and is notably carbon rich. Except at the lowest metallicities ([Fe/H] −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with [C/Fe]1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus [Fe/H]1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE 7.3 and [C/Fe]1D,LTE +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed s le has a power-law slope of Δ(Log N)/Δ[Fe/H] = 1.5 ± 0.1 dex per dex for −4.0 ≤ [Fe/H] ≤ −2.75, but appears to drop abruptly at [Fe/H] ≈ −4.2, in line with previous studies.
Publisher: American Astronomical Society
Date: 05-11-2018
Publisher: EDP Sciences
Date: 10-2016
Publisher: EDP Sciences
Date: 08-2017
Publisher: EDP Sciences
Date: 05-2017
DOI: 10.1051/0004-6361/201629160
Abstract: Context. As observational evidence steadily accumulates, the nature of the Galactic bulge has proven to be rather complex: the structural, kinematic, and chemical analyses often lead to contradictory conclusions. The nature of the metal-rich bulge – and especially of the metal-poor bulge – and their relation with other Galactic components, still need to be firmly defined on the basis of statistically significant high-quality data s les. Aims. We used the fourth internal data release of the Gaia -ESO survey to characterize the bulge metallicity distribution function (MDF), magnesium abundance, spatial distribution, and correlation of these properties with kinematics. Moreover, the homogeneous s ling of the different Galactic populations provided by the Gaia -ESO survey allowed us to perform a comparison between the bulge, thin disk, and thick disk sequences in the [Mg/Fe] vs. [Fe/H] plane in order to constrain the extent of their eventual chemical similarities. Methods. We obtained spectroscopic data for ~2500 red clump stars in 11 bulge fields, s ling the area −10° ≤ l ≤ + 8° and −10° ≤ b ≤ −4° from the fourth internal data release of the Gaia -ESO survey. A s le of ~6300 disk stars was also selected for comparison. Spectrophotometric distances computed via isochrone fitting allowed us to define a s le of stars likely located in the bulge region. Results. From a Gaussian mixture models (GMM) analysis, the bulge MDF is confirmed to be bimodal across the whole s led area. The relative ratio between the two modes of the MDF changes as a function of b , with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. On the other hand, metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. Their Mg enhancement level and general shape in the [Mg/Fe] vs. [Fe/H] plane is comparable to that of the thick disk sequence. The position at which [Mg/Fe] starts to decrease with [Fe/H], called the “knee”, is observed in the metal-poor bulge at [Fe/H] knee = −0.37 ± 0.09, being 0.06 dex higher than that of the thick disk. Although this difference is inside the error bars, it suggest a higher star formation rate (SFR) for the bulge than for the thick disk. We estimate an upper limit for this difference of Δ [Fe/H] knee = 0.24 dex. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast ( Gyr) intense burst of stellar formation that takes place at early epochs. Conclusions. We associate metal-rich stars with the bar boxy eanut bulge formed as the product of secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and α -abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.
Publisher: EDP Sciences
Date: 21-04-2016
Publisher: SPIE
Date: 09-08-2016
DOI: 10.1117/12.2232107
Publisher: Oxford University Press (OUP)
Date: 15-07-2021
Abstract: We present chemical abundances for 21 elements (from Li to Eu) in 150 metal-poor Galactic stars spanning −4.1 & [Fe/H] & −2.1. The targets were selected from the SkyMapper survey and include 90 objects with [Fe/H] ≤ −3 of which some 15 have [Fe/H] ≤ −3.5. When combining the s le with our previous studies, we find that the metallicity distribution function has a power-law slope of Δ(log N)/Δ[Fe/H] = 1.51 ± 0.01 dex per dex over the range −4 ≤ [Fe/H] ≤ −3. With only seven carbon-enhanced metal-poor stars in the s le, we again find that the selection of metal-poor stars based on SkyMapper filters is biased against highly carbon-rich stars for [Fe/H] & −3.5. Of the 20 objects for which we could measure nitrogen, 11 are nitrogen-enhanced metal-poor (NEMP) stars. Within our s le, the high NEMP fraction (55 per cent ± 21 per cent) is compatible with the upper range of predicted values (between 12 per cent and 35 per cent). The chemical abundance ratios [X/Fe] versus [Fe/H] exhibit similar trends to previous studies of metal-poor stars and Galactic chemical evolution models. We report the discovery of nine new r-I stars, four new r-II stars, one of which is the most metal-poor known, nine low-α stars with [α/Fe] ≤ 0.15 as well as one unusual star with [Zn/Fe] = +1.4 and [Sr/Fe] = +1.2 but with normal [Ba/Fe]. Finally, we combine our s le with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy.
Publisher: American Astronomical Society
Date: 07-2021
Publisher: Oxford University Press (OUP)
Date: 22-01-2019
DOI: 10.1093/MNRAS/STZ217
Publisher: American Astronomical Society
Date: 05-2021
Publisher: American Astronomical Society
Date: 25-08-2023
Abstract: The progenitor systems and explosion mechanisms responsible for the thermonuclear events observationally classified as Type Ia supernovae are uncertain and difficult to uniquely constrain using traditional observations of Type Ia supernova host galaxies, progenitors, light curves, and remnants. For the subset of thermonuclear events that are prolific producers of iron, we use published theoretical nucleosynthetic yields to identify a set of elemental abundance ratios infrequently observed in metal-poor stars but shared across a range of progenitor systems and explosion mechanisms: [Na, Mg, Co/Fe] 0. We label stars with this abundance signature “iron-rich metal-poor,” or IRMP stars. We suggest that IRMP stars formed in environments dominated by thermonuclear nucleosynthesis and consequently that their elemental abundances can be used to constrain both the progenitor systems and explosion mechanisms responsible for thermonuclear explosions. We identify three IRMP stars in the literature and homogeneously infer their elemental abundances. We find that the elemental abundances of BD +80 245, HE 0533–5340, and SMSS J034249.53–284216.0 are best explained by the (double) detonations of sub-Chandrasekhar-mass CO white dwarfs. If our interpretation of IRMP stars is accurate, then they should be very rare in globular clusters and more common in the Magellanic Clouds and dwarf spheroidal galaxies than in the Milky Way’s halo. We propose that future studies of IRMP stars will quantify the relative occurrences of different thermonuclear event progenitor systems and explosion mechanisms.
Publisher: Oxford University Press (OUP)
Date: 24-02-2017
DOI: 10.1093/MNRAS/STX480
Publisher: Oxford University Press (OUP)
Date: 05-11-2014
Publisher: Oxford University Press (OUP)
Date: 29-04-2016
Publisher: Springer Science and Business Media LLC
Date: 02-02-2017
Publisher: Oxford University Press (OUP)
Date: 27-09-2016
Publisher: American Astronomical Society
Date: 24-09-2020
Publisher: EDP Sciences
Date: 17-06-2016
Publisher: American Astronomical Society
Date: 13-06-2013
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: EDP Sciences
Date: 09-2018
DOI: 10.1051/0004-6361/201832841
Abstract: Context . Several works have found an increase of the abundances of the s -process neutron-capture elements in the youngest Galactic stellar populations. These trends provide important constraints on stellar and Galactic evolution and they need to be confirmed with large and statistically significant s les of stars spanning wide age and distance intervals. Aims . We aim to trace the abundance patterns and the time evolution of five s -process elements – two belonging to the first peak, Y and Zr, and three belonging to the second peak, Ba, La, and Ce – using the Gaia -ESO IDR 5 results for open clusters and disc stars. Methods . From the UVES spectra of cluster member stars, we determined the average composition of clusters with ages .1 Gyr. We derived statistical ages and distances of field stars, and we separated them into thin and thick disc populations. We studied the time-evolution and dependence on metallicity of abundance ratios using open clusters and field stars whose parameters and abundances were derived in a homogeneous way. Results . Using our large and homogeneous s le of open clusters, thin and thick disc stars, spanning an age range larger than 10 Gyr, we confirm an increase towards young ages of s -process abundances in the solar neighbourhood. These trends are well defined for open clusters and stars located nearby the solar position and they may be explained by a late enrichment due to significant contribution to the production of these elements from long-living low-mass stars. At the same time, we find a strong dependence of the s -process abundance ratios on the Galactocentric distance and on the metallicity of the clusters and field stars. Conclusions . Our results, derived from the largest and most homogeneous s le of s -process abundances in the literature, confirm the growth with decreasing stellar ages of the s -process abundances in both field and open cluster stars. At the same time, taking advantage of the abundances of open clusters located in a wide Galactocentric range, these results offer a new perspective on the dependence of the s -process evolution on the metallicity and star formation history, pointing to different behaviours at various Galactocentric distances.
Publisher: American Astronomical Society
Date: 24-01-2023
Abstract: We present Korg , a new package for 1D LTE spectral synthesis of FGK stars, which computes theoretical spectra from the near-ultraviolet to the near-infrared, and implements both plane-parallel and spherical radiative transfer. We outline the inputs and internals of Korg , and compare synthetic spectra from Korg , Moog , Turbospectrum , and SME . The disagreements between Korg and the other codes are no larger than those between the other codes, although disagreement between codes is substantial. We examine the case of a C 2 band in detail, finding that uncertainties on physical inputs to spectral synthesis account for a significant fraction of the disagreement. Korg is 1–100 times faster than other codes in typical use, compatible with automatic differentiation libraries, and easily extensible, making it ideal for statistical inference and parameter estimation applied to large data sets. Documentation and installation instructions are available at ajwheeler.github.io/Korg.jl/stable/ .
Publisher: American Astronomical Society
Date: 08-2019
Publisher: American Astronomical Society
Date: 08-06-2015
Publisher: Oxford University Press (OUP)
Date: 17-07-2019
Abstract: We report the discovery of SMSS J160540.18−144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure $\\left[\\rm {Fe}/\\rm {H}\\right]= -6.2 \\pm 0.2$ (1D LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, $\\left[\\rm {C}/\\rm {Fe}\\right] = 3.9 \\pm 0.2$, while other abundances are compatible with an α-enhanced solar-like pattern with $\\left[\\rm {Ca}/\\rm {Fe}\\right] = 0.4 \\pm 0.2$, $\\left[\\rm {Mg}/\\rm {Fe}\\right] = 0.6 \\pm 0.2$, $\\left[\\rm {Ti}/\\rm {Fe}\\right] = 0.8 \\pm 0.2$, and no significant s- or r-process enrichment, $\\left[\\rm {Sr}/\\rm {Fe}\\right] \\lt 0.2$ and $\\left[\\rm {Ba}/\\rm {Fe}\\right] \\lt 1.0$ (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about $10\\, \\rm M_\\odot$ imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly $20\\, \\rm M_\\odot$ are incompatible with the observed abundance pattern.
Publisher: EDP Sciences
Date: 28-10-2016
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: 13-08-2018
Publisher: American Astronomical Society
Date: 23-02-2017
Publisher: EDP Sciences
Date: 08-2015
Publisher: Oxford University Press (OUP)
Date: 23-06-2023
Abstract: To accurately measure a star’s atmospheric parameters and chemical abundances, it is crucial to have high-quality spectra. Analysing the detailed chemical abundances of groups of stars can help us better understand nucleosynthesis, galactic chemical enrichment, and stellar evolution. In this study, we explored whether stellar spots can affect a star’s inferred metallicity and, if so, where the impact is the strongest. To investigate this, we created synthetic infrared spectra that included stellar spots for a s le of main-sequence stars younger than the sun. We then applied two models to the data: one that accounted for spots and the other that did not. From this, we can determine the bias introduced when fitting spotted spectra with a non-spotted model and how this bias varies with different parameters. Our findings revealed that fitting spotted spectra with a non-spotted model can introduce a scatter of up to 0.05 dex in the inferred metallicity, especially for stars with high levels of spot coverage. This bias is similar in magnitude to other relevant effects, such as atomic diffusion, radiative levitation, or non-local thermodynamic equilibrium. We also found that the effect is most pronounced in young stars and decreases with age. These results suggest that stellar spots can introduce a systematic uncertainty in metallicity that is not currently accounted for in spectroscopic analysis. This could potentially limit scientific inferences for population-level studies of young stars and differential abundance analyses.
Publisher: Oxford University Press (OUP)
Date: 17-08-2017
Publisher: American Astronomical Society
Date: 03-05-2013
Publisher: Oxford University Press (OUP)
Date: 29-04-2016
Publisher: American Astronomical Society
Date: 03-08-2018
Publisher: American Astronomical Society
Date: 24-01-2018
Publisher: Springer Science and Business Media LLC
Date: 29-07-2020
Publisher: American Astronomical Society
Date: 19-11-2014
Publisher: EDP Sciences
Date: 07-2017
Publisher: American Astronomical Society
Date: 19-04-2018
Publisher: EDP Sciences
Date: 16-05-2016
Publisher: European Southern Observatory (ESO)
Date: 2019
Publisher: American Astronomical Society
Date: 29-08-2018
Publisher: American Astronomical Society
Date: 21-09-2015
Publisher: European Southern Observatory (ESO)
Date: 2019
Publisher: Oxford University Press (OUP)
Date: 10-08-2021
Abstract: In order to accurately determine stellar properties, knowledge of the effective temperature of stars is vital. We implement Gaia and 2MASS photometry in the InfraRed Flux Method and apply it to over 360 000 stars across different evolutionary stages in the GALAH DR3 survey. We derive colour-effective temperature relations that take into account the effect of metallicity and surface gravity over the range $4000\\, \\rm {K}\\lesssim T_{\\rm {eff}}\\lesssim 8000\\, \\rm {K}$, from very metal-poor stars to supersolar metallicities. The internal uncertainty of these calibrations is of order 40–80 K depending on the colour combination used. Comparison against solar-twins, Gaia benchmark stars, and the latest interferometric measurements validates the precision and accuracy of these calibrations from F to early M spectral types. We assess the impact of various sources of uncertainties, including the assumed extinction law, and provide guidelines to use our relations. Robust solar colours are also derived.
Publisher: American Astronomical Society
Date: 21-09-2020
Publisher: EDP Sciences
Date: 2018
DOI: 10.1051/0004-6361/201730412
Abstract: Context. There have been conflicting results with respect to the extent that radial migration has played in the evolution of the Galaxy. Additionally, observations of the solar neighborhood have shown evidence of a merger in the past history of the Milky Way that drives enhanced radial migration. Aims. We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H] 0.1) stars within 2 kpc of the Sun. We also aim to investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the Sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. Methods. We used a s le of more than 3000 stars selected from the fourth internal data release of the Gaia -ESO Survey. We used the stellar parameters from the Gaia -ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Results. Analyzing the kinematics of the most metal-rich stars ([Fe/H] 0.1), we find that more than half have small eccentricities ( e 0.2) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons R p 7 kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by ~10 km s -1 . The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s -1 slower than stars with lower [Mg/Fe] ratios. Conclusions. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.
Publisher: American Astronomical Society
Date: 11-11-2016
Publisher: Springer Science and Business Media LLC
Date: 07-07-2021
DOI: 10.1038/S41586-021-03611-2
Abstract: Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis
Publisher: Oxford University Press (OUP)
Date: 16-04-2021
Abstract: An unprecedented number of exoplanets are being discovered by the Transiting Exoplanet Survey Satellite (TESS). Determining the orbital parameters of these exoplanets, and especially their mass and radius, will depend heavily upon the measured physical characteristics of their host stars. We have cross-matched spectroscopic, photometric, and astrometric data from GALAH Data Release 2, the TESS Input Catalog and Gaia Data Release 2, to create a curated, self-consistent catalogue of physical and chemical properties for 47 285 stars. Using these data, we have derived isochrone masses and radii that are precise to within 5 per cent. We have revised the parameters of three confirmed, and twelve candidate, TESS planetary systems. These results cast doubt on whether CTOI-20125677 is indeed a planetary system, since the revised planetary radii are now comparable to stellar sizes. Our GALAH–TESS catalogue contains abundances for up to 23 elements. We have specifically analysed the molar ratios for C/O, Mg/Si, Fe/Si, and Fe/Mg, to assist in determining the composition and structure of planets with Rp & 4R⊕. From these ratios, 36 per cent fall within 2$\\sigma$ sigma of the Sun/Earth values, suggesting that these stars may host rocky exoplanets with geological compositions similar to planets found within our own Solar system.
Publisher: Oxford University Press (OUP)
Date: 24-04-2018
Publisher: Oxford University Press (OUP)
Date: 05-07-2018
Publisher: American Astronomical Society
Date: 05-11-2021
Abstract: The chemical abundances of a galaxy’s metal-poor stellar population can be used to investigate the earliest stages of its formation and chemical evolution. The Magellanic Clouds are the most massive of the Milky Way’s satellite galaxies and are thought to have evolved in isolation until their recent accretion by the Milky Way. Unlike the Milky Way’s less massive satellites, little is known about the Magellanic Clouds’ metal-poor stars. We have used the mid-infrared metal-poor star selection of Schlaufman & Casey and archival data to target nine LMC and four SMC giants for high-resolution Magellan/MIKE spectroscopy. These nine LMC giants with −2.4 ≲ [Fe/H] ≲ −1.5 and four SMC giants with −2.6 ≲ [Fe/H] ≲ −2.0 are the most metal-poor stars in the Magellanic Clouds yet subject to a comprehensive abundance analysis. While we find that at constant metallicity these stars are similar to Milky Way stars in their α , light, and iron-peak elemental abundances, both the LMC and SMC are enhanced relative to the Milky Way in the r -process element europium. These abundance offsets are highly significant, equivalent to 3.9 σ for the LMC, 2.7 σ for the SMC, and 5.0 σ for the complete Magellanic Cloud s le. We propose that the r -process enhancement of the Magellanic Clouds’ metal-poor stellar population is a result of the Magellanic Clouds’ isolated chemical evolution and long history of accretion from the cosmic web combined with r -process nucleosynthesis on a timescale longer than the core-collapse supernova timescale but shorter than or comparable to the thermonuclear (i.e., Type Ia) supernova timescale.
Publisher: Oxford University Press (OUP)
Date: 17-03-2023
Abstract: Rotationally induced mode splitting frequencies of low-luminosity subgiants suggest that angular momentum transport mechanisms are 1–2 orders of magnitude more efficient in these stars than predicted by theory. Constraints on the rotation profile of low-luminosity subgiants could be used to identify the dominant mechanism for angular momentum transport. We develop a forward model for the rotation profile given observed rotational splittings, assuming a step-like rotation profile. We identify a consistent degeneracy between the position of the profile discontinuity and the surface rotation rate. We perform mock experiments that show the discontinuity position can be better constrained with a prior on the surface rotation rate, which is informed by star spot modulations. We finally apply this approach to KIC 12508433, a well-studied low-luminosity subgiant, as an ex le case. With the observed surface rotation prior, we obtain a factor of 2 increase in precision of the position of strong rotation gradient. We recover the literature values of the core and surface rotation rates and find the highest support for a discontinuity in the radiative zone. Auxiliary measurements of surface rotation could substantially improve inferences on the rotation profile of low-luminosity subgiants with already available data.
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: American Astronomical Society
Date: 13-08-2015
Publisher: EDP Sciences
Date: 30-09-2013
Publisher: American Astronomical Society
Date: 27-02-2014
Publisher: Oxford University Press (OUP)
Date: 03-11-2016
Publisher: American Astronomical Society
Date: 07-2022
Abstract: Of all the light elements, the evolution of lithium (Li) in the Milky Way is perhaps the most difficult to explain. Li is difficult to synthesize and is easily destroyed, making most stellar sites unsuitable for producing Li in sufficient quantities to account for the protosolar abundance. For decades, novae have been proposed as a potential explanation for this “Galactic Li problem,” and the recent detection of 7 Be in the ejecta of multiple nova eruptions has breathed new life into this theory. In this work, we assess the viability of novae as dominant producers of Li in the Milky Way. We present the most comprehensive treatment of novae in a galactic chemical evolution code to date, testing theoretically and observationally derived nova Li yields by integrating metallicity-dependent nova ejecta profiles computed using the binary population synthesis code binary _ c with the galactic chemical evolution code OMEGA+ . We find that our galactic chemical evolution models which use observationally derived Li yields account for the protosolar Li abundance very well, while models relying on theoretical nova yields cannot reproduce the protosolar observation. A brief exploration of physical uncertainties including single-stellar yields, the metallicity resolution of our nova treatment, common-envelope physics, and nova accretion efficiencies indicates that this result is robust to physical assumptions. Scatter within the observationally derived Li yields in novae is identified as the primary source of uncertainty, motivating further observations of 7 Be in nova ejecta.
Publisher: American Astronomical Society
Date: 11-2021
Publisher: American Astronomical Society
Date: 08-2023
Abstract: The eighteenth data release (DR18) of the Sloan Digital Sky Survey (SDSS) is the first one for SDSS-V, the fifth generation of the survey. SDSS-V comprises three primary scientific programs or “Mappers”: the Milky Way Mapper (MWM), the Black Hole Mapper (BHM), and the Local Volume Mapper. This data release contains extensive targeting information for the two multiobject spectroscopy programs (MWM and BHM), including input catalogs and selection functions for their numerous scientific objectives. We describe the production of the targeting databases and their calibration and scientifically focused components. DR18 also includes ∼25,000 new SDSS spectra and supplemental information for X-ray sources identified by eROSITA in its eFEDS field. We present updates to some of the SDSS software pipelines and preview changes anticipated for DR19. We also describe three value-added catalogs (VACs) based on SDSS-IV data that have been published since DR17, and one VAC based on the SDSS-V data in the eFEDS field.
Publisher: Oxford University Press (OUP)
Date: 29-04-2023
Abstract: We present an empirical model of age-dependent photospheric lithium depletion, calibrated using a large homogeneously analysed s le of 6200 stars in 52 open clusters, with ages from 2 to 6000 Myr and −0.3 & [Fe/H] & 0.2, observed in the Gaia-ESO spectroscopic survey. The model is used to obtain age estimates and posterior age probability distributions from measurements of the Li i 6708 Å equivalent width for in idual (pre) main-sequence stars with 3000 & Teff/K & 6500, a domain where age determination from the HR diagram is either insensitive or highly model-dependent. In the best cases, precisions of 0.1 dex in log age are achievable even higher precision can be obtained for coeval groups and associations where the in idual age probabilities of their members can be combined. The method is validated on a s le of exoplanet-hosting young stars, finding agreement with claimed young ages for some, but not others. We obtain better than 10 per cent precision in age, and excellent agreement with published ages, for seven well-studied young moving groups. The derived ages for young clusters (& Gyr) in our s le are also in good agreement with their training ages, and consistent with several published model-insensitive lithium depletion boundary ages. For older clusters, there remain systematic age errors that could be as large as a factor of 2. There is no evidence to link these errors to any strong systematic metallicity dependence of (pre) main-sequence lithium depletion, at least in the range −0.29 & [Fe/H] & 0.18. Our methods and model are provided as software – ‘Empirical AGes from Lithium Equivalent widthS’ (eagles).
Publisher: American Astronomical Society
Date: 20-06-2016
Publisher: Oxford University Press (OUP)
Date: 06-05-2021
Abstract: The ensemble of chemical element abundance measurements for stars, along with precision distances and orbit properties, provides high-dimensional data to study the evolution of the Milky Way. With this third data release of the Galactic Archaeology with HERMES (GALAH) survey, we publish 678 423 spectra for 588 571 mostly nearby stars (81.2 per cent of stars are within & kpc), observed with the HERMES spectrograph at the Anglo-Australian Telescope. This release (hereafter GALAH+ DR3) includes all observations from GALAH Phase 1 (bright, main, and faint survey, 70 per cent), K2-HERMES (17 per cent), TESS-HERMES (5 per cent), and a subset of ancillary observations (8 per cent) including the bulge and & stellar clusters. We derive stellar parameters Teff, log g, [Fe/H], vmic, vbroad, and vrad using our modified version of the spectrum synthesis code Spectroscopy Made Easy (sme) and 1D marcs model atmospheres. We break spectroscopic degeneracies in our spectrum analysis with astrometry from Gaia DR2 and photometry from 2MASS. We report abundance ratios [X/Fe] for 30 different elements (11 of which are based on non-LTE computations) covering five nucleosynthetic pathways. We describe validations for accuracy and precision, flagging of peculiar stars/measurements and recommendations for using our results. Our catalogue comprises 65 per cent dwarfs, 34 per cent giants, and 1 per cent other/unclassified stars. Based on unflagged chemical composition and age, we find 62 per cent young low-$\\alpha$, 9 per cent young high-$\\alpha$, 27 per cent old high-$\\alpha$, and 2 per cent stars with [Fe/H] ≤ −1. Based on kinematics, 4 per cent are halo stars. Several Value-Added-Catalogues, including stellar ages and dynamics, updated after Gaia eDR3, accompany this release and allow chrono-chemodynamic analyses, as we showcase.
Publisher: Oxford University Press (OUP)
Date: 02-03-2017
DOI: 10.1093/MNRAS/STX508
Publisher: EDP Sciences
Date: 07-06-2016
Publisher: Oxford University Press (OUP)
Date: 07-03-2019
DOI: 10.1093/MNRAS/STZ645
Abstract: We present results from the analysis of high-resolution spectra obtained with the Keck HIRES spectrograph for a s le of 17 candidate extremely metal-poor (EMP) stars originally selected from commissioning data obtained with the SkyMapper telescope. Fourteen of the stars have not been observed previously at high dispersion. Three have [Fe/H] ≤ −3.0, while the remainder, with two more metal-rich exceptions, have −3.0 ≤ [Fe/H] ≤ −2.0 dex. Apart from Fe, we also derive abundances for the elements C, N, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, and Zn, and for n-capture elements Sr, Ba, and Eu. None of the current s le of stars is found to be carbon-rich. In general, our chemical abundances follow previous trends found in the literature, although we note that two of the most metal-poor stars show very low [Ba/Fe] (∼−1.7) coupled with low [Sr/Ba] (∼−0.3). Such stars are relatively rare in the Galactic halo. One further star, and possibly two others, meet the criteria for classification as a r-I star. This study, together with that of Jacobson et al. (2015), completes the outcomes of the SkyMapper commissioning data survey for EMP stars.
Publisher: EDP Sciences
Date: 05-2017
Publisher: EDP Sciences
Date: 28-04-2017
Publisher: Oxford University Press (OUP)
Date: 2020
Abstract: The recent identification of a candidate very massive (70 M⊙) black hole (BH) is at odds with our current understanding of stellar winds and pair-instability supernovae. We investigate alternate explanations for this system by searching the bpass v2.2 stellar and population synthesis models for those that match the observed properties of the system. We find binary evolution models that match the LB-1 system, at the reported Gaia distance, with more moderate BH masses of 4–7 M⊙. We also examine the suggestion that the binary motion may have led to an incorrect distance determination by Gaia. We find that the Gaia distance is accurate and that the binary system is consistent with the observation at this distance. Consequently, it is highly improbable that the BH in this system has the extreme mass originally suggested. Instead, it is more likely to be representative of the typical BH binary population expected in our Galaxy.
Publisher: Oxford University Press (OUP)
Date: 23-08-2018
Publisher: Springer Science and Business Media LLC
Date: 11-2015
DOI: 10.1038/NATURE15747
Abstract: The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.
Publisher: Oxford University Press (OUP)
Date: 04-07-2016
Publisher: EDP Sciences
Date: 12-2017
Publisher: American Astronomical Society
Date: 11-12-2019
Publisher: American Astronomical Society
Date: 14-09-2021
Abstract: Statistical studies of cataloged object properties are central to astrophysics. But one cannot model those objects’ population properties without the s le’s selection function, the quantitative understanding of which objects could have ended up in such a catalog. As didactic introductions to this topic are scarce in the astrophysical literature, we provide one here, addressing the following questions: What is a selection function? On what arguments q should it depend? Over what domain must a selection function be defined? What simplifying approximations can be made? And, how is a selection function used in “modeling”? We argue that volume-complete s les, limited by the faintest objects, reflect a highly suboptimal selection function, needlessly reducing the number of bright and usually rare s le members. We illustrate these points by a worked ex le: aia-unlimited/WD-selection-function, deriving the space density of white dwarfs (WDs) in the Galactic neighborhood as a function of their luminosity and color, Φ 0 ( M G , ( B − R )) in [mag −2 pc −3 ]. We construct a s le of 10 5 presumed WDs through straightforward selection cuts on the Gaia EDR3 catalog in magnitude, color, and parallax, q = ( G , ( B − R ), ϖ ). We then combine a simple model for Φ 0 with this selection function’s S ( q ) effective survey volume to estimate Φ 0 ( M G , ( B − R )) precisely and robustly against the detailed choices for S ( q ) . This resulting WD luminosity–color function Φ 0 ( M G , ( B − R )) differs dramatically from the initial number density distribution in the luminosity−color plane: by orders of magnitude in density and by four magnitudes in density peak location.
Publisher: Oxford University Press (OUP)
Date: 12-11-2020
Abstract: We present isochrone ages and initial bulk metallicities ($\\rm [Fe/H]_{bulk}$, by accounting for diffusion) of 163 722 stars from the GALAH Data Release 2, mainly composed of main-sequence turn-off stars and subgiants ($7000\\, \\mathrm{ K}& T_{\\mathrm{ eff}}& 4000\\, \\mathrm{ K}$ and $\\log g& $ dex). The local age–metallicity relationship (AMR) is nearly flat but with significant scatter at all ages the scatter is even higher when considering the observed surface abundances. After correcting for selection effects, the AMR appears to have intrinsic structures indicative of two star formation events, which we speculate are connected to the thin and thick discs in the solar neighbourhood. We also present abundance ratio trends for 16 elements as a function of age, across different $\\rm [Fe/H]_{bulk}$ bins. In general, we find the trends in terms of [X/Fe] versus age from our far larger s le to be compatible with studies based on small (∼100 stars) s les of solar twins, but we now extend them to both sub- and supersolar metallicities. The α-elements show differing behaviour: the hydrostatic α-elements O and Mg show a steady decline with time for all metallicities, while the explosive α-elements Si, Ca, and Ti are nearly constant during the thin-disc epoch (ages $\\lesssim \\! 12$ Gyr). The s-process elements Y and Ba show increasing [X/Fe] with time while the r-process element Eu has the opposite trend, thus favouring a primary production from sources with a short time delay such as core-collapse supernovae over long-delay events such as neutron star mergers.
Publisher: Oxford University Press (OUP)
Date: 19-07-2018
Publisher: Oxford University Press (OUP)
Date: 22-10-2019
Abstract: Here we present the discovery of 895 s-process-rich candidates from 454 180 giant stars observed by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) using a data-driven approach. This s le constitutes the largest number of s-process enhanced stars ever discovered. Our s le includes 187 s-process-rich candidates that are enhanced in both barium and strontium, 49 stars with significant barium enhancement only and 659 stars that show only a strontium enhancement. Most of the stars in our s le are in the range of effective temperature and log g typical of red giant branch (RGB) populations, which is consistent with our observational selection bias towards finding RGB stars. We estimate that only a small fraction (∼0.5 per cent) of binary configurations are favourable for s-process enriched stars. The majority of our s-process-rich candidates (95 per cent) show strong carbon enhancements, whereas only five candidates (& per cent) show evidence of sodium enhancement. Our kinematic analysis reveals that 97 per cent of our s le are disc stars, with the other 3 per cent showing velocities consistent with the Galactic halo. The scaleheight of the disc is estimated to be $z_{\\rm h}=0.634 \\pm {0.063}\\, \\mathrm{kpc}$, comparable with values in the literature. A comparison with yields from asymptotic giant branch (AGB) models suggests that the main neutron source responsible for the Ba and Sr enhancements is the 13C(α,n)16O reaction. We conclude that s-process-rich candidates may have received their overabundances via mass transfer from a previous AGB companion with an initial mass in the range $1\\!-\\!3\\, \\mathrm{M}_{\\odot }$.
Publisher: Oxford University Press (OUP)
Date: 11-05-2017
Publisher: American Astronomical Society
Date: 06-05-2011
Publisher: American Astronomical Society
Date: 23-01-2013
Publisher: American Astronomical Society
Date: 30-11-2017
Publisher: Oxford University Press (OUP)
Date: 25-08-2015
Publisher: Springer Science and Business Media LLC
Date: 02-2014
DOI: 10.1038/NATURE12990
Abstract: The element abundance ratios of four low-mass stars with extremely low metallicities (abundances of elements heavier than helium) indicate that the gas out of which the stars formed was enriched in each case by at most a few--and potentially only one--low-energy supernova. Such supernovae yield large quantities of light elements such as carbon but very little iron. The dominance of low-energy supernovae seems surprising, because it had been expected that the first stars were extremely massive, and that they disintegrated in pair-instability explosions that would rapidly enrich galaxies in iron. What has remained unclear is the yield of iron from the first supernovae, because hitherto no star has been unambiguously interpreted as encapsulating the yield of a single supernova. Here we report the optical spectrum of SMSS J031300.36-670839.3, which shows no evidence of iron (with an upper limit of 10(-7.1) times solar abundance). Based on a comparison of its abundance pattern with those of models, we conclude that the star was seeded with material from a single supernova with an original mass about 60 times that of the Sun (and that the supernova left behind a black hole). Taken together with the four previously mentioned low-metallicity stars, we conclude that low-energy supernovae were common in the early Universe, and that such supernovae yielded light-element enrichment with insignificant iron. Reduced stellar feedback both chemically and mechanically from low-energy supernovae would have enabled first-generation stars to form over an extended period. We speculate that such stars may perhaps have had an important role in the epoch of cosmic reionization and the chemical evolution of early galaxies.
Publisher: Oxford University Press (OUP)
Date: 02-12-2021
Abstract: Pioneering photometric, astrometric, and spectroscopic surveys is helping exoplanetary scientists better constrain the fundamental properties of stars within our galaxy and the planets these stars host. In this study, we use the third data release from the stellar spectroscopic GALAH Survey, coupled with astrometric data of eDR3 from the Gaia satellite, and other data from NASA’s Exoplanet Archive, to refine our understanding of 279 confirmed and candidate exoplanet host stars and their exoplanets. This homogenously analysed data set comprises 105 confirmed exoplanets, along with 146 K2 candidates, 95 TESS Objects of Interest (TOIs), and 52 Community TOIs (CTOIs). Our analysis significantly shifts several previously (unknown) planet parameters while decreasing the uncertainties for others. Our radius estimates suggest that 35 planet candidates are more likely brown dwarfs or stellar companions due to their new radius values. We are able to refine the radii and masses of WASP-47 e, K2-106 b, and CoRoT-7 b to their most precise values yet to less than 2.3 per cent and 8.5 per cent, respectively. We also use stellar rotational values from GALAH to show that most planet candidates will have mass measurements that will be tough to obtain with current ground-based spectrographs. With GALAH’s chemical abundances, we show through chemo-kinematics that there are five planet hosts that are associated with the galaxy’s thick disc, including NGTS-4, K2-183, and K2-337. Finally, we show that there is no statistical difference between the chemical properties of hot Neptune and hot rocky exoplanet hosts, with the possibility that short-period rocky worlds might be the remnant cores of hotter, gaseous worlds.
Publisher: American Astronomical Society
Date: 06-08-2018
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 11-2021
End Date: 11-2024
Amount: $405,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 08-2019
End Date: 12-2022
Amount: $364,259.00
Funder: Australian Research Council
View Funded ActivityStart Date: 12-2009
End Date: 04-2017
Amount: $3,097,098.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2019
End Date: 12-2020
Amount: $159,000.00
Funder: Australian Research Council
View Funded Activity