ORCID Profile
0000-0002-5648-3107
Current Organisations
St. Xavier's College (Autonomous)
,
University of Southern Queensland - Springfield Campus
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Stellar Astronomy and Planetary Systems | Astronomical and Space Sciences | Astrobiology | Astronomical and Space Instrumentation
Publisher: American Astronomical Society
Date: 17-09-2019
Publisher: Oxford University Press (OUP)
Date: 04-09-2020
Abstract: Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spectrum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of 2.8 ± 0.5 kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one δ Sct–roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic d ing in suppressing low-overtone p modes in Ap stars. We compute the magnetic d ing effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths ≲4 kG, while other low-overtone p modes are driven for polar field strengths up to ∼1.5 kG. We find that the high-order g modes commonly observed in γ Dor stars are heavily d ed by polar fields stronger than 1–4 kG, with the d ing being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as γ Dor stars. We use our helium-depleted models to calculate the δ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at ∼5 × 104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars.
Publisher: American Astronomical Society
Date: 12-2022
Abstract: Young stellar populations provide a record of past star formation, and by establishing their members’ dynamics and ages, it is possible to reconstruct the full history of star formation events. Gaia has greatly expanded the number of accessible stellar populations, with one of the most notable recently discovered associations being Cepheus Far North (CFN), a population containing hundreds of members spanning over 100 pc. With its proximity ( d ≲ 200 pc), apparent substructure, and relatively small population, CFN represents a manageable population to study in depth, with enough evidence of internal complexity to produce a compelling star formation story. Using Gaia astrometry and photometry combined with additional spectroscopic observations, we identify over 500 candidate CFN members spread across seven subgroups. Combining ages from isochrones, asteroseismology, dynamics, and lithium depletion, we produce well-constrained ages for all seven subgroups, revealing a largely continuous 10 Myr star formation history in the association. By tracing back the present-day populations to the time of their formation, we identify two spatially and dynamically distinct nodes in which stars form, one associated with β Cephei, which shows mostly co-spatial formation, and one associated with EE Draconis with a more dispersed star formation history. This detailed view of star formation demonstrates the complexity of the star formation process, even in the smallest of regions.
Publisher: Oxford University Press (OUP)
Date: 20-11-2017
Publisher: Oxford University Press (OUP)
Date: 14-01-2023
Abstract: The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation – the so-called hump and spike feature. We investigate whether the rotational modulation (spike) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the litudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anticorrelation between the spike litudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the spike feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in hump and spike stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age).
Publisher: Cambridge University Press (CUP)
Date: 2018
DOI: 10.1017/PASA.2018.5
Abstract: We present the first data release of the SkyMapper Southern Survey, a hemispheric survey carried out with the SkyMapper Telescope at Siding Spring Observatory in Australia. Here, we present the survey strategy, data processing, catalogue construction, and database schema. The first data release dataset includes over 66 000 images from the Shallow Survey component, covering an area of 17 200 deg 2 in all six SkyMapper passbands uvgriz , while the full area covered by any passband exceeds 20 000 deg 2 . The catalogues contain over 285 million unique astrophysical objects, complete to roughly 18 mag in all bands. We compare our griz point-source photometry with Pan-STARRS1 first data release and note an RMS scatter of 2%. The internal reproducibility of SkyMapper photometry is on the order of 1%. Astrometric precision is better than 0.2 arcsec based on comparison with Gaia first data release. We describe the end-user database, through which data are presented to the world community, and provide some illustrative science queries.
Publisher: Oxford University Press (OUP)
Date: 27-02-2020
Abstract: Measuring phase modulation in pulsating stars has proven to be a highly successful way of finding binary systems. The class of pulsating main-sequence A and F variables, known as δ Scuti stars consists of particularly good targets for this, and the Kepler s le of these has been almost fully exploited. However, some Keplerδ Scuti stars have incorrect temperatures in stellar properties catalogues, and were missed in previous analyses. We used an automated pulsation classification algorithm to find 93 new δ Scuti pulsators among tens of thousands of F-type stars, which we then searched for phase modulation attributable to binarity. We discovered 10 new binary systems and calculated their orbital parameters, which we compared with those of binaries previously discovered in the same way. The results suggest that some of the new companions may be white dwarfs.
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 06-08-2020
Abstract: We systematically searched for gravity- and Rossby-mode period spacing patterns in Kepler eclipsing binaries with γ Doradus pulsators. These stars provide an excellent opportunity to test the theory of tidal synchronization and angular momentum transport in F- and A-type stars. We discovered 35 systems that show clear patterns, including the spectroscopic binary KIC 10080943. Combined with 45 non-eclipsing binaries with γ Dor components that have been found using pulsation timing, we measured their near-core rotation rates and asymptotic period spacings. We find that many stars are tidally locked if the orbital periods are shorter than 10 d, in which the near-core rotation periods given by the traditional approximation of rotation are consistent with the orbital period. Compared to the single stars, γ Dor stars in binaries tend to have slower near-core rotation rates, likely a consequence of tidal spin-down. We also find three stars that have extremely slow near-core rotation rates. To explain these, we hypothesize that unstable tidally excited oscillations can transfer angular momentum from the star to the orbit, and slow the star below synchronism, a process we refer to as ‘inverse tides’.
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 15-01-2014
Publisher: Oxford University Press (OUP)
Date: 04-08-2015
Publisher: Oxford University Press (OUP)
Date: 18-10-2019
Abstract: We report our survey of γ Dor stars from the 4-yr Kepler mission. These stars pulsate mainly in g modes and r modes, showing period-spacing patterns in the litude spectra. The period-spacing patterns are sensitive to the chemical composition gradients and the near-core rotation, hence they are essential for understanding the stellar interior. We identified period-spacing patterns in 611 γ Dor stars. Almost every star pulsates in dipole g modes, while about 30% of stars also show clear patterns for quadrupole g modes and 16% of stars present r mode patterns. We measure periods, period spacings, and the gradient of the period spacings. These three observables guide the mode identifications and can be used to estimate the near-core rotation rate. We find many stars are hotter and show longer period-spacing patterns than theory. Using the Traditional Approximation of Rotation (TAR), we inferred the asymptotic spacings, the near-core rotation rates, and the radial orders of the g and r modes. Most stars have a near-core rotation rate around 1 d−1and an asymptotic spacing around 4000 s. We also find that many stars rotate more slowly than predicted by theory for unclear reasons. 11 stars show rotational splittings with fast rotation rates. We compared the observed slope–rotation relation with the theory and find a large spread. We detected rotational modulations in 58 stars and used them to derive the core-to-surface rotation ratios. The interiors rotate faster than the cores in most stars, but by no more than 5%.
Publisher: Oxford University Press (OUP)
Date: 29-11-2016
Publisher: Oxford University Press (OUP)
Date: 04-10-2013
Publisher: Cambridge University Press (CUP)
Date: 2016
DOI: 10.1017/PASA.2016.47
Abstract: This paper presents the first major data release and survey description for the ANU WiFeS SuperNovA Programme. ANU WiFeS SuperNovA Programme is an ongoing supernova spectroscopy c aign utilising the Wide Field Spectrograph on the Australian National University 2.3-m telescope. The first and primary data release of this programme (AWSNAP-DR1) releases 357 spectra of 175 unique objects collected over 82 equivalent full nights of observing from 2012 July to 2015 August. These spectra have been made publicly available via the WISEREP supernova spectroscopy repository. We analyse the ANU WiFeS SuperNovA Programme s le of Type Ia supernova spectra, including measurements of narrow sodium absorption features afforded by the high spectral resolution of the Wide Field Spectrograph instrument. In some cases, we were able to use the integral-field nature of the Wide Field Spectrograph instrument to measure the rotation velocity of the SN host galaxy near the SN location in order to obtain precision sodium absorption velocities. We also present an extensive time series of SN 2012dn, including a near-nebular spectrum which both confirms its ‘super-Chandrasekhar’ status and enables measurement of the sub-solar host metallicity at the SN site.
Publisher: Springer International Publishing
Date: 2015
Publisher: EDP Sciences
Date: 2017
Publisher: Oxford University Press (OUP)
Date: 29-03-2018
DOI: 10.1093/MNRAS/STY784
Publisher: Oxford University Press (OUP)
Date: 08-08-2016
Publisher: Oxford University Press (OUP)
Date: 20-05-2017
Publisher: American Astronomical Society
Date: 29-10-2019
Publisher: Oxford University Press (OUP)
Date: 12-06-2015
Publisher: Oxford University Press (OUP)
Date: 02-03-2019
DOI: 10.1093/MNRAS/STZ590
Publisher: Oxford University Press (OUP)
Date: 28-02-2012
Publisher: EDP Sciences
Date: 11-2020
DOI: 10.1051/0004-6361/202038989
Abstract: Context. Eclipsing binary systems with components that pulsate in gravity modes ( g modes) allow for simultaneous and independent constraints of the chemical mixing profiles of stars. The high precision of the dynamical masses and radii as well as the imposition of identical initial chemical compositions and equivalent ages provide strong constraints during the modelling of g -mode period-spacing patterns. Aims. We aim to assemble a s le of g -mode pulsators in detached eclipsing binaries with the purpose of finding good candidates for future evolutionary and asteroseismic modelling. In addition, we present a case study of the eclipsing binary KIC9850387, identified as our most promising candidate, and detail the results of the observational spectroscopic, photometric, and asteroseismic analysis of the system. Methods. We selected all of the detached eclipsing binaries in the Kepler eclipsing binary catalogue with Kepler Input Catalogue (KIC) temperatures between 6000 K and 10 000 K, and performed a visual inspection to determine the presence and density of g modes, and the presence of g -mode period-spacing patterns in their frequency spectra. We then characterised our s le based on their g -mode pulsational parameters and binary and atmospheric parameters. A spectroscopic follow-up of our most promising candidate was then performed, and the orbital elements of the system were extracted. We then performed spectral disentangling followed by atmospheric modelling and abundance analysis for the primary star. We utilised an iterative approach to simultaneously optimise the pulsational and eclipse models, and subsequently performed an analysis of the pressure- ( p -) and g -mode pulsational frequencies. Results. We compiled a s le of 93 Kepler eclipsing binary stars with g -mode pulsating components and identified clear g -mode period-spacing patterns in the frequency spectra of seven of these systems. We also identified 11 systems that contained hybrid p - and g -mode pulsators. We found that the g -mode pulsational parameters and the binary and atmospheric parameters of our s le are weakly correlated at best, as expected for detached main-sequence binaries. We find that the eclipsing binary KIC9850387 is a double-lined spectroscopic binary in a near-circular orbit with a hybrid p - and g -mode pulsating primary with M p = 1.66 −0.01 +0.01 M ⊙ and R p = 2.154 −0.004 +0.002 R ⊙ , and a solar-like secondary with M s = 1.062 −0.005 +0.003 M ⊙ and R s = 1.081 −0.002 +0.003 R ⊙ . We find ℓ = 1 and ℓ = 2 period-spacing patterns in the frequency spectrum of KIC9850387 spanning more than ten radial orders each, which will allow for stringent constraints of stellar structure during future asteroseismic modelling.
Publisher: American Astronomical Society
Date: 13-12-2012
Publisher: The Open Journal
Date: 10-07-2020
DOI: 10.21105/JOSS.02125
Publisher: Oxford University Press (OUP)
Date: 20-01-2021
Abstract: HD 139614 is known to be a ∼14-Myr-old, possibly pre-main-sequence star in the Sco-Cen OB association in the Upper Centaurus-Lupus subgroup, with a slightly warped circumstellar disc containing ring structures hinting at one or more planets. The star’s chemical abundance pattern is metal-deficient except for volatile elements, which places it in the λ Boo class and suggests it has recently accreted gas-rich but dust-poor material. We identify seven dipole and four radial pulsation modes among its δ Sct pulsations using the TESS light curve and an échelle diagram. Precision modelling with the mesa stellar evolution and gyre stellar oscillation programs confirms it is on the pre-main sequence. Asteroseismic, grid-based modelling suggests an age of 10.75 ± 0.77 Myr, a mass of 1.52 ± 0.02 M ⊙, and a global metal abundance of Z = 0.0100 ± 0.0010. This represents the first asteroseismic determination of the bulk metallicity of a λ Boo star. The precise age and metallicity offer a benchmark for age estimates in Upper Centaurus–Lupus, and for understanding disc retention and planet formation around intermediate-mass stars.
Publisher: Oxford University Press (OUP)
Date: 13-03-2020
Abstract: We report the discovery of the oldest (∼55 Myr) mid-M type star known to host ongoing accretion. 2MASS J15460752–6258042 (2M1546, spectral type M5, 59.2 pc) shows spectroscopic signs of accretion such as strong H α, He i, and [O i] emission lines, from which we estimate an accretion rate of ∼10−10 M⊙ yr−1. Considering the clearly detected infrared excess in all WISE bands, the shape of its spectral energy distribution (SED) and its age, we believe that the star is surrounded by a transitional disc, clearly with some gas still present at inner radii. The position and kinematics of the star from Gaia DR2 and our own radial-velocity measurements suggest membership in the nearby ∼55 Myr-old Argus moving group. At only 59 pc from Earth, 2M1546 is one of the nearest accreting mid-M dwarfs, making it an ideal target for studying the upper limit on the lifetimes of gas-rich discs around low-mass stars.
Publisher: Springer Science and Business Media LLC
Date: 13-10-2018
Publisher: Oxford University Press (OUP)
Date: 15-09-2015
Publisher: Oxford University Press (OUP)
Date: 27-03-2019
DOI: 10.1093/MNRAS/STZ875
Publisher: Oxford University Press (OUP)
Date: 05-11-2019
Abstract: The past two decades have seen dramatic progress in our knowledge of the population of young stars of age $\\lt \\!200\\,$ Myr that lie within $150\\,$ pc of the Sun. These nearby, young stars, most of which are found in loose, comoving groups, provide the opportunity to explore (among many other things) the dissolution of stellar clusters and their diffusion into the field star population. Here, we exploit the combination of astrometric and photometric data from Gaia and photometric data from GALEX (UV) and 2MASS (near-IR) in an attempt to identify additional nearby, young, late-type stars. Specifically, we present a s le of 146 GALEX UV-selected late-type (predominantly K-type) field stars with Gaia-based distances $\\lt \\!125\\,$ pc (based on Gaia Data Release 1) that have isochronal ages $\\lt \\!80\\,$ Myr even if equal-components binaries. We investigate the spectroscopic and kinematic properties of this s le. Despite their young isochronal ages, only ∼10 per cent of stars among this s le can be confidently associated with established nearby, young moving groups (MGs). These candidate MG members include five stars newly identified in this study. The vast majority of our s le of 146 nearby young star candidates have anomalous kinematics relative to the known MGs. These stars may hence represent a previously unrecognized population of young stars that has recently mixed into the older field star population. We discuss the implications and caveats of such a hypothesis – including the intriguing fact that, in addition to their non-young-star-like kinematics, the majority of the UV-selected, isochronally young field stars within $50\\,$ pc appear surprisingly X-ray faint.
Publisher: Oxford University Press (OUP)
Date: 16-06-2017
Publisher: American Astronomical Society
Date: 07-07-2023
Abstract: We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars μ Lep and β Scl, and to the superficially normal star ν Cap. In the spectra of the six stars, the Hg ii line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of P orb = 26.12022 ± 0.00004 day.
Publisher: Oxford University Press (OUP)
Date: 13-06-2019
Abstract: We perform a search for rapidly oscillating Ap stars in the Kepler long-cadence data, where true oscillations above the Nyquist limit of 283.21 $\\mu$Hz can be reliably distinguished from aliases as a consequence of the barycentric time corrections applied to the Kepler data. We find evidence for rapid oscillations in six stars: KIC 6631188, KIC 7018170, KIC 10685175, KIC 11031749, KIC 11296437, and KIC 11409673, and identify each star as chemically peculiar through either pre-existing classifications or spectroscopic measurements. For each star, we identify the principal pulsation mode, and are able to observe several additional pulsation modes in KIC 7018170. We find that KIC 7018170 and KIC 11409673 both oscillate above their theoretical acoustic cut-off frequency, whilst KIC 11031749 oscillates at the cut-off frequency within uncertainty. All but KIC 11031749 exhibit strong litude modulation consistent with the oblique pulsator model, confirming their mode geometry and periods of rotation.
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: Oxford University Press (OUP)
Date: 22-02-2018
DOI: 10.1093/MNRAS/STY471
Publisher: Oxford University Press (OUP)
Date: 23-02-2016
DOI: 10.1093/MNRAS/STW172
Publisher: American Astronomical Society
Date: 30-09-2022
Abstract: In this paper, we study the pulsation properties of KIC 3440495 using Kepler and TESS data. A Fourier analysis of the light curve reveals 24 pulsation modes as well as 29 frequencies associated with rotation. The rotation frequency is derived to be f rot = 2.322909(2) day −1 , and the rotational modulation is determined to be caused by starspots. A large frequency separation of Δ ν = 54.5 μ Hz is found by using a Fourier transform, the autocorrelation function, a histogram of frequency differences, and an échelle diagram. We use the large separation to estimate the refined stellar parameters of the star to be v = [239, 279] km s −1 , M = [1.5, 1.65] M ⊙ , R equator = [2.03, 2.30] R ⊙ , R polar = [1.72, 1.78] R ⊙ , and ω = [0.61, 0.77]. The phase modulations of the pulsating frequencies show a long-term trend which may be attributed to an orbital effect of a binary system hence, the star may be a fast rotating pulsator in a binary system. KIC 3440495 has an litude spectrum similar to Altair, and is identified as a potential sister of Altair. Based on studies of Altair, KIC 3330495 is presumably a young star at a similar evolutionary stage.
Publisher: Oxford University Press (OUP)
Date: 13-10-2017
Publisher: Oxford University Press (OUP)
Date: 29-01-2022
Abstract: We perform mode identification for five δ Scuti stars in the Pleiades star cluster, using custom light curves from K2 photometry. By creating échelle diagrams, we identify radial and dipole mode ridges, comprising a total of 28 radial and 16 dipole modes across the five stars. We also suggest possible identities for those modes that lie offset from the radial and dipole ridges. We calculate non-rotating stellar pulsation models to verify our mode identifications, finding good agreement within the age and metallicity constraints of the cluster. We also find that for all stars, the least dense models are preferred, reflecting the lower density of these oblate, rotating stars. Three of the five stars show rotationally split multiplets. We conclude that the s le shows promise for asteroseismic rotation rates, masses, and ages with rotating models in the future. Our preliminary modelling also indicates some sensitivity to the helium abundance.
Publisher: Springer Science and Business Media LLC
Date: 13-05-2020
Publisher: Elsevier BV
Date: 04-2018
Publisher: Oxford University Press (OUP)
Date: 20-04-2019
Publisher: Oxford University Press (OUP)
Date: 05-2015
DOI: 10.1093/MNRAS/STV528
Publisher: American Astronomical Society
Date: 21-10-2016
Publisher: American Astronomical Society
Date: 06-2018
Publisher: Oxford University Press (OUP)
Date: 07-10-2015
Publisher: American Astronomical Society
Date: 13-02-2019
Publisher: Springer Science and Business Media LLC
Date: 07-2012
DOI: 10.1038/NATURE11210
Abstract: Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.
Publisher: Oxford University Press (OUP)
Date: 09-02-2013
DOI: 10.1093/MNRAS/STT105
Publisher: Oxford University Press (OUP)
Date: 22-05-2015
DOI: 10.1093/MNRAS/STV884
Publisher: Oxford University Press (OUP)
Date: 09-2023
Publisher: Oxford University Press (OUP)
Date: 10-05-2013
DOI: 10.1093/MNRAS/STT587
Publisher: Oxford University Press (OUP)
Date: 13-05-2016
Publisher: EDP Sciences
Date: 2017
Publisher: Oxford University Press (OUP)
Date: 15-07-2016
Publisher: Oxford University Press (OUP)
Date: 13-08-2014
Publisher: Oxford University Press (OUP)
Date: 17-11-2017
Publisher: Oxford University Press (OUP)
Date: 02-06-2022
Abstract: With the availability of large-scale surveys like Kepler and TESS, there is a pressing need for automated methods to classify light curves according to known classes of variable stars. We introduce a new algorithm for classifying light curves that compares 7000 time-series features to find those that most effectively classify a given set of light curves. We apply our method to Kepler light curves for stars with effective temperatures in the range 6500–10 000 K. We show that the s le can be meaningfully represented in an interpretable 5D feature space that separates seven major classes of light curves (δ Scuti stars, γ Doradus stars, RR Lyrae stars, rotational variables, contact eclipsing binaries, detached eclipsing binaries, and non-variables). We achieve a balanced classification accuracy of 82 per cent on an independent test set of Kepler stars using a Gaussian mixture model classifier. We use our method to classify 12 000 Kepler light curves from Quarter 9 and provide a catalogue of the results. We further outline a confidence heuristic based on probability density to search our catalogue and extract candidate lists of correctly classified variable stars.
Publisher: Oxford University Press (OUP)
Date: 10-06-2019
Abstract: We revisit the K -band period–luminosity (P–L) relations of Galactic red supergiants using Gaia Data Release 2 parallaxes and up to 70 yr of photometry from AAVSO and ASAS c aigns. In addition, we examine 206 LMC red supergiants using 50 yr of photometric data from the digitized Harvard Astronomical Plate Collection. We identified periods by computing power spectra and calculated the period–luminosity relations of our s les and compared them with the literature. Newly available data tighten the P–L relations substantially. Identified periods form two groups: one with periods of 300–1000 d, corresponding to pulsations, and another with Long Secondary Periods between 1000 and 8000 d. Among the 48 Galactic objects we find shorter periods in 25 stars and long secondary periods in 23 stars. In the LMC s le we identify 85 and 94 red supergiants with shorter and long secondary periods, respectively. The P–L relation of the Galactic red supergiants is in agreement with the red supergiants in both, the Large Magellanic Cloud and the Andromeda galaxy. We find no clear continuity between the known red giant period–luminosity sequences, and the red supergiant sequences investigated here.
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: Oxford University Press (OUP)
Date: 28-06-2012
Publisher: Oxford University Press (OUP)
Date: 27-07-2021
Abstract: We present TOI-1259Ab, a 1.0RJup gas giant planet transiting a 0.71R⊙ K-dwarf on a 3.48 d orbit. The system also contains a bound white dwarf companion TOI-1259B with a projected distance of ∼1600 au from the planet host. Transits are observed in nine TESS sectors and are 2.7 per cent deep – among the deepest known – making TOI-1259Ab a promising target for atmospheric characterization. Our follow-up radial velocity measurements indicate a variability of semi litude $K=71\\, \\rm m\\, s^{-1}$, implying a planet mass of 0.44MJup. By fitting the spectral energy distribution of the white dwarf, we derive a total age of $4.08^{+1.21}_{-0.53}$ Gyr for the system. The K dwarf’s light curve reveals rotational variability with a period of 28 d, which implies a gyrochronology age broadly consistent with the white dwarf’s total age.
Publisher: Oxford University Press (OUP)
Date: 20-08-2013
Publisher: EDP Sciences
Date: 12-2019
DOI: 10.1051/0004-6361/201935395
Abstract: Context. The TESS satellite was launched in 2018 to perform high-precision photometry from space over almost the whole sky in a search for exoplanets orbiting bright stars. This instrument has opened new opportunities to study variable hot subdwarfs, white dwarfs, and related compact objects. Targets of interest include white dwarf and hot subdwarf pulsators, both carrying high potential for asteroseismology. Aims. We present the discovery and detailed asteroseismic analysis of a new g -mode hot B subdwarf (sdB) pulsator, EC 21494−7018 (TIC 278659026), monitored in TESS first sector using 120-s cadence. Methods. The TESS light curve was analyzed with standard prewhitening techniques, followed by forward modeling using our latest generation of sdB models developed for asteroseismic investigations. By simultaneously best-matching all the observed frequencies with those computed from models, we identified the pulsation modes detected and, more importantly, we determined the global parameters and structural configuration of the star. Results. The light curve analysis reveals that EC 21494−7018 is a sdB pulsator counting up to 20 frequencies associated with independent g -modes. The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy (atmospheric parameters derived from model atmospheres) and astrometry (distance evaluated from Gaia DR2 trigonometric parallax). Several key parameters of the star are derived. Its mass (0.391 ± 0.009 M ⊙ ) is significantly lower than the typical mass of sdB stars and suggests that its progenitor has not undergone the He-core flash therefore this progenitor could originate from a massive (≳2 M ⊙ ) red giant, which is an alternative channel for the formation of sdBs. Other derived parameters include the H-rich envelope mass (0.0037 ± 0.0010 M ⊙ ), radius (0.1694 ± 0.0081 R ⊙ ), and luminosity (8.2 ± 1.1 L ⊙ ). The optimal model fit has a double-layered He+H composition profile, which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope. Moreover, the derived properties of the core indicate that EC 21494−7018 has burnt ∼43% (in mass) of its central helium and possesses a relatively large mixed core ( M core = 0.198 ± 0.010 M ⊙ ), in line with trends already uncovered from other g-mode sdB pulsators analyzed with asteroseismology. Finally, we obtain for the first time an estimate of the amount of oxygen (in mass X (O) core = 0.16 +0.13 −0.05 ) produced at this stage of evolution by an helium-burning core. This result, along with the core-size estimate, is an interesting constraint that may help to narrow down the still uncertain 12 C( α , γ ) 16 O nuclear reaction rate.
Publisher: EDP Sciences
Date: 2015
Publisher: Cambridge University Press (CUP)
Date: 08-2013
DOI: 10.1017/S1743921313015081
Abstract: Basic stellar parameters such as effective temperature, surface gravity, chemical composition, and projected rotational velocity, are important to classify stars and are crucial for successful asteroseismic modelling. However, the Kepler space data do not provide such information. Therefore, ground-based spectral and multi-colour observations of Kepler asteroseismic targets are necessary to complement the space data. For this purpose, in coordination with the KASC ground-based observational Working Groups, high-resolution spectroscopic data for more than 500 B, A, F and G-type stars were collected.
Publisher: Oxford University Press (OUP)
Date: 24-05-2016
Abstract: We study the δ Scuti – red giant (RG) binary KIC 9773821, the first double-pulsator binary of its kind. It was observed by Kepler during its 4-yr mission. Our aims are to ascertain whether the system is bound, rather than a chance alignment, and to identify the evolutionary state of the RG via asteroseismology. An extension of these aims is to determine a dynamical mass and an age prior for a δ Sct star, which may permit mode identification via further asteroseismic modelling. We determine spectroscopic parameters and radial velocities (RVs) for the RG component using HERMES@Mercator spectroscopy. Light arrival-time delays from the δ Sct pulsations are used with the red-giant RVs to determine that the system is bound and to infer its orbital parameters, including the binary mass ratio. We use asteroseismology to model the in idual frequencies of the red giant to give a mass of $2.10^{+0.20}_{-0.10}$ M⊙ and an age of $1.08^{+0.06}_{-0.24}$ Gyr. We find that it is a helium-burning secondary clump star, confirm that it follows the standard $\\nu _{\\rm max}$ scaling relation, and confirm its observed period spacings match their theoretical counterparts in the modelling code mesa. Our results also constrain the mass and age of the δ Sct star. We leverage these constraints to construct δ Sct models in a reduced parameter space and identify four of its five pulsation modes.
Publisher: American Astronomical Society
Date: 29-06-2017
Publisher: Oxford University Press (OUP)
Date: 11-08-2020
Abstract: The λ Boo stars are chemically peculiar A-type stars whose abundance anomalies are associated with the accretion of metal-poor material. We searched for λ Boo stars in the Southern hemisphere in a targeted spectroscopic survey of metal-weak and emission-line stars. Obtaining spectra for 308 stars and classifying them on the MK system, we found or co-discovered 24 new λ Boo stars. We also revised the classifications of 11 known λ Boo stars, one of which turned out to be a chemically normal rapid rotator. We show that stars previously classified in the literature as blue horizontal branch stars or emission-line A stars have a high probability of being λ Boo stars, although this conclusion is based on small-number statistics. Using WISE infrared fluxes, we searched our targets for infrared excesses that might be attributable to protoplanetary or debris discs as the source of the accreted material. Of the 34 λ Boo stars in our s le, 21 at various main-sequence ages have infrared excesses, confirming that not all λ Boo stars are young.
Publisher: Oxford University Press (OUP)
Date: 13-07-2001
Publisher: Springer Science and Business Media LLC
Date: 09-03-2020
Publisher: Springer Science and Business Media LLC
Date: 20-06-2019
DOI: 10.1038/S41598-019-45383-W
Abstract: Mungbean Yellow Mosaic India Virus (MYMIV)-infection creates major hindrance in V . mungo cultivation and poses significant threat to other grain legume production. Symptoms associated include severe patho-physiological alterations characterized by chlorotic foliar lesion accompanied by reduced growth. However, dissection of the host’s defense machinery remains a tough challenge due to limited of host’s genomic resources. A comparative RNA-Seq transcriptomes of resistant (VM84) and susceptible (T9) plants was carried out to identify genes potentially involved in V . mungo resistance against MYMIV. Distinct gene expression landscapes were observed in VM84 and T9 with 2158 and 1679 differentially expressed genes (DEGs), respectively. Transcriptomic responses in VM84 reflect a prompt and intense immune reaction demonstrating an efficient pathogen surveillance leading to activation of basal and induced immune responses. Functional analysis of the altered DEGs identified multiple regulatory pathways to be activated or repressed over time. Up-regulation of DEGs including NB-LRR, WRKY33, ankyrin, argonaute and NAC transcription factor revealed an insight on their potential roles in MYMIV-resistance and qPCR validation shows a propensity of their accumulation in VM84. Analyses of the current RNA-Seq dataset contribute immensely to decipher molecular responses that underlie MYMIV-resistance and will aid in the improvement strategy of V . mungo and other legumes through comparative functional genomics.
Publisher: Oxford University Press (OUP)
Date: 19-11-2012
Publisher: Oxford University Press (OUP)
Date: 29-04-2019
Publisher: Oxford University Press (OUP)
Date: 14-01-2022
Abstract: We present an analysis of the eclipsing single-lined spectroscopic binary system α Dra based on photometry from the Transiting Exoplanet Survey Satellite (TESS) mission and newly acquired spectroscopic measurements. Recently discovered to have eclipses in the TESS data, at a magnitude of V = 3.7, α Dra is now one of the brightest detached eclipsing binary (EB) systems known. We obtain the parameters of this system by simultaneously fitting the TESS light curve in conjunction with radial velocities (RVs) acquired from the SONG spectrograph. We determine the fractional radii (R/a) for the primary and secondary components of the system to be 0.0479 $\\, \\pm \\,$ 0.0003 and 0.0226 $\\, \\pm \\,$ 0.0005, respectively. We constrain the temperature, mass, and luminosity (log(L/L⊙)) of the primary to be $9975\\, \\pm \\, 125$ K, $3.7\\, \\pm \\, 0.1$ M⊙, and $2.49\\, \\pm \\, 0.02$, respectively, using isochrone fitting. Although the secondary is too faint to appear in the spectra, the obtained mass function and observed inclination yields a secondary minimum mass of $M_2=2.5\\, \\pm \\, 0.1$ M⊙, which suggests that it is an A2V type star. We were unable to obtain RVs of the secondary, and are only able to see a weak highly rotationally broadened absorption line, indicating that the secondary is rapidly rotating (vsin i ∼ 200 km s−1). We also perform an abundance analysis of the primary star for 21 chemical elements. We find a complex abundance pattern, with a few elements having mild underabundances while the majority have solar abundances. We make available the python code used in this paper to facilitate future modelling of EBs. anhey/adra
Publisher: Oxford University Press (OUP)
Date: 11-08-2017
Publisher: American Astronomical Society
Date: 29-10-2013
Publisher: Oxford University Press (OUP)
Date: 09-05-2015
DOI: 10.1093/MNRAS/STV868
Publisher: American Astronomical Society
Date: 31-10-2019
Publisher: American Astronomical Society
Date: 21-02-2012
Publisher: Wiley
Date: 12-2012
Publisher: Oxford University Press (OUP)
Date: 22-06-2012
Publisher: EDP Sciences
Date: 02-2020
DOI: 10.1051/0004-6361/201936810
Abstract: Context. Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. Aims. We aim to search for accreting young brown dwarf candidates in a s le of 85 nearby late-M dwarfs. Methods. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late- M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and H α emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Results. Among the 85 late-M dwarfs, only DENIS-P J1538317−103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of ~1 Myr and a mass of 47 M J . The H α emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to 10 −7.9 M ⊙ yr −1 . Conclusions. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.
Publisher: Oxford University Press (OUP)
Date: 18-07-2018
Publisher: Oxford University Press (OUP)
Date: 24-12-2014
Publisher: Oxford University Press (OUP)
Date: 09-06-2016
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: Oxford University Press (OUP)
Date: 29-01-2018
DOI: 10.1093/MNRAS/STY222
Publisher: Cambridge University Press (CUP)
Date: 08-2015
DOI: 10.1017/S1743921316006244
Abstract: Continuous and precise space-based photometry has made it possible to measure the orbital frequency modulation of pulsating stars in binary systems with extremely high precision over long time spans. We present the phase modulation (PM) method for finding binaries among pulsating stars. We demonstrate how the orbital elements of a pulsating binary star can be obtained analytically from photometry alone, without spectroscopic radial velocity measurement. Frequency modulation (FM) caused by binary orbital motion also manifests itself in the Fourier transform, as a multiplet with equal spacing of the orbital frequency. The orbital parameters can also be extracted by analysing the litudes and phases of the peaks in these multiplets. We derive analytically the theoretical relations between the multiplet properties and the orbital parameters, and present a method for determining these parameters, including the eccentricity and the argument of periapsis. This, too, is achievable with the photometry alone, without spectroscopic radial velocity measurements. We apply these two methods to Kepler mission data and demonstrate that the results are in good agreement with each other. These methods are used to search for invisible binary companions, including planets and invisible massive objects such as neutron stars and stellar-mass black holes.
Publisher: Cambridge University Press (CUP)
Date: 2015
DOI: 10.1017/PASA.2015.34
Abstract: The literature on the λ Boo stars has grown to become somewhat heterogenous, as different authors have applied different criteria across the UV, optical, and infrared regions to determine the membership status of λ Boo candidates. We aim to clear up the confusion by consulting the literature on 212 objects that have been considered as λ Boo candidates, and subsequently evaluating the evidence in favour of their admission to the λ Boo class. We obtained new spectra of ~ 90 of these candidates and classified them on the MK system to aid in the membership evaluations. The re-evaluation of the 212 objects resulted in 64 members and 103 non-members of the λ Boo class, with a further 45 stars for which membership status is unclear. We suggest observations for each of the stars in the latter category that will allow them to be confidently included or rejected from the class. Our reclassification facilitates homogenous analysis on group members, and represents the largest collection of confirmed λ Boo stars known.
Publisher: Oxford University Press (OUP)
Date: 25-09-2023
Publisher: EDP Sciences
Date: 2017
Publisher: EDP Sciences
Date: 2015
Publisher: American Astronomical Society
Date: 21-08-2013
Publisher: Oxford University Press (OUP)
Date: 08-12-2018
Publisher: Oxford University Press (OUP)
Date: 10-11-2016
Publisher: EDP Sciences
Date: 2017
Publisher: Oxford University Press (OUP)
Date: 11-10-2019
Publisher: IOP Publishing
Date: 02-2012
DOI: 10.1086/664083
Publisher: Oxford University Press (OUP)
Date: 19-05-2015
DOI: 10.1093/MNRAS/STV875
Publisher: Oxford University Press (OUP)
Date: 12-2018
Publisher: Oxford University Press (OUP)
Date: 08-06-2010
Publisher: Springer Science and Business Media LLC
Date: 14-04-2022
Publisher: American Astronomical Society
Date: 19-10-2021
Publisher: American Astronomical Society
Date: 21-08-2013
Publisher: American Astronomical Society
Date: 12-2022
Abstract: The study of young associations is essential for building a complete record of local star formation processes. The Fornax–Horologium association (FH), including the χ 1 Fornacis cluster, represents one of the nearest young stellar populations to the Sun. This association has recently been linked to the Tuc-Hor, Carina, and Columba associations, building an extensive “Austral Complex” almost entirely within 150 pc. Using Gaia astrometry and photometry in addition to new spectroscopic observations, we perform the deepest survey of FH to date, identifying over 300 candidate members, nearly doubling the known population. By combining this s le with literature surveys of the other constituent populations, we produce a contiguous stellar population covering the entire Austral Complex, allowing the definitions of subpopulations to be reassessed along with connections to external populations. This analysis recovers new definitions for FH, Tuc-Hor, Columba, and Carina, while also revealing a connection between the Austral complex and the Sco-Cen-affiliated Platais 8 cluster. This suggests that the Austral complex may be just a small component of a much larger and more erse star formation event. Computing ages and tracing stellar populations back to formation reveals two distinct nodes of cospatial and continuous formation in the Austral Complex, one containing Tuc-Hor, and the other containing FH, Carina, and Columba. This mirrors recent work showing similar structure elsewhere, suggesting that these nodes, which only emerge through the use of traceback, may represent the clearest discrete unit of local star formation, and a key building block needed to reconstruct larger star-forming events.
Publisher: Oxford University Press (OUP)
Date: 31-05-2021
Abstract: We present the results of a systematic search for new rapidly oscillating Ap (roAp) stars using the 2-min cadence data collected by the Transiting Exoplanet Survey Satellite (TESS) during its Cycle 1 observations. We identify 12 new roAp stars. Amongst these stars we discover the roAp star with the longest pulsation period, another with the shortest rotation period, and six with multiperiodic variability. In addition to these new roAp stars, we present an analysis of 44 known roAp stars observed by TESS during Cycle 1, providing the first high-precision and homogeneous s le of a significant fraction of the known roAp stars. The TESS observations have shown that almost 60 per cent (33) of our s le of stars are multiperiodic, providing excellent cases to test models of roAp pulsations, and from which the most rewarding asteroseismic results can be gleaned. We report four cases of the occurrence of rotationally split frequency multiplets that imply different mode geometries for the same degree modes in the same star. This provides a conundrum in applying the oblique pulsator model to the roAp stars. Finally, we report the discovery of non-linear mode interactions in α Cir (TIC 402546736, HD 128898) around the harmonic of the principal mode – this is only the second case of such a phenomenon.
Publisher: Oxford University Press (OUP)
Date: 19-08-2016
Publisher: American Astronomical Society
Date: 13-11-2014
Publisher: Oxford University Press (OUP)
Date: 02-03-2017
DOI: 10.1093/MNRAS/STX535
Publisher: Oxford University Press (OUP)
Date: 23-12-2010
Publisher: Oxford University Press (OUP)
Date: 31-07-2022
Abstract: We have used NASA’s TESS mission to study catalogued δ Scuti stars. We examined TESS light curves for 434 stars, including many for which few previous observations exist. We found that 62 are not δ Scuti pulsators, with most instead showing variability from binarity. For the 372 δ Scuti stars, we provide a catalogue of the period and litude of the dominant pulsation mode. Using Gaia DR3 parallaxes, we place the stars in the period–luminosity (P–L) diagram and confirm previous findings that most stars lie on a ridge that corresponds to pulsation in the fundamental radial mode, and that many others fall on a second ridge that is a factor two shorter in period. This second ridge is seen more clearly than before, thanks to the revised periods and distances. We demonstrate the value of the P–L diagram in distinguishing δ Scuti stars from short-period RR Lyrae stars, and we find several new ex les of high-frequency δ Scuti stars with regular sequences of overtone modes, including XX Pyx and 29 Cyg. Finally, we revisit the s le of δ Scuti stars observed by Kepler and show that they follow a tight period–density relation, with a pulsation constant for the fundamental mode of Q = 0.0315 d.
Publisher: Oxford University Press (OUP)
Date: 06-07-2016
Publisher: Oxford University Press (OUP)
Date: 05-12-2018
Publisher: AOA Academic Open Access Ltd.
Date: 12-2013
DOI: 10.18052/WWW.SCIPRESS.COM/ILNS.7.23
Abstract: HIV-1 or Human Immuno Deficiency Virus-1 is the main causative agent of Acquired Immuno Deficiency Syndrome (AIDS). Human host infected with HIV - 1 extensively harbours many viral variants but very little is known about the difference in pattern [17] of evolution of phylogenetic lineages of HIV-1 non recombinant, normal inter subtype recombinant and main two specific recombinant forms of HIV-1 i.e., Circulating Recombinant Forms (CRFs) and Unique Recombinant Forms (URFs). This study is mainly concerned with study of the difference in evolutionary lineages of non-recombinant and recombinant sequences of HIV-1 genome sequences and identification of geographically rich areas which has reported high degree of HIV-1 occurrence and variety. Total 1550 HIV-1 genome sequences were obtained from HIV Los Alamos Database. The sequences were aligned using MAFFT (Multiple Alignment using Fast Fourier Transform) web server tool. Alignment was carried out using 10 different set of alignment parameter values. After alignment the aligned file was used for constructing N-J phylogenetic tree using Clustal X2 tool. Phylogenetic analysis was performed keeping in mind the category to which the sequence belongs. Upon analysis it was observed that the clade containing the probable ancestor belongs remained constant in all cases of different alignment values. Non recombinant isolates, inter subtype recombinants, CRFs, URFs all followed different patterns of evolution. Non recombinant sequences were found geographically specific and subtype specific to some extent whereas, normal recombinants were subtype specific and less geographically specific. CRFs showed variation among the pattern of their evolution. At some instances the sequences occurred as sister taxa of non-recombinant or normal inter subtype recombinant sequences, while at some instances as sister taxa of other CRFs where they were geographically specific. Three CRFs existed as completely erged sequences. URFs were four in number two of them were Indian isolates of while other two were Japanese isolates. URFs were found to be totally geographically specific. Geography wise high rate of variation was observed in India and Japan as these two countries had sequences belonging to all of the above categories. Cameroon and South Africa have very large number isolates and a considerable amount of genetic variation among isolates but they lack URFs.
Publisher: IEEE
Date: 03-2016
Publisher: Oxford University Press (OUP)
Date: 23-10-2017
Publisher: Oxford University Press (OUP)
Date: 23-12-2022
Abstract: We announce the discovery of pulsations in HD 23642, the only bright eclipsing system in the Pleiades, based on light curves from the Transiting Exoplanet Survey Satellite (TESS). We measure 46 pulsation frequencies and attribute them to δ Scuti pulsations in the secondary component. We find four ℓ = 1 doublets, three of which have frequency splittings consistent with the rotation rate of the star. The dipole mode litude ratios are consistent with a high stellar inclination angle and the stellar rotation period agrees with the orbital period. Together, these suggest that the spin axis of the secondary is aligned with the orbital axis. We also determine precise effective temperatures and a spectroscopic light ratio, and use the latter to determine the physical properties of the system alongside the TESS data and published radial velocities. We measure a distance to the system in agreement with the Gaia parallax, and an age of 170 ± 20 Myr based on a comparison to theoretical stellar evolutionary models.
Publisher: Oxford University Press (OUP)
Date: 29-01-2015
Publisher: Oxford University Press (OUP)
Date: 21-04-2021
Abstract: Among the intermediate mass pulsating stars known as δ Sct stars is a subset of high- litude and predominantly radial-mode pulsators known as high- litude δ Sct (HADS) stars. From more than 2000 δ Sct stars observed by the Kepler space mission, only two HADS stars were detected. We investigate the more perplexing of these two HADS stars, KIC 5950759. We study its variability using ground- and space-based photometry, determine its atmospheric parameters from spectroscopy and perform asteroseismic modelling to constrain its mass and evolutionary stage. From spectroscopy, we find that KIC 5950759 is a metal-poor star, which is in agreement with the inferred metallicity needed to reproduce its pulsation mode frequencies from non-adiabatic pulsation models. Furthermore, we combine ground-based WASP and Kepler space photometry, and measure a linear change in period of order $\\dot{P}/P \\simeq 10^{-6}$ yr−1 for both the fundamental and first overtone radial modes across a time base of several years, which is at least two orders of magnitude larger than predicted by evolution models, and is the largest measured period change in a δ Sct star to date. Our analysis indicates that KIC 5950759 is a metal-poor HADS star near the short-lived contraction phase and the terminal-age main sequence, with its sub-solar metallicity making it a candidate SX Phe star. KIC 5950759 is a unique object among the thousands of known δ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.
Publisher: American Astronomical Society
Date: 09-08-2016
Publisher: Elsevier BV
Date: 06-2017
Publisher: American Astronomical Society
Date: 10-04-2020
Publisher: American Astronomical Society
Date: 25-08-2011
Publisher: Oxford University Press (OUP)
Date: 08-05-2020
Abstract: We analyse TESS light curves for 70 southern λ Boo stars to identify binaries and to determine which of them pulsate as δ Scuti stars. We find two heartbeat stars and two eclipsing binaries among the s le. We calculate that 81 per cent of λ Boo stars pulsate as δ Sct variables, which is about twice that of normal stars over the same parameter space. We determine the temperatures and luminosities of the λ Boo stars from photometry and Gaia DR2 parallaxes. A subset of 40 λ Boo stars have 2-min TESS data, reliable temperatures and luminosities, and δ Sct pulsation. We use Petersen diagrams (period ratios), échelle diagrams, and the period–luminosity relation to identify the fundamental mode in 20 of those 40 stars and conclude that a further 8 stars are not pulsating in this mode. For the remaining 12, the fundamental mode cannot be unambiguously identified. Further mode identification is possible for 12 of the fundamental mode pulsators that have regular sequences of pulsation overtones in their échelle diagrams. We use stellar evolution models to determine statistically that the λ Boo stars are only superficially metal weak. Simple pulsation models also better fit the observations at a metallicity of Z = 0.01 than at Z = 0.001. The TESS observations reveal the great potential of asteroseismology on λ Boo stars, for determining precise stellar ages and shedding light on the origin(s) of the λ Boo phenomenon.
Publisher: EDP Sciences
Date: 06-2020
DOI: 10.1051/0004-6361/202037470
Abstract: Context. We present our findings on 18 previously known ZZ Ceti stars observed by the TESS space telescope in 120 s cadence mode during the survey observation of the southern ecliptic hemisphere. Aims. We focus on the frequency analysis of the space-based observations, comparing the results with findings of previous ground-based measurements. The frequencies detected by the TESS observations can serve as inputs for future asteroseismic analyses. Methods. We performed standard pre-whitening of the data sets to derive the possible pulsation frequencies of the different targets. In some cases, we fit Lorentzians to the frequency groups that emerged as the result of short-term litude or phase variations that occurred during the TESS observations. Results. We detected more than 40 pulsation frequencies in seven ZZ Ceti stars observed in the 120 s cadence by TESS, with precision better than 0.1 μ Hz. We found that HE 0532−5605 may be a new outbursting ZZ Ceti. Ten targets do not show any significant pulsation frequencies in their Fourier transforms, due to a combination of their intrinsic faintness and/or crowding on the large TESS pixels. We also detected possible litude or phase variations during the TESS observations in some cases. Such behaviour in these targets was not previously identified from ground-based observations.
Publisher: Oxford University Press (OUP)
Date: 19-11-2014
Publisher: Oxford University Press (OUP)
Date: 05-2021
Abstract: We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($d\\sim 460\\, \\rm pc$), bright (V ≃ 8.3 mag), evolved (Teff, giant ≃ 4440 K, and Lgiant ≃ 173 L⊙) red giant in a high mass function, f(M) = 1.72 ± 0.01 M⊙, nearly circular binary (P = 59.9 d, e ≃ 0). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey, Kilodegree Extremely Little Telescope, and Transiting Exoplanet Survey Satellite light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities, and stellar temperature give an inclination of $87.0^{\\circ ^{+1.7^\\circ }}_{-1.4^\\circ }$, a mass ratio of q ≃ 0.33 ± 0.02, a companion mass of Mcomp = 3.04 ± 0.06 M⊙, a stellar radius of Rgiant = 24.9 ± 0.7 R⊙, and a giant mass of Mgiant = 1.00 ± 0.07 M⊙. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of ${\\sim }63{{\\ \\rm per\\ cent}}$ and ${\\sim }24{{\\ \\rm per\\ cent}}$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion discs. The X-ray luminosity of the system is $L_{\\rm X}\\simeq 7.6\\times 10^{29}~\\rm ergs~s^{-1}$, corresponding to L/Ledd ∼ 10−9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the ‘mass gap’.
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: Elsevier BV
Date: 12-2017
Publisher: Cambridge University Press (CUP)
Date: 11-2015
DOI: 10.1017/S1743921315006031
Abstract: We describe the serendipitous discovery of two new lithium-rich M5 members of the Scorpius-Centaurus OB Association (Sco-Cen). Both stars exhibit large 12 and 22 μm excesses and strong, variable Hα emission which we attribute to accretion from circumstellar discs. Such stars are thought to be incredibly rare at the ~16 Myr median age of much of Sco-Cen. The serendipitous discovery of two accreting stars hosting large quantities of circumstellar material may be indicative of a sizeable age spread in Sco-Cen, or further evidence that disc dispersal and planet formation time-scales are longer around lower-mass stars.
Publisher: American Astronomical Society
Date: 03-2023
Abstract: We studied 89 A- and F-type members of the Pleiades open cluster, including five escaped members. We measured projected rotational velocities ( v sin i ) for 49 stars and confirmed that stellar rotation causes a broadening of the main sequence in the color–magnitude diagram. Using time-series photometry from NASA’s TESS Mission (plus one star observed by Kepler/K2), we detected δ Scuti pulsations in 36 stars. The fraction of Pleiades stars in the middle of the instability strip that pulsate is unusually high (over 80%), and their range of effective temperatures agrees well with theoretical models. On the other hand, the characteristics of the pulsation spectra are varied and do not correlate with stellar temperature, calling into question the existence of a useful ν max relation for δ Scutis, at least for young main-sequence stars. By including δ Scuti stars observed in the Kepler field, we show that the instability strip is shifted to the red with increasing distance by interstellar reddening. Overall, this work demonstrates the power of combining observations with Gaia and TESS for studying pulsating stars in open clusters.
Publisher: EDP Sciences
Date: 10-2011
Publisher: EDP Sciences
Date: 17-11-2015
Publisher: Zenodo
Date: 2021
Publisher: Oxford University Press (OUP)
Date: 16-01-2015
Publisher: Cambridge University Press (CUP)
Date: 08-2012
DOI: 10.1017/S1743921313012970
Abstract: We develop a means to distinguish between warm dusty circumstellar debris disks produced by steady state collisional evolution of a planetesimal belt or through transient events.
Publisher: Oxford University Press (OUP)
Date: 31-08-2015
Publisher: Oxford University Press (OUP)
Date: 28-03-2016
DOI: 10.1093/MNRAS/STW705
Publisher: Oxford University Press (OUP)
Date: 20-05-2014
DOI: 10.1093/MNRAS/STU765
Publisher: Oxford University Press (OUP)
Date: 05-12-2016
Publisher: American Association for the Advancement of Science (AAAS)
Date: 14-04-2023
Abstract: Direct imaging of gas giant exoplanets provides information on their atmospheres and the architectures of planetary systems. However, few planets have been detected in blind surveys with direct imaging. Using astrometry from the Gaia and Hipparcos spacecraft, we identified dynamical evidence for a gas giant planet around the nearby star HIP 99770. We confirmed the detection of this planet with direct imaging using the Subaru Coronagraphic Extreme Adaptive Optics instrument. The planet, HIP 99770 b, orbits 17 astronomical units from its host star, receiving an amount of light similar to that reaching Jupiter. Its dynamical mass is 13.9 to 16.1 Jupiter masses. The planet-to-star mass ratio [(7 to 8) × 10 −3 ] is similar to that of other directly imaged planets. The planet’s atmospheric spectrum indicates an older, less cloudy analog of the previously imaged exoplanets around HR 8799.
Location: India
Location: Australia
Start Date: 2018
End Date: 12-2021
Amount: $322,713.00
Funder: Australian Research Council
View Funded ActivityStart Date: 2022
End Date: 12-2025
Amount: $661,020.00
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: 2018
End Date: 07-2023
Amount: $792,859.00
Funder: Australian Research Council
View Funded Activity