arXiv subjects

Timothy R. White

Publications and source records attributed to Timothy R. White.

At least 19 recordsLinked to original sources

arXiv2026

Diameters and Temperatures VII: High-angular resolution measurements of Solar-type stars with the CHARA Array

We present interferometric measurements of angular diameters for 27 nearby solar-type stars obtained with the Precision Astronomical Visible Observations (PAVO) beam combiner at the CHARA Array. The sample spans a broad range of metallicities, includes several known exoplanet hosts, and covers evolutionary stages from the zero-age main sequence to mildly evolved subgiants. Uniform-disk and limb-darkened angular diameters were measured for each target and combined with bolometric fluxes and Gaia parallaxes to determine precise, model-independent stellar radii, effective temperatures, and luminosities. We achieve typical uncertainties of $\sim1$\% in radius and $\sim1.5$\% in effective temperature. Comparisons with multiple stellar evolutionary model grids yield mass and age estimates and enable assessment of grid-to-grid systematics, highlighting sensitivities to abundances, evolutionary stage, and proximity to grid boundaries. Our results provide empirical benchmarks for testing stellar evolutionary theory, refining surface brightness-color relations, and improving the characterization of exoplanet host stars.

astro-ph.SR↗

arXiv2026

CHARA Interferometry and TESS Asteroseismology of the Core-Helium Burning Red Giant $\kappa$ Cyg

We present a detailed study of the secondary red clump star, $\kappa$ Cyg, by combining long-baseline visible interferometry using the PAVO beam combiner at the CHARA Array with high-precision asteroseismology from TESS. This dual approach allowed for a stringent test of stellar evolutionary models in the core helium-burning phase, which remains a regime of significant theoretical uncertainty. Using the PAVO interferometric data and fitting the limb-darkened intensity profile directly, we measured $R = 8.65\pm0.10 \rm R_\odot$. We fitted the spectral energy distribution (SED) using Phoenix model atmospheres and calculated $L = 44.46 \pm 1.09 \rm L_\odot$ and $T_{\rm eff} = 5066^{+47}_{-50} \mathrm{K}$. Using 16 sectors of TESS photometry, we detected clear solar-like oscillations in $\kappa$ Cyg. Through comparison of oscillation frequencies with MESA grids using either predictive mixing (PM) or exponential overshooting (OS), we found that models reproducing the oscillation frequencies systematically overestimate the stellar radius, with overshooting models performing only marginally better. The same models also under-predict the observed dipole-mode period spacing ($\Delta\Pi_1$). By inspecting the phase offset ($\epsilon_p$), we conclude that models misrepresent the interior structure of the star. Our results demonstrate that matching envelope-dominated asteroseismic observables alone is insufficient to ensure a correct core or even global structure, and highlight the need for improved treatments of convective boundary mixing in the models of core helium-burning (CHeB) stars.

astro-ph.SR↗

arXiv2026

Halo Photometry and Asteroseismology for 98 of the Brightest Stars Observed by TESS

The Transiting Exoplanet Survey Satellite (TESS) mission has facilitated studies of asteroseismology, eclipsing binaries, and transits in many stars. However, the brightest stars saturate TESS, yet they are the most amenable to photon-hungry high-resolution studies and have long observational histories. In this work, we adapted the halo photometry used in $K$2 to extract light curves from the unsaturated halo pixels of the star's point spread function. We used this method to extract light curves for 98 of the brightest stars observed by TESS in Sectors 1-93. These bright stars include 15 red giants, five $\delta$ Scuti variables, eight stochastic low-frequency variables, eight eclipsing binaries, and 46 other variables. We measured $\nu_{\rm max}$ for 13 red giants using pyMON and $\Delta\nu$ for one of them, $\beta$ Gem (Pollux). For five of them, this represents the first time that oscillations were detected. We derived their stellar masses using the measured $\nu_{\rm max}$ and previous interferometric and radiometric angular diameters. We also discovered $\delta$ Scuti and $\gamma$ Doradus variability in $\alpha$ Cep, possible asteroseismic binary signatures in $\epsilon$ Car, and a new eclipsing binary, $\gamma$ And. Furthermore, we identified 18 stars in our sample that will be observed by the future PLAnetary Transits and Oscillations of stars (PLATO) mission, and 69 stars that have Stellar Observations Network Group (SONG) observations, including some simultaneous with TESS. The light curves are publicly available on the Mikulski Archive for Space Telescopes.

astro-ph.SR↗

arXiv2024

The K2 Asteroseismic KEYSTONE sample of Dwarf and Subgiant Solar-Like Oscillators. I: Data and Asteroseismic parameters

The KEYSTONE project aims to enhance our understanding of solar-like oscillators by delivering a catalogue of global asteroseismic parameters (${Δν}$ and ${ν_{\rm max}}$) for 173 stars, comprising mainly dwarfs and subgiants, observed by the K2 mission in its short-cadence mode during campaigns 6-19. We derive atmospheric parameters and luminosities using spectroscopic data from TRES, astrometric data from $\textit{Gaia}$, and the infrared flux method (IRFM) for a comprehensive stellar characterisation. Asteroseismic parameters are robustly extracted using three independent methods, complemented by an iterative refinement of the spectroscopic analyses using seismic ${\log g}$ values to enhance parameter accuracy. Our analysis identifies new detections of solar-like oscillations in 159 stars, providing an important complement to already published results from previous campaigns. The catalogue provides homogeneously derived atmospheric parameters and luminosities for the majority of the sample. Comparison between spectroscopic ${T_{\rm eff}}$ and those obtained from the IRFM demonstrates excellent agreement. The iterative approach to spectroscopic analysis significantly enhances the accuracy of the stellar properties derived.

astro-ph.SR↗

arXiv2024

A New Asteroseismic $\textit{Kepler}$ Benchmark Constrains the Onset of Weakened Magnetic Braking in Mature Sun-Like Stars

Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin down rate. However, a large number of precise asteroseismic ages are needed for mature ($\geq$ 3Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright ($K_{p}$ = 9.6 mag) solar-type star with well-measured rotation period (21.1$\pm$0.8 days) from spot modulation using 4 years of Kepler long cadence data. We combine asteroseismology and spectroscopy to obtain $T_{eff}=5888\pm100$ K, $\rm{[Fe/H]} = 0.30 \pm 0.06\,$ dex, $M = 1.24 \pm 0.05 M_{\odot}$, $R = 1.34 \pm 0.02 R_{\odot}$ and age of 4.0 $\pm$ 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars.

astro-ph.SR↗

arXiv2023

A prescription for the asteroseismic surface correction

In asteroseismology, the surface effect refers to a disparity between the observed and the modelled frequencies in stars with solar-like oscillations. It originates from improper modelling of the surface layers. Correcting the surface effect usually requires using functions with free parameters, which are conventionally fitted to the observed frequencies. On the basis that the correction should vary smoothly across the H--R diagram, we parameterize it as a simple function of surface gravity, effective temperature, and metallicity. We determine this function by fitting a wide range of stars. The absolute amount of the surface correction decreases with luminosity, but the ratio between it and $ν_{\rm max}$ increases, suggesting the surface effect is more important for red giants than dwarfs. Applying the prescription can eliminate unrealistic surface correction, which improves parameter estimations with stellar modelling. Using two open clusters, we found a reduction of scatter in the model-derived ages for each star in the same cluster. As an important application, we provide a new revision for the $Δν$ scaling relation that, for the first time, accounts for the surface correction. The values of the correction factor, $f_{Δν}$, are up to 2\% smaller than those determined without the surface effect considered, suggesting decreases of up to 4\% in radii and up to 8\% in masses when using the asteroseismic scaling relations. This revision brings the asteroseismic properties into an agreement with those determined from eclipsing binaries. The new correction factor and the stellar models with the corrected frequencies are available at {https://www.github.com/parallelpro/surface}.

astro-ph.SR↗

arXiv2022

TESS observations of the Pleiades cluster: a nursery for delta Scuti stars

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 delta 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 nu_max relation for delta Scutis, at least for young stars. By including delta 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.

astro-ph.SR↗

arXiv2022

A Catalogue of Solar-Like Oscillators Observed by TESS in 120-second and 20-second Cadence

The Transiting Exoplanet Survey Satellite (TESS) mission has provided photometric light curves for stars across nearly the entire sky. This allows for the application of asteroseismology to a pool of potential solar-like oscillators that is unprecedented in size. We aim to produce a catalogue of solar-like oscillators observed by TESS in the 120-second and 20-second cadence modes. The catalogue is intended to highlight stars oscillating at frequencies above the TESS 30-minute cadence Nyquist frequency with the purpose of encompassing the main sequence and subgiant evolutionary phases. We aim to provide estimates for the global asteroseismic parameters $ν_{\mathrm{max}}$ and $Δν$. We apply a new probabilistic detection algorithm to the 120-second and 20-second light curves of over 250,000 stars. This algorithm flags targets that show characteristic signatures of solar-like oscillations. We manually vet the resulting list of targets to confirm the presence of solar-like oscillations. Using the probability densities computed by the algorithm, we measure the global asteroseismic parameters $ν_{\mathrm{max}}$ and $Δν$. We produce a catalogue of 4,177 solar-like oscillators, reporting $Δν$ and $ν_{\mathrm{max}}$ for $98\%$ of the total star count. The asteroseismic data reveals vast coverage of the HR diagram, populating the red giant branch, the subgiant regime and extending toward the main sequence. A crossmatch with external catalogs shows that 25 of the detected solar-like oscillators are a component of a spectroscopic binary, and 28 are confirmed planet host stars. These results provide the potential for precise, independent asteroseismic constraints on these and any additional TESS targets of interest.

astro-ph.SR↗

arXiv2022

Pulsating B stars in the Scorpius-Centaurus Association with TESS

We study 119 B stars located in the Scorpius-Centaurus Association using data from NASA's TESS Mission. We see pulsations in 81 stars (68%) across the full range of effective temperatures. In particular, we confirm previous reports of low-frequency pulsations in stars whose temperatures fall between the instability strips of SPB stars (slowly pulsating B stars) and $\delta$ Scuti stars. By taking the stellar densities into account, we conclude that these cannot be p modes and confirm previous suggestions that these are probably rapidly-rotating SPB stars. We also confirm that they follow two period-luminosity relations that are consistent with prograde sectoral g modes that are dipole ($l=m=1$) and quadrupole ($l=m=2$), respectively. One of the stars ($\xi^2$ Cen) is a hybrid pulsator that shows regular spacings in both g and p modes. We confirm that $\alpha$ Cru has low-amplitude p-mode pulsations, making it one of the brightest $\beta$ Cephei stars in the sky. We also find several interesting binaries, including a very short-period heartbeat star (HD 132094), a previously unknown eclipsing binary ($\pi$ Lup), and an eclipsing binary with high-amplitude tidally driven pulsations (HR 5846). The results clearly demonstrate the power of TESS for studying variability in stellar associations.

astro-ph.SR↗

arXiv2022

Five young $δ$ Scuti stars in the Pleiades seen with Kepler/K2

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 sample 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.

astro-ph.SR↗

arXiv2021

A 20-Second Cadence View of Solar-Type Stars and Their Planets with TESS: Asteroseismology of Solar Analogs and a Re-characterization of pi Men c

We present an analysis of the first 20-second cadence light curves obtained by the TESS space telescope during its extended mission. We find a precision improvement of 20-second data compared to 2-minute data for bright stars when binned to the same cadence (~10-25% better for T<~8 mag, reaching equal precision at T~13 mag), consistent with pre-flight expectations based on differences in cosmic ray mitigation algorithms. We present two results enabled by this improvement. First, we use 20-second data to detect oscillations in three solar analogs (gamma Pav, zeta Tuc and pi Men) and use asteroseismology to measure their radii, masses, densities and ages to ~1%, ~3%, ~1% and ~20% respectively, including systematic errors. Combining our asteroseismic ages with chromospheric activity measurements we find evidence that the spread in the activity-age relation is linked to stellar mass and thus convection-zone depth. Second, we combine 20-second data and published radial velocities to re-characterize pi Men c, which is now the closest transiting exoplanet for which detailed asteroseismology of the host star is possible. We show that pi Men c is located at the upper edge of the planet radius valley for its orbital period, confirming that it has likely retained a volatile atmosphere and that the "asteroseismic radius valley" remains devoid of planets. Our analysis favors a low eccentricity for pi Men c (<0.1 at 68% confidence), suggesting efficient tidal dissipation (Q/k <~ 2400) if it formed via high-eccentricity migration. Combined, these early results demonstrate the strong potential of TESS 20-second cadence data for stellar astrophysics and exoplanet science.

astro-ph.SR↗

arXiv2021

Asteroseismology of iota Draconis and Discovery of an Additional Long-Period Companion

Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host stars are also some of the brightest in the sky and so enable high signal-to-noise follow-up measurements. Here we report on new observations of the bright (V ~ 3.3) giant star $ι$ Draconis ($ι$ Dra), known to host a planet in a highly eccentric ~511 day period orbit. TESS observations of the star over 137 days reveal asteroseismic signatures, allowing us to constrain the stellar radius, mass, and age to ~2%, ~6%, and ~28%, respectively. We present the results of continued radial velocity monitoring of the star using the Automated Planet Finder over several orbits of the planet. We provide more precise planet parameters of the known planet and, through the combination of our radial velocity measurements with Hipparcos and Gaia astrometry, we discover an additional long-period companion with an orbital period of ~$68^{+60}_{-36}$ years. Mass predictions from our analysis place this sub-stellar companion on the border of the planet and brown dwarf regimes. The bright nature of the star combined with the revised orbital architecture of the system provides an opportunity to study planetary orbital dynamics that evolve as the star moves into the giant phase of its evolution.

astro-ph.EP↗

arXiv2021

TESS Data for Asteroseismology: Photometry

Over the last two decades, asteroseismology has increasingly proven to be the observational tool of choice for the study of stellar physics, aided by the high quality of data available from space-based missions such as CoRoT, Kepler, K2 and TESS. TESS in particular will produce more than an order of magnitude more such data than has ever been available before. While the standard TESS mission products include light curves from 120-sec observations suitable for both exoplanet and asteroseismic studies, they do not include light curves for the vastly larger number of targets observed by the mission at a longer 1800-sec cadence in Full Frame Images (FFIs). To address this lack, the TESS Data for Asteroseismology (T'DA) group under the TESS Asteroseismic Science Consortium (TASC), has constructed an open-source pipeline focused on producing light curves for all stars observed by TESS at all cadences, currently including stars down to a TESS magnitude of 15. The pipeline includes target identification, background estimation and removal, correction of FFI timestamps, and a range of potential photometric extraction methodologies, though aperture photometry is currently the default approach. For the brightest targets, we transparently apply a halo photometry algorithm to construct a calibrated light curve from unsaturated pixels in the image. In this paper, we describe in detail the algorithms, functionality, and products of this pipeline, and summarize the noise metrics for the light curves. Companion papers will address the removal of systematic noise sources from our light curves, and a stellar variability classification from these.

astro-ph.IM↗

arXiv2021

A precise asteroseismic age and metallicity for HD 139614: a pre-main-sequence star with a protoplanetary disc in Upper-Centaurus Lupus

HD\,139614 is known to be a $\sim$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 $10.75\pm0.77$ Myr, a mass of $1.52\pm0.02$ M$_{\odot}$, and a global metal abundance of $Z=0.0100\pm0.0010$. 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.

astro-ph.SR↗

arXiv2020

Very regular high-frequency pulsation modes in young intermediate-mass stars

Asteroseismology is a powerful tool for probing the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs. However, a large group of pulsating stars of intermediate mass--the so-called delta Scuti stars--have rich pulsation spectra for which systematic mode identification has not hitherto been possible. This arises because only a seemingly random subset of possible modes are excited, and because rapid rotation tends to spoil the regular patterns. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, allowing definitive mode identification. Some of these stars have space motions that indicate they are members of known associations of young stars, and modelling of their pulsation spectra confirms that these stars are indeed young.

astro-ph.SR↗

arXiv2020

Asteroseismology of 36 \emph{Kepler} subgiants -- I. Oscillation frequencies, linewidths and amplitudes

The presence of mixed modes makes subgiants excellent targets for asteroseismology, providing a probe for the internal structure of stars. Here we study 36 \emph{Kepler} subgiants with solar-like oscillations and report their oscillation mode parameters. We performed a so-called peakbagging exercise, i.e. estimating oscillation mode frequencies, linewidths, and amplitudes with a power spectrum model, fitted in the Bayesian framework and sampled with a Markov Chain Monte Carlo algorithm. The uncertainties of the mode frequencies have a median value of 0.180 $μ$Hz. We obtained seismic parameters from the peakbagging, analysed their correlation with stellar parameters, and examined against scaling relations. The behaviour of seismic parameters (e.g. $Δν$, $ν_{\rm max}$, $ε_p$) is in general consistent with theoretical predictions. We presented the observational p--g diagrams: $γ_1$--$Δν$ for early subgiants and $ΔΠ_1$--$Δν$ for late subgiants, and demonstrate their capability to estimate stellar mass. We also found a $\log g$ dependence on the linewidths and a mass dependence on the oscillation amplitudes and the widths of oscillation excess. This sample will be valuable constraints for modelling stars and studying mode physics such as excitation and damping.

astro-ph.SR↗

arXiv2020

Precision angular diameters for 16 southern stars with VLTI/PIONIER

In the current era of Gaia and large, high signal to noise stellar spectroscopic surveys, there is an unmet need for a reliable library of fundamentally calibrated stellar effective temperatures based on accurate stellar diameters. Here we present a set of precision diameters and temperatures for a sample of 6 dwarf, 5 sub-giant, and 5 giant stars observed with the PIONIER beam combiner at the VLTI. Science targets were observed in at least two sequences with five unique calibration stars each for accurate visibility calibration and to reduce the impact of bad calibrators. We use the standard PIONIER data reduction pipeline, but bootstrap over interferograms, in addition to employing a Monte-Carlo approach to account for correlated errors by sampling stellar parameters, limb darkening coefficients, and fluxes, as well as predicted calibrator angular diameters. The resulting diameters were then combined with bolometric fluxes derived from broadband Hipparcos-Tycho photometry and MARCS model bolometric corrections, plus parallaxes from Gaia to produce effective temperatures, physical radii, and luminosities for each star observed. Our stars have mean angular diameter and temperatures uncertainties of 0.8% and 0.9% respectively, with our sample including diameters for 10 stars with no pre-existing interferometric measurements. The remaining stars are consistent with previous measurements, with the exception of a single star which we observe here with PIONIER at both higher resolution and greater sensitivity than was achieved in earlier work.

astro-ph.SR↗

arXiv2020

Age dating of an early Milky Way merger via asteroseismology of the naked-eye star $ν$ Indi

Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. While these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to precisely date the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to a substantial pollution of the chemical and dynamical properties of the Milky Way. Here, we identify the very bright, naked-eye star $ν$\,Indi as a probe of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric, and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be $11.0 \pm 0.7$ (stat) $\pm 0.8$ (sys)$\,\rm Gyr$. The star bears hallmarks consistent with it having been kinematically heated by the \textit{Gaia}-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95% confidence, respectively. Input from computations based on hierarchical cosmological models tightens (i.e. reduces) slightly the above limits.

astro-ph.GA↗