arXiv subjects

Keaton J. Bell

Publications and source records attributed to Keaton J. Bell.

At least 19 recordsLinked to original sources

arXiv2026

Improvements to Asteroseismic Fitting of White Dwarfs in the Gaia Era

White dwarf asteroseismology aims to constrain the interior structures of pulsating white dwarf stars by fitting observed pulsation periods to those computed for stellar models. However, there remain several obstacles for achieving reliable results with the model grid-fitting approach. We simulate data for a parameter recovery experiment in one and two dimensions (mass and effective temperature) to demonstrate improved methodologies that address challenges for characterizing degenerate asteroseismic solutions. We show that interpolating model periods onto a finer grid can adequately resolve asteroseismic solutions that would be missed with sparse model grids. Incorporating absolute magnitude from Gaia astrometry into the statistical fitting is shown to reduce solution degeneracy. We define a seismic solution as a solution to the mode identification problem, and we show that fitting to consistent model pulsation modes can isolate each candidate solution in degenerate solution space. A criterion based on $χ^2$ for what should be considered a reasonable fit is adopted, and we identify all combinations of model periods that meet this criterion, not only those nearest to the measured periods. Finally, we demonstrate how fitting Gaussians to probability distributions allows for the robust characterization of each candidate solution, including uncertainties. Our accurate characterization of the degenerate solution landscape is supported by the comparison of our solutions to a direct marginalization of the likelihood function. These fitting approaches can be generalized to higher dimensions, where there are more than two free parameters.

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arXiv2026

A Seismic Technique for Obtaining White Dwarf Fundamental Parameters from Mean Period Spacings and Astrometry

We present a new statistical technique that utilizes the synergy of precision astrometry and time series photometry from modern space missions to obtain reliable physical parameters of pulsating white dwarf stars. We compute a grid of white dwarf structural models that span the helium-atmosphere pulsating white dwarf (DBV) instability strip, showing that mean period spacings between adjacent pulsation modes and absolute magnitudes derived from Gaia astrometry vary monotonically and in opposing directions across parameter space. While most efforts in white dwarf asteroseismology to directly fit individual pulsation periods to stellar models result in degenerate and poorly resolved solutions, the "seismic technique" produces unique and reliable seismic solutions for global parameters of mass and effective temperature when a reliable mean period spacing is detected. Our models sample various physically plausible interior chemical composition profiles based on modern evolutionary models to propagate uncertainty from the precise structures of actual stars. Once the global stellar parameters are tightly constrained, seismically resolving white dwarf interior structures becomes more computationally tractable, and degeneracies can be resolved. This new seismic technique is largely insensitive to the precise absorption line profiles interpreted by the widely used spectroscopic technique, and therefore provides complementary constraints that can be used to test spectroscopic methods. We demonstrate the method for the pulsating helium-atmosphere white dwarf WD 0158-160 observed by TESS, obtaining $T_\mathrm{eff} = 24584\pm 971$ K and $M_\star = 0.608\pm 0.013$ $M_\odot$.

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arXiv2026

Constraints from parallaxes and average period spacings in the asteroseismic study of 8 DAVs

With space missions such as Kepler, TESS, and Gaia, we have a wealth of data on pulsating white dwarfs that can be leveraged in white dwarf asteroseismology. We address the question of the proportion of white dwarfs with thin hydrogen layers versus those with thick hydrogen layers. We also provide a mass-radius relation for carbon-oxygen core, hydrogen atmosphere white dwarfs. Such a relationship can be used in conjunction with magnitudes and distance measurements to constrain the mass and effective temperature of the white dwarfs. We select eight hydrogen atmosphere, pulsating white dwarfs (DAVs), for their rich pulsation spectra. From such pulsation spectra, we can derive an asymptotic period spacing, which in turn allows us to determine the thickness of the hydrogen and helium envelope of the models, without having to perform period by period fitting. We find that the majority of the white dwarfs have thicker hydrogen layers and determine an upper limit of Mr = 1 - 10 to the -2.2 for the location of the base of the helium layer, in accordance with stellar evolution models. We confirm a finding from earlier studies that used a mass-radius relation and Gaia data to determine the effective temperatures of white dwarfs. The Gaia data systematically points to white dwarfs of lower effective temperature than indicated by the spectroscopy. Our results also support the hypothesis that white dwarfs with thicker hydrogen layers are more common than those with thinner layers.

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arXiv2026

Unveiling the properties of pulsating low-mass helium-core white dwarfs through TESS asteroseismology I. First results

Recent space-based photometry, particularly from TESS, has opened new possibilities for probing the internal structure of low-mass (LM) helium (He)-core white dwarfs (WDs). We present a homogeneous asteroseismological analysis of six pulsating LM WD stars, based on new and updated TESS photometry. We processed short- and ultra-short-cadence TESS observations of TIC 290904838 (J1112), TIC 156064657, TIC 33717565, TIC 344130696, TIC 72637474, and TIC 188087204, and analyzed the resulting pulsation spectra. We then carried out a detailed asteroseismological analysis using fully evolutionary models of LM He-core WDs that allow for varying hydrogen (H)-envelope thicknesses. We also estimated spectroscopic/photometric stellar masses when atmospheric parameters are available. We report the first TESS-based frequencies for J1112 and provide revised or expanded frequency solutions for the remaining targets. The asteroseismological analysis yields relatively well-constrained solutions for three stars, a representative but more tentative solution for one target, and constrained ranges for the remaining two. The inferred solutions span a broad range of H-envelope thicknesses, although some of the asteroseismological inferences remain tentative because of the limited number of observed periods available for the analysis. For most objects, the derived spectroscopic/photometric stellar masses are broadly compatible with the asteroseismological values. This is the first homogeneous TESS-based asteroseismological study of a small sample of pulsating LM WDs. Our results suggest that LM WDs can harbor H envelopes with a range of thicknesses, from canonical (thick) to very thin, as in average-mass H-rich pulsating WDs. They also provide a useful reference point for future studies of larger samples, which will hopefully benefit from richer mode sets and improved mode identification.

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arXiv2026

Observing bright pulsating white dwarfs with PLATO: A new window into the late stages of stellar evolution

We present the scientific case for exploiting the capabilities of the PLATO mission to study bright pulsating white dwarfs across a wide spectral range, including hydrogen-deficient types (GW Vir and DBV stars) and hydrogen-rich classes (classical DAVs, pulsating extremely low-mass DA white dwarfs, and ultra-massive DA white dwarfs). PLATOs exceptional photometric precision, long-duration continuous monitoring, and extensive sky coverage promise transformative advances in white dwarf asteroseismology. Our key objectives include probing the internal structure and chemical stratification of white dwarfs, detecting secular changes in pulsation modes over extended timescales, and discovering rare or previously unknown classes of pulsators. To assess feasibility, we constructed a sample of 650 white dwarf candidates identified within PLATOs Southern LOPS2 field using the PLATO complementary science catalogue combined with Gaia DR3, and derived atmospheric parameters through photometric modeling. This sample comprises 118 DA white dwarfs (including 23 ZZ Ceti candidates), and 41 non-DAs (including 35 DBV candidates). Simulated observations using PlatoSim demonstrate that PLATO will be capable of detecting white dwarf pulsation modes with amplitudes as low as 0.1 mma depending on stellar magnitude, observation duration, pixel location, and the number of contributing cameras. We provide detailed detection limits and visibility forecasts for known pulsators across a representative range of these parameters. Furthermore, we emphasize strong synergies with Gaia astrometry, TESS photometry, and targeted spectroscopic campaigns, which together will enable robust mode identification and detailed stellar modeling. Collectively, these efforts will unlock unprecedented insights into white dwarf origins, evolution and internal physics, and the fate of their planetary systems.

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arXiv2026

Mode Instability and a Massive, Isolated Outburst in the Pulsating White Dwarf GD 1212

We analyze a large brightening event that lasted for roughly half a day in the pulsating hydrogen-atmosphere white dwarf GD 1212 during K2 Campaign 12 of the extended Kepler mission. For the other 80 days of K2 observations, GD 1212 exhibited a rich spectrum of long-period (~1100 s) pulsations that underwent rapid variations in frequency and amplitude but did not exhibit any additional outbursts. We refine previous attempts at mode identification and find a likely sequence of dipole and quadrupole splittings that reveal an overall rotation rate of roughly 17.0 hr. The outburst at Day 61 is fully resolved by the 60-second-cadence K2 data, with the entire white dwarf becoming up to 17.5% brighter overall, from an approximately 850 K increase in effective temperature, with pulsational variability during the outburst showing shorter periods and higher amplitudes. Outbursts are believed to be the result of nonlinear mode coupling via parametric instability, whereby energy stored in linearly excited parent modes is rapidly transferred to damped child modes that dissipate near the surface. Additionally, we characterize a "failed" outburst that caused correlated pulsation frequency changes, an approximately 5 microHz increase, with a small approximately 0.35% corresponding brightness increase. GD 1212 is now the eighth pulsating hydrogen-atmosphere DAV white dwarf to show outburst behavior, although it exhibited the largest outburst yet and has the longest inferred recurrence timescale. This high-signal-to-noise record tracing pulsations through both large and small temperature excursions in GD 1212 provides unique insights into parametric resonance and nonlinear mode coupling in white dwarf pulsations.

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arXiv2026

Kepler Discovery of GW Vir Pulsations of the Central Star of Planetary Nebula Kn 61

We report the discovery of pulsations in the N-rich PG 1159-type central star of the planetary nebula Kn 61 based on one month of Kepler Short Cadence observations. We detect four significant peaks in the frequency range consistent with g-modes excited in GW Vir stars. From the detected modes, we identify a mean period spacing of $ΔΠ=21.526(6)$ s for a sequence of three $\ell=1$ modes. This allows us to derive the asteroseismic mass of the star, which we estimate to be $0.551(6)~\mathrm{M}_{\odot}$, consistent with the one derived from the evolutionary tracks. We also characterize sporadic brightening events in the Long Cadence Kepler light curve of Kn 61. If we assume these are caused by increases in effective temperature, we estimate their energies to be $\sim10^{40}$ erg, though this may not be accurate as the mechanism for releasing so much energy is still unknown.

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arXiv2025

Gravitational Influence from Planets on the Measured Rates of Period Change of Pulsating White Dwarfs

The measured rates of period change, $\dot{P}$, in the signals of pulsating white dwarf stars are often interpreted as direct detections of structural changes from secular cooling. Due to the intrinsic nature of this quantity, $\dot{P}$ analysis has been used to probe fundamental physics, such as constraining the mass of hypothetical axion particles. However, most white dwarfs are expected to host planets that could induce an external source of period change, caused by the light travel time variations from reflex motion about the system barycenter. Assuming a plausible distribution of planets that could orbit white dwarf stars, we quantify the amount of reflex motion expected from undiscovered planets as an important source of extrinsic error in $\dot{P}$ analyses. While the median error from reflex motion is $\sim10^{-15}$ ss$^{-1}$ (similar to the secular $\dot{P}$ rates expected for cool DAV pulsators), individual close-in planets could cause $\dot{P}$ errors as large as $10^{-11}$ ss$^{-1}$

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arXiv2025

White Dwarf Variability

There are a few different mechanisms that can cause white dwarf stars to vary in brightness, providing opportunities to probe the physics, structures, and formation of these compact stellar remnants. The observational characteristics of the three most common types of white dwarf variability are summarized: stellar pulsations, rotation, and ellipsoidal variations from tidal distortion in binary systems. Stellar pulsations are emphasized as the most complex type of variability, which also has the greatest potential to reveal the conditions of white dwarf interiors.

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arXiv2025

Photometric White Dwarf Rotation

We present a census of photometrically detected rotation periods for white dwarf (WD) stars. We analyzed the light curves of 9285 WD stars observed by the Transiting Exoplanet Survey Satellite up to Sector 69. Using Fourier transform analyses and the TESS_localize software, we detected variability periods for 318 WD stars. The 115 high-probability likely single WDs in our sample have a median rotational period of 3.9 hr and a median absolute deviation of 3.5 hr. Our distribution is significantly different from the distribution of the rotational period from asteroseismology, which exhibits a longer median period of 24.2 hr and a median absolute deviation of 12.1 hr. In addition, we reported nonpulsating periods for three known pulsating WDs with rotational periods previously determined by asteroseismology: NGC 1501, TIC 7675859, and G226-29. We also calculated evolutionary models including six angular momentum transfer mechanisms from the literature throughout evolution in an attempt to reproduce our findings. Our models indicate that the temperature-period relation of most observational data is best fitted by models with low metallicity, probably indicating problems with the computations of angular momentum loss during the high-mass-loss phase. Our models also generate internal magnetic fields through the Tayler-Spruit dynamo.

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arXiv2024

APOKASC-3: The Third Joint Spectroscopic and Asteroseismic catalog for Evolved Stars in the Kepler Fields

In the third APOKASC catalog, we present data for the complete sample of 15,808 evolved stars with APOGEE spectroscopic parameters and Kepler asteroseismology. We used ten independent asteroseismic analysis techniques and anchor our system on fundamental radii derived from Gaia $L$ and spectroscopic $T_{\rm eff}$. We provide evolutionary state, asteroseismic surface gravity, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them for 12,418 stars. This includes 10,036 exceptionally precise measurements, with median fractional uncertainties in \nmax, \dnu, mass, radius and age of 0.6\%, 0.6\%, 3.8\%, 1.8\%, and 11.1\% respectively. We provide more limited data for 1,624 additional stars which either have lower quality data or are outside of our primary calibration domain. Using lower red giant branch (RGB) stars, we find a median age for the chemical thick disk of $9.14 \pm 0.05 ({\rm ran}) \pm 0.9 ({\rm sys})$ Gyr with an age dispersion of 1.1 Gyr, consistent with our error model. We calibrate our red clump (RC) mass loss to derive an age consistent with the lower RGB and provide asymptotic GB and RGB ages for luminous stars. We also find a sharp upper age boundary in the chemical thin disk. We find that scaling relations are precise and accurate on the lower RGB and RC, but they become more model dependent for more luminous giants and break down at the tip of the RGB. We recommend the usage of multiple methods, calibration to a fundamental scale, and the usage of stellar models to interpret frequency spacings.

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arXiv2024

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS VI. Asteroseismology of the GW Vir-type central star of the Planetary Nebula NGC 246

Significant advances have been achieved through the latest improvements in the photometric observations accomplished by the recent space missions, substantially boosting the study of pulsating stars via asteroseismology. The TESS mission has already proven to be of relevance for pulsating white dwarf and pre-white dwarf stars. We report a detailed asteroseismic analysis of the pulsating PG 1159 star NGC 246 (TIC3905338), the central star of the planetary nebula NGC 246, based on high-precision photometric data gathered by the TESS space mission. We reduced TESS observations of NGC 246 and performed a detailed asteroseismic analysis using fully evolutionary PG 1159 models computed accounting for the complete prior evolution of their progenitors. We constrained the mass of this star by comparing the measured mean period spacing with the average of the computed period spacings of the models and also employed the observed individual periods to search for a seismic stellar model. We extracted 17 periodicities from the TESS light curves from the two sectors where NGC246 was observed. All the oscillation frequencies are associated with g-mode pulsations, with periods spanning from ~1460 to ~1823s. We found a constant period spacing of $ΔΠ= 12.9$s, allowing us to deduce that the stellar mass is larger than ~0.87 Mo if the period spacing is assumed to be associated with l= 1 modes, and ~ 0.568 Mo if it is associated with l= 2 modes. The less massive models are more consistent with the distance constraint from Gaia parallax. Although we were not able to find a unique asteroseismic model for this star, the period-to-period fit analyses suggest a high-stellar mass ($\gtrsim$0.74 Mo) when the observed periods are associated with modes with l= 1 only, and both a high ($\gtrsim$ 0.74 Mo) and intermediate (~0.57 Mo) stellar mass when the observed periods are associated with modes with l= 1 and 2.

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arXiv2024

Rotation plays a role in the generation of magnetic fields in single white dwarfs

Recent surveys of close white dwarf binaries as well as single white dwarfs have provided evidence for the late appearance of magnetic fields in white dwarfs, and a possible generation mechanism a crystallization and rotation-driven dynamo has been suggested. A key prediction of this dynamo is that magnetic white dwarfs rotate, at least on average, faster than their non-magnetic counterparts and/or that the magnetic field strength increases with rotation. Here we present rotation periods of ten white dwarfs within 40 pc measured using photometric variations. Eight of the light curves come from TESS observations and are thus not biased towards short periods, in contrast to most period estimates that have been reported previously in the literature. These TESS spin periods are indeed systematically shorter than those of non-magnetic white dwarfs. This means that the crystallization and rotation-driven dynamo could be responsible for a fraction of the magnetic fields in white dwarfs. However, the full sample of magnetic white dwarfs also contains slowly rotating strongly magnetic white dwarfs which indicates that another mechanism that leads to the late appearance of magnetic white dwarfs might be at work, either in addition to or instead of the dynamo. The fast-spinning and massive magnetic white dwarfs that appear in the literature form a small fraction of magnetic white dwarfs, and probably result from a channel related to white dwarf mergers.

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arXiv2023

Asteroseismological analysis of the polluted ZZ Ceti star G29-38 with TESS

G\,29$-$38 (TIC~422526868) is one of the brightest ($V=13.1$) and closest ($d = 17.51$\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29$-$38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\sl TESS} data set spans $\sim 51$ days in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with $g$-mode pulsations, with periods spanning from $\sim$ 260 s to $\sim$ 1400 s. We identified %three distinct rotational frequency triplets with a mean separation $δν_{\ell=1}$ of 4.67 $μ$Hz and a quintuplet with a mean separation $δν_{\ell=2}$ of 6.67 $μ$Hz, from which we estimated a rotation period of about $1.35 \pm 0.1$ days. We determined a constant period spacing of 41.20~s for $\ell= 1$ modes and 22.58\,s for $\ell= 2$ modes. We performed period-to-period fit analyses and found an asteroseismological model with $M_{\star}/M_{\odot}=0.632 \pm 0.03$, $T_{\rm eff}=11\, 635\pm 178$ K, and $\log{g}=8.048\pm0.005$ (with a hydrogen envelope mass of $M_{\rm H}\sim 5.6\times 10^{-5}M_{\star}$), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by {\sl Gaia} (17.51 pc).

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arXiv2023

Anomalous flux event in the TESS Sector 43 light curve of the white dwarf photometric standard HZ 4 was caused by a passing asteroid

Frymire & Ardila (2023) reported an anomalous flux variation in the Transiting Exoplanet Survey Satellite (TESS) Sector 43 light curve of the white dwarf HZ 4. We show that this flux variation was caused by the main-belt asteroid 4382 Stravinsky traversing the nearby TESS pixels, and it is therefore not a cause for concern regarding the continued use of HZ 4 as a photometric standard star.

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arXiv2023

Localizing Sources of Variability in Crowded TESS Photometry

The Transiting Exoplanet Survey Satellite (TESS) has an exceptionally large plate scale of 21"/px, causing most TESS light curves to record the blended light of multiple stars. This creates a danger of misattributing variability observed by TESS to the wrong source, which would invalidate any analysis. We develop a method that can localize the origin of variability on the sky to better than one fifth of a pixel. Given measured frequencies of observed variability (e.g., from periodogram analysis), we show that the corresponding best-fit sinusoid amplitudes to raw light curves extracted from each pixel are distributed the same as light from the variable source. The primary assumption of this method is that other nearby stars are not variable at the same frequencies. Essentially, we are using the high frequency resolution of TESS to overcome limitations from its low spatial resolution. We have implemented our method in an open source Python package, TESS Localize (github.com/Higgins00/TESS-Localize), that determines the location of a variable source on the sky given TESS pixel data and a set of observed frequencies of variability. Our method utilizes the TESS Pixel Response Function models, and we characterize systematics in the residuals of fitting these models to data. Given the ubiquity of source blending in TESS light curves, verifying the source of observed variability should be a standard step in TESS analyses.

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arXiv2023

Contamination in TESS light curves: The case of the Fast Yellow Pulsating Supergiants

Given its large plate scale of 21" / pixel, analyses of data from the TESS space telescope must be wary of source confusion from blended light curves, which creates the potential to attribute observed photometric variability to the wrong astrophysical source. We explore the impact of light curve contamination on the detection of fast yellow pulsating supergiant (FYPS) stars as a case study to demonstrate the importance of confirming the source of detected signals in the TESS pixel data. While some of the FYPS signals have already been attributed to contamination from nearby eclipsing binaries, others are suggested to be intrinsic to the supergiant stars. In this work, we carry out a detailed analysis of the TESS pixel data to fit the source locations of the dominant signals reported for 17 FYPS stars with the Python package TESS_localize. We are able to reproduce the detections of these signals for 14 of these sources, obtaining consistent source locations for four. Three of these originate from contaminants, while the signal reported for BZ Tuc is likely a spurious frequency introduced to the light curve of this 127-day Cepheid by the data processing pipeline. Other signals are not significant enough to be localized with our methods, or have long periods that are difficult to analyze given other TESS systematics. Since no localizable signals hold up as intrinsic pulsation frequencies of the supergiant targets, we argue that unambiguous detection of pulsational variability should be obtained before FYPS are considered a new class of pulsator.

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arXiv2022

Asteroseismology of PG 1541$+$651 and BPM 31594 with TESS

We present the photometric data from TESS for two known ZZ Ceti stars, PG 1541+651 and BPM 31594. Before TESS, both objects only had observations from short runs from ground-based facilities, with three and one period detected, respectively. The TESS data allowed the detection of multiple periodicities, 12 for PG 1541$+$651, and six for BPM 31594, which enables us to perform a detailed asteroseismological study. For both objects we found a representative asteroseismic model with canonical stellar mass ~ 0.61 Msun and thick hydrogen envelopes, thicker than 10^(-5.3) M_*. The detection of triplets in the Fourier transform also allowed us to estimate mean rotation periods, being ~22 h for PG 1541+651 and 11.6 h for BPM 31594, which is consistent with range of values reported for other ZZ Ceti stars.

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