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1. Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: V. Discovery of two new DBV pulsators, WDJ152738.4−450207.4 and WD 1708−871, and asteroseismology of the already known DBV stars PG 1351+489, EC 20058−5234, and EC 04207−4748

   https://doi.org/10.1051/0004-6361/202244198  

   Córsico, A. H.; Uzundag, M.; Kepler, S. O.; Althaus, L. G.; Silvotti, R.; Bradley, P. A.; Baran, A. S.; Koester, D.; Bell, K. J.; Romero, A. D.; et al (December 2022, Astronomy & Astrophysics)

   Context. The TESS space mission has recently demonstrated its great potential to discover new pulsating white dwarf and pre-white dwarf stars, and to detect periodicities with high precision in already known white-dwarf pulsators. Aims. We report the discovery of two new pulsating He-rich atmosphere white dwarfs (DBVs) and present a detailed asteroseismological analysis of three already known DBV stars employing observations collected by the TESS mission along with ground-based data. Methods. We processed and analyzed TESS observations of the three already known DBV stars PG 1351+489 (TIC 471015205), EC 20058−5234 (TIC 101622737), and EC 04207−4748 (TIC 153708460), and the two new DBV pulsators WDJ152738.4−50207.4 (TIC 150808542) and WD 1708−871 (TIC 451533898), whose variability is reported for the first time in this paper. We also carried out a detailed asteroseismological analysis using fully evolutionary DB white-dwarf models built considering the complete evolution of the progenitor stars. We constrained the stellar mass of three of these target stars by means of the observed period spacing, and derived a representative asteroseismological model using the individual periods, when possible. Results. We extracted frequencies from the TESS light curves of these DBV stars using a standard pre-whitening procedure to derive the potential pulsation frequencies. All the oscillation frequencies that we found are associated with g -mode pulsations with periods spanning from ∼190 s to ∼936 s. We find hints of rotation from frequency triplets in some of the targets, including the two new DBVs. For three targets, we find constant period spacings, which allowed us to infer their stellar masses and constrain the harmonic degree ℓ of the modes. We also performed period-to-period fit analyses and found an asteroseismological model for three targets, with stellar masses generally compatible with the spectroscopic masses. Obtaining seismological models allowed us to estimate the seismological distances and compare them with the precise astrometric distances measured with Gaia . We find a good agreement between the seismic and the astrometric distances for three stars (PG 1351+489, EC 20058-5234, and WD 1708-871); although, for the other two stars (EC 04207-4748 and WD J152738.4-50207), the discrepancies are substantial. Conclusions. The high-quality data from the TESS mission continue to provide important clues which can be used to help determine the internal structure of pulsating pre-white dwarf and white dwarf stars through the tools of asteroseismology. 

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2. TESS first look at evolved compact pulsators: Asteroseismology of the pulsating helium-atmosphere white dwarf TIC 257459955

   https://doi.org/10.1051/0004-6361/201936340  

   Bell, Keaton J.; Córsico, Alejandro H.; Bischoff-Kim, Agnès; Althaus, Leandro G.; Bradley, Paul A.; Calcaferro, Leila M.; Montgomery, Michael H.; Uzundag, Murat; Baran, Andrzej S.; Bognár, Zsófia; et al (December 2019, Astronomy & Astrophysics)

   Context. Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky. Aims. As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3. Methods. We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies were compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models from LPCODE and the structural models from WDEC . Results. We detected and measured the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158−160 to ∼0.01  μ Hz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole (ℓ = 1) modes with a mean period spacing of 38.1 ± 1.0 s. The global best-fit seismic models from both LPCODE and WDEC have effective temperatures that are ≳3000 K hotter than archival spectroscopic values of 24 100–25 500 K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints from Gaia astrometry. Conclusions. Our results demonstrate the value of the TESS data for DBV white dwarf asteroseismology. The extent of the short-cadence photometry enables reliably accurate and extremely precise pulsation frequency measurements. Similar subsets of both the LPCODE and WDEC models show good agreement with these measurements, supporting that the asteroseismic interpretation of DBV observations from TESS is not dominated by the set of models used. However, given the sensitivity of the observed set of pulsation modes to the stellar structure, external constraints from spectroscopy and/or astrometry are needed to identify the best seismic solutions. 

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3. Discovery of the Richest Pulsating Ultramassive White Dwarf

   https://doi.org/10.3847/2041-8213/adad73  

   De_Gerónimo, Francisco C; Uzundag, Murat; Rebassa-Mansergas, Alberto; Brown, Alex; Kilic, Mukremin; Córsico, Alejandro H; Jewett, Gracyn C; Moss, Adam G (February 2025, The Astrophysical Journal Letters)

   Abstract The discovery of pulsations in ultramassive (UM) white dwarfs (WDs) can help to probe their interiors and unveil their core composition and crystallized mass fraction through asteroseismic techniques. To date, the richest pulsating UM WD known is BPM 37093 with 8 modes detected, for which detailed asteroseismic analysis has been performed in the past. In this work, we report the discovery of 19 pulsation modes in the UM WD star WD J0135+5722, making it the richest pulsating hydrogen-atmosphere UM WD known to date. This object exhibits multiperiodic luminosity variations with periods ranging from 137 to 1345 s, typical of pulsating WDs in the ZZ Ceti instability strip, which is centered atT_eff ∼ 12,000 K. We estimate the stellar mass of WD J0135+5722 by different methods, resulting inM_⋆ ∼ 1.12–1.14M_⊙if the star’s core is made of oxygen and neon orM_⋆ ∼ 1.14–1.15M_⊙if the star hosts a carbon oxygen core. Future analysis of the star periods could shed light on the core chemical composition through asteroseismology. 

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4. Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: III. Asteroseismology of the DBV star GD 358

   https://doi.org/10.1051/0004-6361/202142153  

   Córsico, A. H.; Uzundag, M.; Kepler, S. O.; Silvotti, R.; Althaus, L. G.; Koester, D.; Baran, A. S.; Bell, K. J.; Bischoff-Kim, A.; Hermes, J. J.; et al (March 2022, Astronomy & Astrophysics)

   Context. The collection of high-quality photometric data by space telescopes, such as the completed Kepler mission and the ongoing TESS program, is revolutionizing the area of white-dwarf asteroseismology. Among the different kinds of pulsating white dwarfs, there are those that have He-rich atmospheres, and they are called DBVs or V777 Her variable stars. The archetype of these pulsating white dwarfs, GD 358, is the focus of the present paper. Aims. We report a thorough asteroseismological analysis of the DBV star GD 358 (TIC 219074038) based on new high-precision photometric data gathered by the TESS space mission combined with data taken from the Earth. Methods. We reduced TESS observations of the DBV star GD 358 and performed a detailed asteroseismological analysis using fully evolutionary DB white-dwarf models computed accounting for the complete prior evolution of their progenitors. We assessed the mass of this star by comparing the measured mean period separation with the theoretical averaged period spacings of the models, and we used the observed individual periods to look for a seismological stellar model. We detected potential frequency multiplets for GD 358, which we used to identify the harmonic degree ( ℓ ) of the pulsation modes and rotation period. Results. In total, we detected 26 periodicities from the TESS light curve of this DBV star using standard pre-whitening. The oscillation frequencies are associated with nonradial g (gravity)-mode pulsations with periods from ∼422 s to ∼1087 s. Moreover, we detected eight combination frequencies between ∼543 s and ∼295 s. We combined these data with a huge amount of observations from the ground. We found a constant period spacing of 39.25 ± 0.17 s, which helped us to infer its mass ( M ⋆  = 0.588 ± 0.024  M ⊙ ) and constrain the harmonic degree ℓ of the modes. We carried out a period-fit analysis on GD 358, and we were successful in finding an asteroseismological model with a stellar mass ( M ⋆ = 0.584 −0.019 +0.025   M ⊙ ), compatible with the stellar mass derived from the period spacing, and in line with the spectroscopic mass ( M ⋆  = 0.560 ± 0.028  M ⊙ ). In agreement with previous works, we found that the frequency splittings vary according to the radial order of the modes, suggesting differential rotation. Obtaining a seismological model made it possible to estimate the seismological distance ( d seis  = 42.85 ± 0.73 pc) of GD 358, which is in very good accordance with the precise astrometric distance measured by Gaia EDR3 ( π  = 23.244 ± 0.024,  d Gaia  = 43.02 ± 0.04 pc). Conclusions. The high-quality data measured with the TESS space telescope, used in combination with data taken from ground-based observatories, provides invaluable information for conducting asteroseismological studies of DBV stars, analogously to what happens with other types of pulsating white-dwarf stars. The currently operating TESS mission, together with the advent of other similar space missions and new stellar surveys, will give an unprecedented boost to white dwarf asteroseismology. 

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5. Massive White Dwarfs in the 100 pc Sample: Magnetism, Rotation, Pulsations, and the Merger Fraction

   https://doi.org/10.3847/1538-4357/ad6905  

   Jewett, Gracyn; Kilic, Mukremin; Bergeron, Pierre; Moss, Adam; Blouin, Simon; Brown, Warren_R; Kosakowski, Alekzander; Toonen, Silvia; Agüeros, Marcel_A (October 2024, The Astrophysical Journal)

   Abstract We present a detailed model atmosphere analysis of massive white dwarfs withM> 0.9M_⊙andT_eff≥ 11,000 K in the Montreal White Dwarf Database 100 pc sample and the Pan-STARRS footprint. We obtained follow-up optical spectroscopy of 109 objects with no previous spectral classification in the literature. Our spectroscopic follow-up is now complete for all 204 objects in the sample. We find 118 normal DA white dwarfs, including 45 massive DAs near the ZZ Ceti instability strip. There are no normal massive DBs: the six DBs in the sample are strongly magnetic and/or rapidly rotating. There are 20 massive DQ white dwarfs in our sample, and all are found in the crystallization sequence. In addition, 66 targets are magnetic (32% of the sample). We use magnetic white dwarf atmosphere models to constrain the field strength and geometry using offset dipole models. We also use magnetism, kinematics, and rotation measurements to constrain the fraction of merger remnant candidates among this population. The merger fraction of this sample increases from 25% for 0.9–1M_⊙white dwarfs to 49% for 1.2–1.3M_⊙. However, this fraction is as high as ${78}_{-7}^{+4}$% for 1.1–1.2M_⊙white dwarfs. Previous works have demonstrated that 5%–9% of high-mass white dwarfs stop cooling for ∼8 Gyr due to the²²Ne distillation process, which leads to an overdensity of Q-branch stars in the solar neighborhood. We demonstrate that the overabundance of the merger remnant candidates in our sample is likely due to the same process. 

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