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1. Two new white dwarfs with variable magnetic Balmer emission lines

   https://doi.org/10.1093/mnras/stad760  

   Reding, Joshua S. ; Hermes, J. J. ; Clemens, J. C. ; Hegedus, R. J. ; Kaiser, B. C. ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   We report the discovery of two apparently isolated stellar remnants that exhibit rotationally modulated magnetic Balmer emission, adding to the emerging DAHe class of white dwarf stars. While the previously discovered members of this class show Zeeman-split triplet emission features corresponding to single magnetic field strengths, these two new objects exhibit significant fluctuations in their apparent magnetic field strengths with variability phase. The Zeeman-split hydrogen emission lines in LP 705−64 broaden from 9.4 to 22.2 MG over an apparent spin period of 72.629 min. Similarly, WD J143019.29−562358.33 varies from 5.8  to 8.9 MG over its apparent 86.394 min rotation period. This brings the DAHe class of white dwarfs to at least five objects, all with effective temperatures within 500 K of 8000 K and masses ranging from $0.65\,\,{\text{to}}\,\,0.83\, {\rm M}_{\odot }$.
2. 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. Allmore »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.« less
3. 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 Hzmore »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.« less
4. A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

   https://doi.org/10.1051/0004-6361/202039831  

   Calcaferro, Leila M. ; Córsico, Alejandro H. ; Althaus, Leandro G. ; Bell, Keaton J. ( March 2021 , Astronomy & Astrophysics)

   Context. Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant in shedding some light on the internal structure of these stars through asteroseismology, similarly to what occurs with other classes of pulsating white dwarfs. Aims. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the ε mechanism is able to excite gravity-mode pulsations. We assess the ranges of unstable periods and the corresponding instability domain in the log g  −  T eff plane. Methods. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models with stellar masses between 0.2025 and 0.3630  M ⊙ going through CNO flashes during their early cooling phases. Results. We found that the ε mechanism due to stable hydrogen burning can excite low-order ( ℓ  = 1, 2) gravity modes with periods between ∼80 and 500 s for stars with 0.2025 ≲  M ⋆ / M ⊙  ≲ 0.3630 locatedmore »in an extended region of the log g  −  T eff diagram, with effective temperature and surface gravity in the ranges 15 000 ≲  T eff  ≲ 38 000 K and 5.8 ≲ log g  ≲ 7.1, respectively. For the sequences that experience multiple CNO flashes, we found that with every consecutive flash, the region of instability becomes wider and the modes are more strongly excited. The magnitudes of the rate of period change for these modes are in the range of ∼10 −10 –10 −11  [s/s]. Conclusions. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. Given the current problems in distinguishing some stars that populate the same region of the log g  −  T eff plane, the eventual detection of short-period pulsations may help in the classification of such stars. Furthermore, if a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the ε mechanism. In addition, confirming a rapid rate of period change in these pulsations would support the idea that these stars actually experience CNO flashes, as has been predicted by evolutionary calculations.« less
5. A Magnetic White Dwarf with Five Hα Components

   https://doi.org/10.1093/mnras/stz2394  

   Kilic, Mukremin ; Rolland, B ; Bergeron, P ; Vanderbosch, Z ; Benni, P ; Garlitz, J ( August 2019 , Monthly Notices of the Royal Astronomical Society)

   G183−35 is an unusual white dwarf that shows an Hα line split into five components, instead of the usual three components seen in strongly magnetic white dwarfs. Potential explanations for the unusual set of lines includes a double degenerate system containing two magnetic white dwarfs and/or rotational modulation of a complex magnetic field structure. Here we present time-resolved spectroscopy of G183−35 obtained at the Gemini Observatory. These data reveal two sets of absorption lines that appear and disappear over a period of about 4 hours. We also detect low-level (0.2%) variability in optical photometry at the same period. We demonstrate that the spectroscopic and photometric variability can be explained by the presence of spots on the surface of the white dwarf and a change in the average field strength from about 4.6 MG to 6.2 MG. The observed variability is clearly due to G183−35’s relatively short spin period. However, rotational modulation of a complex magnetic field by itself cannot explain the changes seen in the central Hα component. An additional source of variability in the line profiles, most likely due to a chemically inhomogeneous surface composition, is also needed. We propose further observations of similar objects to test this scenario.