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  1. Abstract

    The asteroseismic radius determination previously reported for the pulsating helium-atmosphere white dwarf star KIC 08626021 is 6σdiscrepant with constraints from Gaia astrometry. This calls into question the other results of the asteroseismic analysis, especially the high (central) oxygen abundance that stellar evolutionary models have been unable to reproduce.

  2. Abstract

    PG 1159-035 is the prototype of the PG 1159 hot (pre-)white dwarf pulsators. This important object was observed during the Kepler satellite K2 mission for 69 days in 59 s cadence mode and by the TESS satellite for 25 days in 20 s cadence mode. We present a detailed asteroseismic analysis of those data. We identify a total of 107 frequencies representing 32= 1 modes, 27 frequencies representing 12= 2 modes, and eight combination frequencies. The combination frequencies and the modes with very highkvalues represent new detections. The multiplet structure reveals an average splitting of 4.0 ± 0.4μHz for= 1 and 6.8 ± 0.2μHz for= 2, indicating a rotation period of 1.4 ± 0.1 days in the region of period formation. In the Fourier transform of the light curve, we find a significant peak at 8.904 ± 0.003μHz suggesting a surface rotation period of 1.299 ± 0.002 days. We also present evidence that the observed periods change on timescales shorter than those predicted by current evolutionary models. Our asteroseismic analysis finds an average period spacing for= 1 of 21.28 ± 0.02 s. The= 2 modes have a mean spacing of 12.97 ± 0.4 s. We performed a detailed asteroseismicmore »fit by comparing the observed periods with those of evolutionary models. The best-fit model hasTeff= 129, 600 ± 11 100 K,M*= 0.565 ± 0.024M, andlogg=7.410.54+0.38, within the uncertainties of the spectroscopic determinations. We argue for future improvements in the current models, e.g., on the overshooting in the He-burning stage, as the best-fit model does not predict excitation for all of the pulsations detected in PG 1159-035.

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  3. Abstract

    We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1 m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000 K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of DBVs known between 20,000 and 24,000 K. We do not find any new DBVs hotter than PG 0112+104, the current hottest DBV is atTeff≈ 31,000 K, but do find pulsations in four DBVs with temperatures between 27,000 and 30,000 K, improving empirical constraints on the poorly defined blue edge. We investigate the ensemble pulsation properties of all currently known DBVs, finding that the weighted mean period and total pulsation power exhibit trends with effective temperature that are qualitatively similar to the pulsating hydrogen-atmosphere white dwarfs.

  4. 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. ABSTRACT Long, high-quality time-series data provided by previous space missions such as CoRoT and Kepler have made it possible to derive the evolutionary state of red giant stars, i.e. whether the stars are hydrogen-shell burning around an inert helium core or helium-core burning, from their individual oscillation modes. We utilize data from the Kepler mission to develop a tool to classify the evolutionary state for the large number of stars being observed in the current era of K2, TESS, and for the future PLATO mission. These missions provide new challenges for evolutionary state classification given the large number of stars being observed and the shorter observing duration of the data. We propose a new method, Clumpiness, based upon a supervised classification scheme that uses ‘summary statistics’ of the time series, combined with distance information from the Gaia mission to predict the evolutionary state. Applying this to red giants in the APOKASC catalogue, we obtain a classification accuracy of $\sim 91{{\ \rm per\ cent}}$ for the full 4 yr of Kepler data, for those stars that are either only hydrogen-shell burning or also helium-core burning. We also applied the method to shorter Kepler data sets, mimicking CoRoT, K2, and TESS achieving anmore »accuracy $\gt 91{{\ \rm per\ cent}}$ even for the 27 d time series. This work paves the way towards fast, reliable classification of vast amounts of relatively short-time-span data with a few, well-engineered features.« less
  6. In a search for eclipsing white dwarfs using the Zwicky Transient Facility lightcurves, we identified a deep eclipsing white dwarf with a dark, substellar companion. The lack of an infrared excess and an orbital period of 10 hours made this a potential exoplanet candidate. We obtained high-speed photometry and radial velocity measurements to characterize the system. The white dwarf has a mass of 0.50±0.02M⊙ and a temperature of 10900±200K. The companion has a mass of 0.059±0.004M⊙ and a small radius of 0.0783±0.0013R⊙. It is one of the smallest transiting brown dwarfs known and likely old, ≳8Gyr. The ZTF discovery efficiency of substellar objects transiting white dwarfs is limited by the number of epochs and as ZTF continues to collect data we expect to find more of these systems. This will allow us to measure period and mass distributions and allows us to understand the formation channels of white dwarfs with substellar companions.