Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: I. Asteroseismology of the GW Vir stars RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16

Córsico, Alejandro H.; Uzundag, Murat; Kepler, S. O.; Althaus, Leandro G.; Silvotti, Roberto; Baran, Andrzej S.; Vučković, Maja; Werner, Klaus; Bell, Keaton J.; Higgins, Michael

doi:10.48550/arxiv.2011.03629

Cited by 2 publications

(2 citation statements)

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“…What remains to be discussed is the cause of the photometric variability and how it is linked to the occurrence of the UHE features and He II line problem. The photometric periods of all stars in our sample are well above the theoretical upper limit of 10 4 s predicted for non-radial gmode pulsations that are frequently observed amongst PG 1159 stars (most of them having periods below 3000 s, Quirion et al 2007;Córsico et al 2019Córsico et al , 2020. Thus, we see two possible scenarios that could instead account for the photometric variability in our stars -one is linked to close binaries, and the other one related to magnetic fields.…”

Section: Discussionsupporting

confidence: 69%

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Reindl,

Schaffenroth,

Filiz

et al. 2021

Preprint

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Context. About 10% of all stars exhibit absorption lines of ultra-high excited (UHE) metals (e.g. O viii) in their optical spectra when entering the white dwarf cooling sequence. This is something that has never been observed in any other astrophysical object, and challenges our understanding of the late stages of stellar evolution since decades. The recent discovery of a both spectroscopic and photometric variable UHE white dwarf led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. Aims. We aim to gain a better understanding of these mysterious objects by studying the photometric variability of the whole population of UHE white dwarfs, and white dwarfs showing only the He ii line problem, as both phenomena are believed to be connected. Methods. We investigate (multi-band) light curves from several ground-and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the He ii line problem. Results. We find that 75 +8 −13 % of the UHE white dwarfs, and 75 +9 −19 % of the He ii line problem white dwarfs are significantly photometrically variable, with periods ranging from a 0.22 d to 2.93 d and amplitudes from a few tenth to a few hundredth mag. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find 9 +4 −2 %), marking UHE and He ii line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. Conclusions. We found further evidence that UHE and He ii line problem white dwarfs are indeed related, as concluded from their overlap in the Gaia HRD, similar photometric variability rates, light curve shapes and amplitudes, as well as period distributions. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the non-detection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible.

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Section: Discussionsupporting

confidence: 69%

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Reindl,

Schaffenroth,

Filiz

et al. 2021

Preprint

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“…From the continuous light curves produced by the space missions such as Kepler and K2 and now TESS, it has been observed that oscillation frequencies in compact stars, including pulsating sdB stars, DBV and DOV pulsating white dwarfs, may not be stable (Silvotti et al 2019;Zong et al 2016b;Córsico et al 2020). It is known that the frequency and amplitude variations mostly occur due to beatings of unresolved peaks or unresolved multiplets.…”

Section: Frequency and Amplitude Variationsmentioning

confidence: 99%

Asteroseismic analysis of variable hot subdwarf stars observed with TESS I. The mean g-mode period spacings in hot subdwarf B stars

Uzundag,

Vuckovic,

Nemeth

et al. 2021

Preprint

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Context. We present photometric and spectroscopic analyses of gravity (g-mode) long-period pulsating hot subdwarf B (sdB) stars, also called V1093 Her stars, observed by the TESS space telescope in both 120 s short-cadence and 20 s ultra-short-cadence mode during the survey observation and the extended mission of the southern ecliptic hemisphere. Aims. We perform a detailed asteroseismic and spectroscopic analysis of five pulsating sdB stars observed with TESS aiming at the global comparison of the observations with the model predictions based on our stellar evolution computations coupled with the adiabatic pulsation computations. Methods. We process and analyze TESS observations of long-period pulsating hot subdwarf B stars. We perform standard prewhitening techniques on the datasets to extract the pulsation periods from the TESS light curves. We apply standard seismic tools for mode identification, including asymptotic period spacings and rotational frequency multiplets. Based on the values obtained from Kolmogorov-Smirnov and Inverse Variance tests, we search for a constant period spacing for dipole (l = 1) and quadrupole (l = 2) modes. We calculate the mean period spacing for l = 1 and l = 2 modes and estimate the errors by means of a statistical resampling analysis. For all stars, atmospheric parameters were derived by fitting synthetic spectra to the newly obtained low-resolution spectra. We have computed stellar evolution models using LPCODE stellar evolution code, and computed l = 1 g-mode frequencies with the adiabatic non-radial pulsation code LP-PUL. Derived observational mean period spacings are then compared to the mean period spacings from detailed stellar evolution computations coupled with the adiabatic pulsation computations of g-modes.Results. We detect 73 frequencies, most of which are identified as dipole and quadrupole g-modes with periods spanning from ∼ 3 000 s to ∼ 14 500 s. The derived mean period spacing of dipole modes is concentrated in a narrow region ranging from 251 s to 256 s, while the mean period spacing for quadrupole modes spans from 145 s to 154 s. The atmospheric parameters derived from spectroscopic data are typical of long-period pulsating sdB stars with the effective temperature ranging from 23 700 K to 27 600 K and surface gravity spanning from 5.3 dex to 5.5 dex. In agreement with the expectations from theoretical arguments and previous asteroseismological works, we find that the mean period spacings obtained for models with small convective cores, as predicted by a pure Schwarzschild criterion, are incompatible with the observations. We find that models with a standard/modest convective boundary mixing at the boundary of the convective core are in better agreement with the observed mean period spacings and are therefore more realistic. Conclusions. Using high-quality space-based photometry collected by the TESS mission coupled with low-resolution spectroscopy from the ground, we have provided a global comparison of the observations with model predictions by means of a r...

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Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: I. Asteroseismology of the GW Vir stars RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16

Abstract: Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS

Cited by 2 publications

References 60 publications

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Asteroseismic analysis of variable hot subdwarf stars observed with TESS I. The mean g-mode period spacings in hot subdwarf B stars

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