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1. Two new double-lined spectroscopic binary white dwarfs

   https://doi.org/10.1093/mnras/staa466  

   Kilic, Mukremin ; Bédard, A ; Bergeron, P ; Kosakowski, Alekzander ( April 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present radial velocity observations of four binary white dwarf candidates identified through their overluminosity. We identify two new double-lined spectroscopic binary systems, WD 0311–649 and WD 1606+422, and constrain their orbital parameters. WD 0311–649 is a 17.7 h period system with a mass ratio of 1.44 ± 0.06 and WD 1606+422 is a 20.1 h period system with a mass ratio of 1.33 ± 0.03. An additional object, WD 1447–190, is a 43 h period single-lined white dwarf binary, whereas WD 1418–088 does not show any significant velocity variations over time-scales ranging from minutes to decades. We present an overview of the 14 overluminous white dwarfs that were identified by Bédard et al., and find the fraction of double- and single-lined systems to be both 31 per cent. However, an additional 31 per cent of these overluminous white dwarfs do not show any significant radial velocity variations. We demonstrate that these must be in long-period binaries that may be resolved by Gaia astrometry. We also discuss the overabundance of single low-mass white dwarfs identified in the SPY survey, and suggest that some of those systems are also likely long-period binary systems of more massive white dwarfs.
2. Discovery of a Double-detonation Thermonuclear Supernova Progenitor

   https://doi.org/10.3847/2041-8213/ac48f1  

   Kupfer, Thomas ; Bauer, Evan B. ; van Roestel, Jan ; Bellm, Eric C. ; Bildsten, Lars ; Fuller, Jim ; Prince, Thomas A. ; Heber, Ulrich ; Geier, Stephan ; Green, Matthew J. ; et al ( January 2022 , The Astrophysical Journal Letters)

   We present the discovery of a new double-detonation progenitor system consisting of a hot subdwarf B (sdB) binary with a white dwarf companion with aP_orb= 76.34179(2) minutes orbital period. Spectroscopic observations are consistent with an sdB star during helium core burning residing on the extreme horizontal branch. Chimera light curves are dominated by ellipsoidal deformation of the sdB star and a weak eclipse of the companion white dwarf. Combining spectroscopic and light curve fits, we find a low-mass sdB star,M_sdB= 0.383 ± 0.028M_⊙with a massive white dwarf companion,M_WD= 0.725 ± 0.026M_⊙. From the eclipses we find a blackbody temperature for the white dwarf of 26,800 K resulting in a cooling age of ≈25 Myr whereas ourMESAmodel predicts an sdB age of ≈170 Myr. We conclude that the sdB formed first through stable mass transfer followed by a common envelope which led to the formation of the white dwarf companion ≈25 Myr ago. Using theMESAstellar evolutionary code we find that the sdB star will start mass transfer in ≈6 Myr and in ≈60 Myr the white dwarf will reach a total mass of 0.92M_⊙with a thick helium layer of 0.17M_⊙. This will lead to a detonation that will likely destroymore »the white dwarf in a peculiar thermonuclear supernova. PTF1 J2238+7430 is only the second confirmed candidate for a double-detonation thermonuclear supernova. Using both systems we estimate that at least ≈1% of white dwarf thermonuclear supernovae originate from sdB+WD binaries with thick helium layers, consistent with the small number of observed peculiar thermonuclear explosions.

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3. Predicting the LISA white dwarf binary population in the Milky Way with cosmological simulations

   https://doi.org/10.1093/mnras/stz2834  

   Lamberts, Astrid ; Blunt, Sarah ; Littenberg, Tyson B. ; Garrison-Kimmel, Shea ; Kupfer, Thomas ; Sanderson, Robyn E. ( October 2019 , Monthly Notices of the Royal Astronomical Society)

   White dwarf binaries with orbital periods below 1 h will be the most numerous sources for the space-based gravitational wave detector Laser Interferometer Space Antenna (LISA). Based on thousands of individually resolved systems, we will be able to constrain binary evolution and provide a new map of the Milky Way and its close surroundings. In this paper we predict the main properties of populations of different types of detached white dwarf binaries detected by LISA over time. For the first time, we combine a high-resolution cosmological simulation of a Milky Way-mass galaxy (taken from the FIRE project) with a binary population synthesis model for low- and intermediate-mass stars. Our Galaxy model therefore provides a cosmologically realistic star formation and metallicity history for the Galaxy and naturally produces its different components such as the thin and thick disc, the bulge, the stellar halo, and satellite galaxies and streams. Thanks to the simulation, we show how different Galactic components contribute differently to the gravitational wave signal, mostly due to their typical age and distance distributions. We find that the dominant LISA sources will be He–He double white dwarfs (DWDs) and He–CO DWDs with important contributions from the thick disc and bulge. The resulting sky map ofmore »the sources is different from previous models, with important consequences for the searches for electromagnetic counterparts and data analysis. We also emphasize that much of the science-enabling information regarding white dwarf binaries, such as the chirp mass and the sky localization, becomes increasingly rich with long observations, including an extended mission up to 8 yr.

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4. Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: II. Discovery of two new GW Vir stars: TIC 333432673 and TIC 095332541

   https://doi.org/10.1051/0004-6361/202141253  

   Uzundag, M. ; Córsico, A. H. ; Kepler, S. O. ; Althaus, L. G. ; Werner, K. ; Reindl, N. ; Bell, K. J. ; Higgins, M. ; da Rosa, G. O. ; Vučković, M. ; et al ( November 2021 , Astronomy & Astrophysics)

   Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossoming area of asteroseismology, particularly of pulsating white dwarfs and pre-white dwarfs, thus continuing the impulse of its predecessor, the Kepler mission. Aims. In this paper, we present the observations from the extended TESS mission in both 120 s short-cadence and 20 s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We identify them as two new GW Vir stars, TIC 333432673 and TIC 095332541. We apply the tools of asteroseismology with the aim of deriving their structural parameters and seismological distances. Methods. We carried out a spectroscopic analysis and a spectral fitting of TIC 333432673 and TIC 095332541. We also processed and analyzed the high-precision TESS photometric light curves of the two target stars, and derived their oscillation frequencies. We performed an asteroseismological analysis of these stars on the basis of PG 1159 evolutionary models that take into account the complete evolution of the progenitor stars. We searched for patterns of uniform period spacings in order to constrain the stellar mass of the stars. We employed the individual observed periods to search for a representative seismological model. Results. The analysis of the TESS light curves ofmore »TIC 333432673 and TIC 095332541 reveals the presence of several oscillations with periods ranging from 350 to 500 s associated to typical gravity ( g )-modes. From follow-up ground-based spectroscopy, we find that both stars have a similar effective temperature ( T eff  = 120 000 ± 10 000 K) and surface gravity (log g  = 7.5 ± 0.5), but a different He/C composition of their atmosphere. On the basis of PG 1159 evolutionary tracks, we derived a spectroscopic mass of M ⋆ = 0.58 −0.08 +0.16   M ⊙ for both stars. Our asteroseismological analysis of TIC 333432673 allowed us to find a constant period spacing compatible with a stellar mass M ⋆  ∼ 0.60 − 0.61  M ⊙ , and an asteroseismological model for this star with a stellar mass M ⋆ = 0.589 ± 0.020 M ⊙ , as well as a seismological distance of d = 459 −156 +188 pc. For this star, we find an excellent agreement between the different methods to infer the stellar mass, and also between the seismological distance and that measured with Gaia ( d Gaia = 389 −5.2 +5.6 pc). For TIC 095332541, we have found a possible period spacing that suggests a stellar mass of M ⋆  ∼ 0.55 − 0.57  M ⊙ . Unfortunately, we have not been able to find an asteroseismological model for this star. Conclusions. Using the high-quality data collected by the TESS space mission and follow-up spectroscopy, we have been able to discover and characterize two new GW Vir stars. The TESS mission is having, and will continue to have, an unprecedented impact on the area of white-dwarf asteroseismology.« less
5. Gamma Cas Stars as Be+White Dwarf Binary Systems

   https://doi.org/10.3847/2041-8213/acaaa1  

   Gies, Douglas R. ; Wang, Luqian ; Klement, Robert ( December 2022 , The Astrophysical Journal Letters)

   The origin of the bright and hard X-ray emission flux among theγCas subgroup of B-emission line (Be) stars may be caused by gas accretion onto an orbiting white dwarf (WD) companion. Such Be+WD binaries are the predicted outcome of a second stage of mass transfer from a helium star mass donor to a rapidly rotating mass gainer star. The stripped donor stars become small and hot white dwarfs that are extremely faint compared to their Be star companions. Here we discuss model predictions about the physical and orbital properties of Be+WD binaries, and we show that current observational results onγCas systems are consistent with the expected large binary frequency, companion faintness and small mass, and relatively high mass range of the Be star hosts. We determine that the companions are probably not stripped helium stars (hot subdwarf sdO stars), because these are bright enough to detect in ultraviolet spectroscopy, yet their spectroscopic signatures are not observed in studies ofγCas binaries. Interferometry of relatively nearby systems provides the means to detect very faint companions including hot subdwarf and cooler main-sequence stars. Preliminary observations of fiveγCas binaries with the CHARA Array interferometer show no evidence of the companion flux, leaving whitemore »dwarfs as the only viable candidates for the companions.

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