More Like this

1. Hidden in plain sight: a double-lined white dwarf binary 26 pc away and a distant cousin

   https://doi.org/10.1093/mnras/stab439  

   Kilic, Mukremin; Bédard, A; Bergeron, P (February 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present high-resolution spectroscopy of two nearby white dwarfs with inconsistent spectroscopic and parallax distances. The first one, PG 1632+177, is a 13th magnitude white dwarf only 25.6 pc away. Previous spectroscopic observations failed to detect any radial velocity changes in this star. Here, we show that PG 1632+177 is a 2.05-d period double-lined spectroscopic binary (SB2) containing a low-mass He-core white dwarf with a more-massive, likely CO-core white dwarf companion. After L 870−2, PG 1632+177 becomes the second closest SB2 white dwarf currently known. Our second target, WD 1534+503, is also an SB2 system with an orbital period of 0.71 d. For each system, we constrain the atmospheric parameters of both components through a composite model-atmosphere analysis. We also present a new set of non-local thermodynamic equilibrium (NLTE) synthetic spectra appropriate for modelling high-resolution observations of cool white dwarfs, and show that NLTE effects in the core of the H α line increase with decreasing effective temperature. We discuss the orbital period and mass distribution of SB2 and eclipsing double white dwarfs with orbital constraints, and demonstrate that the observed population is consistent with the predicted period distribution from the binary population synthesis models. The latter predict more massive CO + CO white dwarf binaries at short (<1 d) periods, as well as binaries with several day orbital periods; such systems are still waiting to be discovered in large numbers. 

   more » « less
2. Hot subdwarfs in close binaries observed from space: I. Orbital, atmospheric, and absolute parameters, and the nature of their companions

   https://doi.org/10.1051/0004-6361/202244214  

   Schaffenroth, V.; Pelisoli, I.; Barlow, B. N.; Geier, S.; Kupfer, T. (October 2022, Astronomy & Astrophysics)

   Context. About a third of the hot subdwarfs of spectral type B (sdBs), which are mostly core-helium-burning objects on the extreme horizontal branch, are found in close binaries with cool, low-mass stellar, substellar, or white dwarf companions. They can show light variations due to different phenomena. Aims. Many hot subdwarfs now have space-based light curves with a high signal-to-noise ratio available. We used light curves from the Transiting Exoplanet Survey Satellite and the K2 space mission to look for more sdB binaries. Their light curves can be used to study the hot subdwarf primaries and their companions, and obtained orbital, atmospheric, and absolute parameters for those systems, when combined with other analysis methods. Methods. By classifying the light variations and combining these with the fit of the spectral energy distribution, the distance derived by the parallaxes obtained by Gaia , and the atmospheric parameters, mainly from the literature, we could derive the nature of the primaries and secondaries in 122 (75%) of the known sdB binaries and 82 newly found reflection effect systems. We derived absolute masses, radii, and luminosities for a total of 39 hot subdwarfs with cool, low-mass companions, as well 29 known and newly found sdBs with white dwarf companions. Results. The mass distribution of hot subdwarfs with cool, low-mass stellar and substellar companions, differs from those with white dwarf companions, implying they come from different populations. By comparing the period and minimum companion mass distributions, we find that the reflection effect systems all have M dwarf or brown dwarf companions, and that there seem to be several different populations of hot subdwarfs with white dwarf binaries – one with white dwarf minimum masses around 0.4  M ⊙ , one with longer periods and minimum companion masses up to 0.6  M ⊙ , and at the shortest period, another with white dwarf minimum masses around 0.8  M ⊙ . We also derive the first orbital period distribution for hot subdwarfs with cool, low-mass stellar or substellar systems selected from light variations instead of radial velocity variations. It shows a narrower period distribution, from 1.5 h to 35 h, compared to the distribution of hot subdwarfs with white dwarfs, which ranges from 1 h to 30 days. These period distributions can be used to constrain the previous common-envelope phase. 

   more » « less
3. 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)

   Abstract 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 destroy 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. 

   more » « less
4. The OmegaWhite survey for short-period variable stars – VII. High amplitude short-period blue variables

   https://doi.org/10.1093/mnras/stac1000  

   Ramsay, Gavin; Woudt, Patrick A.; Kupfer, Thomas; van Roestel, Jan; Paterson, Kerry; Warner, Brian; Buckley, David A. H.; Groot, Paul J.; Heber, Ulrich; Irrgang, Andreas; et al (April 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Blue Large-Amplitude Pulsators (BLAPs) are a relatively new class of blue variable stars showing periodic variations in their light curves with periods shorter than a few tens of minutes and amplitudes of more than 10 per cent. We report nine blue variable stars identified in the OmegaWhite survey conducted using ESO’s VST, which shows a periodic modulation in the range 7–37 min and an amplitude in the range 0.11–0.28 mag. We have obtained a series of followup photometric and spectroscopic observations made primarily using SALT and telescopes at SAAO. We find four stars which we identify as BLAPs, one of which was previously known. One star, OW  J0820–3301, appears to be a member of the V361 Hya class of pulsating stars and is spatially close to an extended nebula. One further star, OW J1819–2729, has characteristics similar to the sdAV pulsators. In contrast, OW J0815–3421 is a binary star containing an sdB and a white dwarf with an orbital period of 73.7 min, making it only one of six white dwarf-sdB binaries with an orbital period shorter than 80 min. Finally, high cadence photometry of four of the candidate BLAPs show features that we compare with notch-like features seen in the much longer period Cepheid pulsators. 

   more » « less
5. Expanding the Ultracompacts: Gravitational-wave-driven Mass Transfer in the Shortest-period Binaries with Accretion Disks

   https://doi.org/10.3847/1538-4357/ad9563  

   Chakraborty, Joheen; Burdge, Kevin B; Rappaport, Saul A; Munday, James; Chen, Hai-Liang; Rodríguez-Gil, Pablo; Dhillon, V S; Hughes, Scott A; Nelemans, Gijs; Kara, Erin; et al (December 2024, The Astrophysical Journal)

   Abstract We report the discovery of three ultracompact binary white dwarf systems hosting accretion disks, with orbital periods of 7.95, 8.68, and 13.15 minutes. This significantly augments the population of mass-transferring binaries at the shortest periods, and provides the first evidence that accretors in ultracompacts can be dense enough to host accretion disks even below 10 minutes (where previously only direct-impact accretors were known). In the two shortest-period systems, we measured changes in the orbital periods driven by the combined effect of gravitational-wave emission and mass transfer. We find $\dot{P}$is negative in one case, and positive in the other. This is only the second system measured with a positive $\dot{P}$, and it is the most compact binary known that has survived a period minimum. Using these systems as examples, we show how the measurement of $\dot{P}$is a powerful tool in constraining the physical properties of binaries, e.g., the mass and mass–radius relation of the donor stars. We find that the chirp masses of ultracompact binaries at these periods seem to cluster around ${\mathcal{M}}_c\sim 0.3M_{\odot }$, perhaps suggesting a common origin for these systems or a selection bias in electromagnetic discoveries. Our new systems are among the highest-amplitude known gravitational-wave sources in the millihertz regime, providing an exquisite opportunity for multimessenger study with future space-based observatories such as LISA and TianQin. We discuss how such systems provide fascinating laboratories to study the unique regime where the accretion process is mediated by gravitational waves. 

   more » « less