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1. An Infrared Census of R Coronae Borealis Stars II—Spectroscopic Classifications and Implications for the Rate of Low-mass White Dwarf Mergers

   https://doi.org/10.1088/1538-3873/ad6210  

   Karambelkar, Viraj_R ; Kasliwal, Mansi_M ; Tisserand, Patrick ; Anand, Shreya ; Ashley, Michael_C_B ; Bildsten, Lars ; Clayton, Geoffrey_C ; Crawford, Courtney_C ; De, Kishalay ; Earley, Nicholas ; et al ( August 2024 , Publications of the Astronomical Society of the Pacific)

   We present results from a systematic infrared (IR) census of R Coronae Borealis (RCB) stars in the Milky Way, using data from the Palomar Gattini IR (PGIR) survey. RCB stars are dusty, erratic variable stars presumably formed from the merger of a He-core and a CO-core white dwarf (WD). PGIR is a 30 cmJ-band telescope with a 25 deg²camera that surveys 18,000 deg²of the northern sky (δ> −28°) at a cadence of 2 days. Using PGIRJ-band lightcurves for ∼60 million stars together with mid-IR colors from WISE, we selected a sample of 530 candidate RCB stars. We obtained near-IR spectra for these candidates and identified 53 RCB stars in our sample. Accounting for our selection criteria, we find that there are a total of$\approx {350}_{-100}^{+150}$RCB stars in the Milky Way. Assuming typical RCB lifetimes, this corresponds to an RCB formation rate of 0.8–5 × 10⁻³yr⁻¹, consistent with observational and theoretical estimates of the He-CO WD merger rate. We searched for quasi-periodic pulsations in the PGIR lightcurves of RCB stars and present pulsation periods for 16 RCB stars. We also examined high-cadenced TESS lightcurves for RCB and the chemically similar, but dustless hydrogen-deficient carbon (dLHdC) stars. We find that dLHdC stars show variations on timescales shorter than RCB stars, suggesting that they may have lower masses than RCB stars. Finally, we identified 3 new spectroscopically confirmed and 12 candidate Galactic DY Per type stars—believed to be colder cousins of RCB star—doubling the sample of Galactic DY Per type stars.

    

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2. Gamma Cas Stars as Be+White Dwarf Binary Systems

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

   Gies, Douglas R. ; Wang 王, Luqian 璐茜 ; 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 white dwarfs as the only viable candidates for the companions.

    

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3. Dynamical He Flashes in Double White Dwarf Binaries

   https://doi.org/10.3847/1538-4357/acce9d  

   Wong, Tin_Long_Sunny ; Bildsten, Lars ( June 2023 , The Astrophysical Journal)

   The detonation of an overlying helium layer on a 0.8–1.1M_⊙carbon–oxygen (CO) white dwarf (WD) can detonate the CO WD and create a thermonuclear supernova (SN). Many authors have recently shown that when the mass of the He layer is low (≲0.03M_⊙), the ashes from its detonation minimally impact the spectra and light curve from the CO detonation, allowing the explosion to appear remarkably similar to Type Ia SNe. These new insights motivate our investigation of dynamical He shell burning and our search for a binary scenario that stably accumulates thermally unstable He shells in the 0.01–0.08M_⊙range, thick enough to detonate, but also often thin enough for minimal impact on the observables. We first show that our improved nonadiabatic evolution of convective He shell burning in this range of shell mass leads to conditions ripe for a He detonation. We also find that a stable mass transfer scenario with a high-entropy He WD donor of mass 0.15–0.25M_⊙yields the He shell masses needed to achieve the double detonations. This scenario also predicts that the surviving He donor leaves with a spatial velocity consistent with the unusual runaway object, D6-2. We find that hot He WD donors originate in common-envelope events when a 1.3–2.0M_⊙star fills its Roche lobe at the base of the red giant branch at orbital periods of 1–10 days with the CO WD.

    

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4. On Trapped Modes in Variable White Dwarfs as Probes of the 12 C(α, γ) 16 O Reaction Rate

   https://doi.org/10.3847/1538-4357/ac7ec3  

   Chidester, Morgan T. ; Farag, Ebraheem ; Timmes, F. X. ( August 2022 , The Astrophysical Journal)

   Abstract We seek signatures of the current experimental 12 C α , γ 16 O reaction rate probability distribution function in the pulsation periods of carbon–oxygen white dwarf (WD) models. We find that adiabatic g-modes trapped by the interior carbon-rich layer offer potentially useful signatures of this reaction rate probability distribution function. Probing the carbon-rich region is relevant because it forms during the evolution of low-mass stars under radiative helium-burning conditions, mitigating the impact of convective mixing processes. We make direct quantitative connections between the pulsation periods of the identified trapped g-modes in variable WD models and the current experimental 12 C α , γ 16 O reaction rate probability distribution function. We find an average spread in relative period shifts of Δ P / P ≃ ±2% for the identified trapped g-modes over the ±3 σ uncertainty in the 12 C α , γ 16 O reaction rate probability distribution function—across the effective temperature range of observed DAV and DBV WDs and for different WD masses, helium shell masses, and hydrogen shell masses. The g-mode pulsation periods of observed WDs are typically given to six to seven significant figures of precision. This suggests that an astrophysical constraint on the 12 C α , γ 16 O reaction rate could, in principle, be extractable from the period spectrum of observed variable WDs. 

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5. Convection and spin-up during common envelope evolution: the formation of short-period double white dwarfs

   Wilson, E. C. Nordhaus ( July 2020 , Monthly notices of the Royal Astronomical Society)

   The formation channels and predicted populations of double white dwarfs (DWDs) are important because a subset will evolve to be gravitational-wave sources and/or progenitors of Type Ia supernovae. Given the observed population of short-period DWDs, we calculate the outcomes of common envelope (CE) evolution when convective effects are included. For each observed white dwarf (WD) in a DWD system, we identify all progenitor stars with an equivalent proto-WD core mass from a comprehensive suite of stellar evolution models. With the second observed WD as the companion, we calculate the conditions under which convection can accommodate the energy released as the orbit decays, including (if necessary) how much the envelope must spin-up during the CE phase. The predicted post-CE final separations closely track the observed DWD orbital parameter space, further strengthening the view that convection is a key ingredient in CE evolution. 

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