Post-merger evolution of carbon–oxygen + helium white dwarf binaries and the origin of R Coronae Borealis and extreme helium stars

Abstract

Orbital decay by gravitational-wave radiation will cause some close-binary white dwarfs (WDs) to merge within a Hubble time. The results from previous hydrodynamical WD-merger simulations have been used to guide calculations of the post-merger evolution of carbon-oxygen + helium (CO+He) WD binaries. Our models include the formation of a hot corona in addition to a Keplerian disk. We introduce a "destroyeddisk" model to simulate the effect of direct disk ingestion into the expanding envelope. These calculations indicate significant lifetimes in the domain of the rare R Coronae Borealis (RCB) stars, before a fast evolution through the domain of the hotter extreme helium (EHe) stars. Surface chemistries of the resulting giants are in partial agreement with the observed abundances of RCB and EHe stars. The production of 3 He, 18 O and 19 F are discussed. Evolutionary timescales combined with binary white-dwarf merger rates from binary-star population synthesis are consistent with present-day numbers of RCBs and EHes, provided that the majority come from relatively recent (< 2Gyr) star formation. However, most RCBs should be produced by CO-WD + low-mass He-WD mergers, with the He-WD having a mass in the range 0.20 − 0.35 M ⊙ . Whilst, previously, a high He-WD mass ( 0.40 M ⊙ ) was required to match the carbon-rich abundances of RCB stars, the "destroyed-disk" model yields a high-carbon product with He-WD mass 0.30 M ⊙ , in better agreement with population synthesis results.

HubbleSpace Telescope ultraviolet spectroscopy of the hottest known helium-rich pre-white dwarf KPD 0005+5106

“…A comparison of the results for KPD 0005+5106 confirms earlier suggestions Wassermann et al 2010) that KPD 0005+5106 is indeed an evolved RCB star. The calculations of Zhang et al (2014) show that the observed C abundance in RCB stars can only be produced by CO WDs in a very narrow mass range (0.55 ± 0.02 M ) merging with a He-WD with a mass in the range 0.3−0.45 M . The total mass should therefore be in the range 0.85−1.1 M , which is similar to observed masses of RCB stars, deduced from the luminosity and evolution calculations (0.8−1.0 M ).…”

“…However, based on arguments from binary population synthesis, only 1% of the RCB stars may form from double He-WDs, and the majority forms from a He-WD + CO-WD merger (see Zhang et al 2014, and references therein). In the cited work, it is demonstrated that post-merger evolution calculations predict surface abundances that can partially explain the observations in RCB stars, in particular for the elements studied in KPD 0005+5106 in the present paper.…”

“…In the context of the O(He) stars it is also worth emphasizing the finding by Zhang et al (2014) that CO+He WD mergers may also result in stars that are not carbon rich. Thus one could conceive that many more of the O(He) stars are the result of such mergers and not of He+He WD mergers, although their masses appear too low.…”

Evolution Models of Helium White Dwarf–Main-sequence Star Merger Remnants

“…We use a rapid binary evolution code (BSE, Hurley et al 2000Hurley et al , 2002 to evolve these binary systems for 13 Gyr and record the properties of HeWD+MS and HeWD+HeWD mergers. The parameters in the rapid evolution code in this work are chosen to be the same as those previously used to model the rate of double WD mergers in the Galaxy (Han 1998;Zhang et al 2014). Our results are processed to find the properties of a model population with an age of 13 Gyr and a constant star formation rate history of 5 M ⊙ yr −1 , intended to represent the Galaxy (Yungelson & Livio 1998).…”

Radial velocity variable, hot post-AGB stars from the MUCHFUSS project

“…If the system contains two sufficiently massive white dwarfs or a subdwarf star and a sufficiently massive white dwarf, this evolutionary path can lead to a type Ia supernova (Webbink 1984), the so-called double degenerate scenario. If the total mass of the two merging stars does not exceed the Chandrasekhar limit, He-dominated stars, i.e., R Coronae Borealis stars (RCB), extreme helium (EHe) stars, He-rich subdwarf O (He-sdO) stars or O(He) stars may be produced (Webbink 1984;Iben & Tutukov 1984;Justham et al 2011;Zhang & Jeffery 2012b,a;Zhang et al 2014;Reindl et al 2014b). The existence of planetary nebulae (PNe) around every other O(He) star and the [WN] type central stars, however, put in question the possibility that all He-dominated stars have a merger origin.…”