Evolutionary Models for R Coronae Borealis Stars

Abstract

We use Modules for Experiments in Stellar Astrophysics (MESA) to construct stellar evolution models that reach a hydrogen-deficient, carbon-rich giant phase like the R Coronae Borealis (R CrB) stars. These models use opacities from OPAL and AESOPUS that cover the conditions in the cool, H-deficient, CNO-enhanced envelopes of these stars. We compare models that begin from homogeneous He stars with models constructed to reproduce the remnant structure shortly after the merger of a He and a CO white dwarf (WD). We emphasize that models originating from merger scenarios have a thermal reconfiguration phase that can last up to ≈ 1 kyr post merger, suggesting some galactic objects should be in this phase. We illustrate the important role of mass loss in setting the lifetimes of the R CrB stars. Using AGB-like mass loss prescriptions, models with CO WD primaries 0.7 M typically leave the R CrB phase with total masses ≈ 0.6 − 0.7 M , roughly independent of their total mass immediately post-merger. This implies that the descendants of the R CrB stars may have a relatively narrow range in mass and luminosity as extreme He stars and a relatively narrow range in mass as single WDs.

The Zwicky Transient Facility Census of the Local Universe. I. Systematic Search for Calcium-rich Gap Transients Reveals Three Related Spectroscopic Subclasses

“…Similar detonation conditions can also be achieved in the case of mergers of He WDs with CO WDs (e.g., Pakmor et al 2013), where a variety of detonation conditions could be achieved depending on how the He layer settles on the surface of the more massive WD (Fryer et al 2010;Dessart & Hillier 2015). The merger rate of He + CO WD binaries in the field for a Milky Way-like galaxy is ∼3×10 −3 yr −1 (Brown et al 2016) or ≈10% of the SN Ia rate, suggesting that most He + CO WD binaries end up merging to produce thermonuclear transients or stable long-lived remnants (e.g., R Coronae Borealis stars; Clayton 1996;Schwab 2019). The high rate of these mergers within Milky Way-like galaxies is however in contradiction with the preference for large-offset distributions (long delay times) of the Ca-rich gap transient population (Shen et al 2019).…”

A New Class of Roche Lobe–filling Hot Subdwarf Binaries

“…Our models predict a mass transfer rate of more than 10 −9 M e yr −1 . Due to its high total mass the system will merge in ≈30 Myr and either explode as subluminous type Ia supernova (Perets et al 2019;Zenati et al 2019) or merge and form a massive single WD (Webbink 1984;Saio 2008;Clayton 2012;Schwab 2019). ZTF J2055+4651 and ZTF J2130+4420 are the only known binaries with a Roche lobe-filling hot subdwarf donor.…”

Stellar triples on the edge; Comprehensive overview of the evolution of destabilised triples leading to stellar and binary exotica

“…These include torques from the Milky Way's tide, rare, but strong encounters with passing field stars, or a cumulative effect from many weak encounters which change the periastron of the widest orbits through eccentricity pumping (Kaib & Raymond 2014;Michaely & Perets 2016). Recently, especially the latter effect has been provoked to lead to the formation of compact binaries and mergers (Michaely & Perets 2016, 2019Hamers & Thompson 2019;Safarzadeh et al 2020;Michaely & Perets 2020).…”