Partial-envelope stripping and nuclear-timescale mass transfer from evolved supergiants at low metallicity

Abstract

Stable mass transfer from a massive post-main sequence (post-MS) donor is thought to be a short-lived event of thermal-timescale mass transfer (∼ 10 −3 M yr −1 ) which within 10 4 yr strips the donor star of nearly its entire H-rich envelope, producing a hot, compact helium star. This long-standing picture is based on stellar models with rapidly-expanding Hertzprung gap (HG) donor stars. Motivated by a finding that in low-metallicity binaries, post-MS mass transfer may instead be initiated by donors already at the corehelium burning (CHeB) stage, we use the MESA stellar-evolution code to compute grids of detailed massive binary models at three metallicities: those of the Sun, the Large Magellanic Cloud (LMC, Z Fe;LMC /Z Fe; ≈ 0.36), and the Small Magellanic Cloud (SMC, Z Fe;SMC /Z Fe; ≈ 0.2). Our grids span a wide range in orbital periods (∼ 3 to 5000 days) and initial primary masses (10 M to 36-53 M , depending on metallicity). We find that metallicity strongly influences the course and outcome of mass-transfer evolution. We identify two novel types of post-MS mass transfer: (a) mass exchange on the long nuclear timescale (∆T MT 10 5 yr, Ṁ ∼ 10 −5 M yr −1 ) that continues until the end of the CHeB phase, and (b) rapid mass transfer leading to detached binaries with mass-losers that are only partially stripped of their envelopes. At LMC and SMC compositions, the majority of binary models with donor masses ≥ 17 M follow one of these two types of evolution. In neither (a) or (b) does the donor become a fully stripped helium star by the end of CHeB. Boundaries between the different types of post-MS mass transfer evolution are associated with the degree of rapid post-MS expansion of massive stars and, for a given metallicity, are sensitive to the assumptions about internal mixing. At low metallicity, due to partial envelope stripping, we predict fewer hot fully stripped stars formed through binary interactions as well as higher compactness of the pre-supernova core structures of mass losers. Nuclear-timescale post-MS mass transfer suggests a strong preference for metal-poor host galaxies of ultra-luminous X-ray sources with black-hole (BH) accretors and massive donors, some of which might be the immediate progenitors of binary BH mergers. It also implies a population of interacting binaries with blue and yellow supergiant donors. Partially-stripped stars could potentially explain the puzzling nitrogen-enriched slowly-rotating (super)giants in the LMC.

Luminous Fast Blue Optical Transients and Type Ibn/Icn SNe from Wolf-Rayet/Black Hole Mergers

“…The present paper focuses on a second promising class of models, which generally begins with common envelope (CE) interaction following unstable mass-transfer from a giant star onto a BH or NS binary companion (Soker et al 2019;Uno & Maeda 2020;Schrøder et al 2020; see also Chevalier 2012). After the BH/NS plunges into the envelope of the giant and spirals towards its center, the gravitational energy released is generally expected to unbind the stellar envelope (e.g., MacLeod et al 2018;Law-Smith et al 2020; however note that there exist ways to tighten the binary through stable mass-transfer without a CE event, which we shall return to below; e.g., Pavlovskii et al 2017;Klencki et al 2021;Marchant et al 2021;van Son et al 2021). If the envelope cannot be removed (i.e., the CE is a "failure"), past works envision that the BH/NS will enter the He-rich core of the giant, triggering a high accretion rate onto the BH/NS and powering a "merger-driven" explosion (e.g., Fryer & Woosley 1998;Chevalier 2012;Soker et al 2019;Schrøder et al 2020).…”

“…In some systems the CE may be replaced or preceded by a phase of thermal-timescale stable masstransfer (e.g., Pavlovskii et al 2017;van den Heuvel 2017;Klencki et al 2021;Marchant et al 2021;van Son et al 2021;Bavera et al 2021;Gallegos-Garcia et al 2021), which also has the effect of at least partially tightening the binary on a relatively short timescale (and may leave a relic gaseous disk due to RLOF spillover from the binary during the mass-transfer process; see below).…”

Luminous Fast Blue Optical Transients and Type Ibn/Icn SNe from Wolf-Rayet/Black Hole Mergers

“…The present paper focuses on a second promising class of models, which generally begins with common envelope (CE) interaction following unstable mass transfer from a giant star onto a BH or NS binary companion (Soker et al 2019;Schrøder et al 2020;Uno & Maeda 2020; see also Chevalier 2012). After the BH/NS plunges into the envelope of the giant and spirals toward its center, the gravitational energy released is generally expected to unbind the stellar envelope (e.g., MacLeod et al 2018;Law-Smith et al 2020; however, note that there exist ways to tighten the binary through stable mass transfer without a CE event; e.g., Pavlovskii et al 2017;van den Heuvel 2017;Neijssel et al 2019;Klencki et al 2021;Marchant et al 2021;van Son et al 2022). If the envelope cannot be removed (i.e., the CE is a "failure"), past works envision that the BH/NS will enter the He-rich core of the giant, triggering a high accretion rate onto the BH/NS and powering a "merger-driven" explosion (e.g., Fryer & Woosley 1998;Chevalier 2012;Soker et al 2019;Schrøder et al 2020).…”

Stripped-Envelope Stars in Different Metallicity Environments I. Evolutionary Phases, Classification and Populations

“…Binary evolutionary calculations with different initial metallicities (Klencki et al 2021) suggest that this layer is efficiently removed in systems with Solar metallicity and above, suggesting that binary stripping is more efficient as metallicity increases. Therefore, we are confident that our models reproduce the properties of stars that have been stripped by Case A or Case B mass transfer, as well as successful CE ejections that occur soon after the ignition of hydrogen shell burning at the metallicities employed in our grids.…”