The Incidence of Stellar Mergers and Mass Gainers Among Massive Stars

Abstract

Because the majority of massive stars are born as members of close binary systems, populations of massive main-sequence stars contain stellar mergers and products of binary mass transfer. We simulate populations of massive stars accounting for all major binary evolution effects based on the most recent binary parameter statistics and extensively evaluate the effect of model uncertainties.Assuming constant star formation, we find that 8 +9 −4 % of a sample of early type stars to be the product of a merger resulting from a close binary system. In total we find that 30 +10 −15 % of massive main-sequence stars are the product of binary interaction.We show that the commonly adapted approach to minimize the effects of binaries on an observed sample by excluding systems detected as binaries through radial velocity campaigns can be counterproductive. Systems with significant radial velocity variations are mostly pre-interaction systems. Excluding them substantially enhances the relative incidence of mergers and binary products in the non radial velocity variable sample.This poses a challenge for testing single stellar evolutionary models. It also raises the question of whether certain peculiar classes of stars, such as magnetic O-stars, are the result of binary interaction and it emphasizes the need to further study the effect of binarity on the diagnostics that are used to derive the fundamental properties (star-formation history, initial mass function, mass to light ratio) of stellar populations nearby and at high redshift.

TheGaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster

“…Perhaps an alternative explanation that might be explored is whether additional mass transfer between the components of γ 2 Vel could make them appear younger or whether the system was initially a triple and the present Wolf-Rayet component is the rejuvenated remnant of a merged close binary (e.g. de Mink et al 2013), although the small separation of the current components may make the latter unlikely.…”

The VLT-FLAMES Tarantula Survey

“…Finally it should be noted that all the above analyses make the crucial assumption that single-star evolutionary models are appropriate and that there are no significant effects due to binarity (see, for example, de Mink et al 2011Mink et al , 2014.…”

“…Such a search will be complicated by the possibility that some of the single supergiants may currently be an undetected binary or may have evolved from a binary system (see, for example, de Mink et al 2014). For the former, undetected binaries will be weighted towards those having compact companions or long-period systems , which may have effectively evolved as single stars.…”

Asymmetric Accretion Flows Within a Common Envelope

“…A common envelope (CE) phase occurs when one star grows to the point that it engulfs its more compact companion within a shared envelope (Paczynski 1976). This process is not rare (Kochanek et al 2014); the majority of stars exist in binary or multiple systems (e.g., Duchêne and Kraus 2013), and interactions or mergers should mark the evolution of 30% of massive stars (Sana et al 2012;de Mink et al 2014). During the CE phase, orbital energy and angular momentum are shared with the surrounding gaseous envelope tightening the orbit of the embedded binary cores (Paczynski 1976;Iben & Livio 1993;Taam & Sandquist 2000;Ivanova et al 2013).…”

“…CE episodes, and their outcomes, are therefore critical in shaping populations of close binaries (e.g., Belczynski et al 2002;Stairs 2004;Kalogera et al 2007;Toonen & Nelemans 2013) as well as their merger products (Sana et al 2012;de Mink et al 2013de Mink et al , 2014. Substantial progress has been made by constraining the efficiencies of CE ejection compared to the change in orbital energy via the parameter a CE (Webbink 1984;Iben & Livio 1993), or the change in orbital angular momentum with the parameter g CE (Nelemans et al 2000).…”