On the kinematics of a runaway Be star population

Abstract

We explore the hypothesis that B type emission-line stars (Be stars) have their origin in mass-transfer binaries by measuring the fraction of runaway Be stars. We assemble the largest-to-date catalogue of 632 Be stars with 6D kinematics, exploiting the precise astrometry of the Tycho-Gaia Astrometric Solution (TGAS) from the first Gaia Data Release. Using binary stellar evolution simulations, we make predictions for the runaway and equatorial rotation velocities of a runaway Be star population. Accounting for observational biases, we calculate that if all classical Be stars originated through mass transfer in binaries, then 17.5% of the Be stars in our catalogue should be runaways. The remaining 82.5% should be in binaries with subdwarfs, white dwarfs or neutron stars, because those systems either remained bound post-supernova or avoided the supernova entirely. Using a Bayesian methodology, we compare the hypothesis that each Be star in our catalogue is a runaway to the null hypothesis that it is a member of the Milky Way disc. We find that 13.1 +2.6 −2.4 % of the Be stars in our catalogue are runaways, and identify a subset of 40 high-probability runaways. We argue that deficiencies in our understanding of binary stellar evolution, as well as the degeneracy between velocity dispersion and number of runaway stars, can explain the slightly lower runaway fraction. We thus conclude that all Be stars could be explained by an origin in mass-transfer binaries. This conclusion is testable with the second Gaia data release (DR2).

The VLT-FLAMES Tarantula Survey

“…We find no evidence of runaway RSGs, which have RVs offset by more than 30 km s −1 compared to the surrounding population (Blaauw 1961;Eldridge et al 2011;Boubert & Evans 2018;Renzo et al 2018). Based on the binary supernova scenario, Renzo et al (2018) found that the number of runaway stars is dwarfed by an order of magnitude by the number of slower moving -nevertheless unbound -walkaway stars.…”

Massive runaway and walkaway stars

“…Blaauw (1961) introduced the term "runaway stars" for those in the fast tail of the velocity distribution for a given spectral type. The typical threshold adopted to define the tail of this distribution for O and B-type stars is v 30 km s −1 (e.g., Blaauw 1956;Gies & Bolton 1986;De Donder et al 1997;Hoogerwerf et al 2000Hoogerwerf et al , 2001Dray et al 2005;Eldridge et al 2011), although sometimes other values have been considered (e.g., 40 km s −1 in Blaauw 1961;de Wit et al 2005;Boubert & Evans 2018). However, as we will argue based on simulations presented in this work, the majority of unbound companions are expected to exhibit velocities well below these thresholds.…”

“…Various earlier studies have discussed the evolution and interaction of populations of massive binary systems. Examples include, but are not limited to, Vanbeveren (1982), De Donder et al (1997, Fryer & Kalogera (1997, 2001, Fryer et al (2012), Belczynski et al (2012), Repetto et al (2012), Fragos et al (2013), Grudzinska et al (2015), Boubert & Evans (2018). Most of these studies focus on the minor fraction of systems that remain bound and are the progenitors of X-ray binaries and/or gravitational wave sources.…”

“…Their velocities can either be detected as proper motions (i.e., their motion in the plane of the sky measured directly from the displacement of the star) or as radial velocities (i.e., their motion perpendicular to the plane of the sky, measured from the Doppler shift of the spectral lines). Large spatial velocities have been inferred for a significant sub-population of young massive stars (e.g., Blaauw 1961;Cruz-González et al 1974;Gies & Bolton 1986;Gies 1987;Hoogerwerf et al 2000Hoogerwerf et al , 2001Tetzlaff et al 2011;Boubert & Evans 2018). Blaauw (1961) introduced the term "runaway stars" for those in the fast tail of the velocity distribution for a given spectral type.…”

Short-term variability and mass loss in Be stars

“…In observations of open star clusters, it has been found that Be stars already have high angular momentum when they reach the zero-age main sequence (Martayan et al 2007, and references therein). However, it has also been variously suggested (Pols et al 1991;Harmanec et al 2002;Boubert & Evans 2018, and references therein) that binarity plays a formative role in the Be phenomenon. In fact, Be stars with subluminous companions (Wang et al 2018) and Be X-ray binaries with compact companions (Reig 2011) prove that there are Be stars that must have gone through a phase of strong interactions with these companion stars.…”