Kinematics of the Central Stars Powering Bowshock Nebulae and the Large Multiplicity Fraction of Runaway OB Stars

Abstract

OB stars powering stellar bowshock nebulae (SBNe) have been presumed to have large peculiar velocities. We measured peculiar velocities of SBN central stars to assess their kinematics relative to the general O-star population using Gaia EDR3 data for 267 SBN central stars and a sample of 455 Galactic O stars to derive projected velocities v 2D. For a subset of each sample, we obtained new optical spectroscopy to measure radial velocities and identify multiple-star systems. We find a minimum multiplicity fraction of 36% ± 6% among SBN central stars, consistent with >28% among runaway Galactic O stars. The large multiplicity fraction among runaways implicates very efficient dynamical ejection rather than binary-supernova origins. The median v 2D of SBN central stars is v 2D = 14.6 km s−1, larger than the median v 2D = 11.4 km s−1 for non-bowshock O stars. Central stars of SBNe have a runaway (v 2D > 25 km s−1) fraction of 24 − 7 + 9 %, consistent with the 22 − 3 + 3 % for control-sample O stars. Most (76%) SBNe central stars are not runaways. Our analysis of alignment (ΔPA) between the nebular morphological and v 2D kinematic position angles reveals two populations: a highly aligned (σ PA = 25°) population that includes stars with the largest v 2D (31% of the sample) and a random (nonaligned) population (69%). SBNe that lie within or near H ii regions comprise a larger fraction of this latter component than SBNe in isolated environments, implicating localized ISM flows as a factor shaping their orientations and morphologies. We outline a new conceptual approach to computing the solar local standard of rest motion, yielding [U ⊙, V ⊙, W ⊙] = [5.5, 7.5,4.5] km s−1.

A Catalog of Early-type Runaway Stars from LAMOST DR8

“…Our understanding of runaway stars has been improved significantly in recent years thanks to the extensive collection and the unprecedented accuracy of astrometric observations released by the Gaia mission. Boubert & Evans (2018) Many of the above studies lack RV measurements (Tetzlaff et al 2011;Maíz Apellániz et al 2018;Schoettler et al 2020;Kobulnicky & Chick 2022), restricting the identification of runaway stars solely to peculiar tangential velocity analysis. Alternatively, some obtain RV information can be obtained through crossmatching from different literature sources and SIMBAD (Boubert & Evans 2018;Bhat et al 2022).…”

“…The limitation caused by inconsistency in RV measurements may introduce errors into the analysis. Therefore, spectroscopic studies with homogeneous RV measurements are necessary (Bhat et al 2022;Kobulnicky & Chick 2022). The recent release of a large collection of spectroscopic observations from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) has offered us an opportunity to search for new OB massive runaway stars using this extensive and homogeneous database (Cui et al 2012;Deng et al 2012;Zhao et al 2012;Liu et al 2020).…”