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$\pm$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}$% for control-sample O stars. Most (76%) of SBNe central stars are not runaways. Our analysis of alignment (${\Delta }_{\mathrm{PA}}$) between the nebular morphological and $v_{2D}$ kinematic position angles reveals two populations: a highly aligned (${\sigma }_{PA}$=25${}^{\circ }$) population that includes stars with the largest $v_{2D}$ (31% of the sample) and a random (non-aligned) population (69% of the sample). SBNe that lie within or near HII 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 LSR motion, yielding [U${}_{\odot }$, V${}_{\odot }$, W${}_{\odot }$]= [5.5, 7.5, 4.5] km s${}^{-1}$.

Accepted for publication in AAS journals

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