Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud

TL;DR

This study uses Gaia DR2 data to analyze the kinematics of OB stars in the Small Magellanic Cloud, revealing distinct motions between regions and identifying the origins of runaway stars.

Contribution

It provides the first detailed kinematic analysis of runaway OB stars in the SMC, distinguishing between dynamical ejections and supernova kicks as their origins.

Findings

The SMC Wing moves away from the Bar at ~64 km/s.

About half of the OB stars are unbound runaways.

Dynamical ejections dominate the fast runaway population.

Abstract

We use GAIA DR2 proper motions of the RIOTS4 field OB stars in the Small Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar of (v_RA, v_Dec) = (62 +/-7, -18+/-5) km/s and relative radial velocity +4.5 +/- 5.0 km/s. This unambiguously demonstrates that these two regions are kinematically distinct: the Wing is moving away from the Bar, and towards the Large Magellanic Cloud with a 3-D velocity of 64 +/- 10 km/s. This is consistent with models for a recent, direct collision between the Clouds. We present transverse velocity distributions for our field OB stars, confirming that unbound runaways comprise on the order of half our sample, possibly more. Using eclipsing binaries and double-lined spectroscopic binaries as tracers of dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers of runaways accelerated by supernova kicks, we find significant contributions from both populations. The data suggest that HMXBs have lower velocity dispersion relative to dynamically ejected binaries, consistent with the former corresponding to less energetic supernova kicks that failed to unbind the components. Evidence suggests that our fast runaways are dominated by dynamical, rather than supernova, ejections.