Dynamical vs Supernova Acceleration of OB Stars in the Small Magellanic Cloud

TL;DR

This study analyzes the origins of OB star runaways in the Small Magellanic Cloud, finding that dynamical ejection is the dominant acceleration mechanism over supernova explosions, with implications for stellar evolution models.

Contribution

It provides a comparative analysis of dynamical versus supernova acceleration mechanisms using Gaia data, highlighting the predominance of dynamical ejections in a low-metallicity galaxy.

Findings

Dynamical ejection dominates by a factor of 2-3.

Two-step acceleration involving SN kicks is significant.

Updated Gaia DR3 data refines previous results.

Abstract

We use the RIOTS4 sample of SMC field OB stars to determine the origin of massive runaways in this low-metallicity galaxy using Gaia proper motions, together with stellar masses obtained from RIOTS4 data. These data allow us to estimate the relative contributions of stars accelerated by the dynamical ejection vs binary supernova mechanisms, since dynamical ejection favors faster, more massive runaways, while SN ejection favors the opposite trend. In addition, we use the frequencies of classical OBe stars, high-mass X-ray binaries, and non-compact binaries to discriminate between the mechanisms. Our results show that the dynamical mechanism dominates by a factor of 2 - 3. This also implies a significant contribution from two-step acceleration that occurs when dynamically ejected binaries are followed by SN kicks. We update our published quantitative results from Gaia DR2 proper motions with new data from DR3.