A Sample of OB Stars That Formed in the Field

TL;DR

This study identifies 14 OB stars in the Small Magellanic Cloud that formed in extremely sparse, isolated conditions, challenging existing theories linking cluster mass to maximum stellar mass.

Contribution

It provides evidence that OB stars can form in isolation in sparse environments, contradicting traditional cluster-based star formation models.

Findings

14 OB stars in the SMC formed in isolation

Stars show no signs of being runaway stars

Some stars have low-mass companions, supporting in-situ formation

Abstract

We present a sample of 14 OB stars in the Small Magellanic Cloud that meet strong criteria for having formed under extremely sparse star-forming conditions in the field. These stars are a minimum of 28 pc in projection from other OB stars, and they are centered within symmetric, round HII regions. They show no evidence of bow shocks, implying that the targets are not transverse runaway stars. Their radial velocities relative to local HI also indicate that they are not line-of-sight runaway stars. A friends-of-friends analysis shows that 9 of the objects present a few low-mass companion stars, with typical mass ratios for the two highest-mass stars of around 0.1. This further substantiates that these OB stars formed in place, and that they can and do form in extremely sparse conditions. This poses strong constraints on theories of star formation and challenges proposed relations between cluster mass and maximum stellar mass.