The Origin of B-Type Runaway Stars: Non-LTE Abundances as a Diagnostic

TL;DR

This study uses non-LTE atmospheric modeling to analyze the surface abundances of B-type runaway stars, testing whether their compositions support the binary supernova or cluster ejection scenarios.

Contribution

It applies advanced non-LTE abundance analysis to runaways, revealing no significant abundance anomalies and supporting the idea that their compositions reflect Galactic abundance gradients.

Findings

Runaway stars show no significant abundance anomalies.

Surface compositions reflect Galactic abundance gradients.

Abundance ratios are uniform across the sample.

Abstract

There are two accepted mechanisms to explain the origin of runaway OB-type stars: the Binary Supernova Scenario (BSS), and the Cluster Ejection Scenario (CES). In the former, a supernova explosion within a close binary ejects the secondary star, while in the latter close multi-body interactions in a dense cluster cause one or more of the stars to be ejected from the region at high velocity. Both mechanisms have the potential to affect the surface composition of the runaway star. TLUSTY non-LTE model atmosphere calculations have been used to determine atmospheric parameters and carbon, nitrogen, magnesium and silicon abundances for a sample of B-type runaways. These same analytical tools were used by Hunter et al. (2009) for their analysis of 50 B-type open cluster Galactic stars (i.e. non-runaways). Effective temperatures were deduced using the silicon-ionization balance technique, surface gravities from Balmer line profiles and microturbulent velocities derived using the Si spectrum. The runaways show no obvious abundance anomalies when compared with stars in the open clusters. The runaways do show a spread in composition which almost certainly reflects the Galactic abundance gradient and a range in the birthplaces of the runaways in the Galactic disk. Since the observed Galactic abundance gradients of C, N, Mg and Si are of a similar magnitude, the abundance ratios (e.g., N/Mg) are, as obtained, essentially uniform across the sample.