The HD5980 multiple system: Masses and evolutionary status

TL;DR

This study uses spectroscopic data to determine the masses and evolutionary status of the luminous stars in the HD5980 system, suggesting they may be progenitors of gamma-ray bursts or supernovae.

Contribution

It provides the first mass estimates for the stars in HD5980 and proposes a quasi-chemically homogeneous evolution scenario for the binary system.

Findings

Masses of 61 and 66 solar masses for the two main stars.

Identification of a third star in a long, eccentric orbit.

Evidence supporting a hierarchical quadruple system.

Abstract

New spectroscopic observations of the LBV/WR multiple system HD5980 in the Small Magellanic Cloud are used to address the question of the masses and evolutionary status of the two very luminous stars in the 19.3d eclipsing binary system. Two distinct components of the N V 4944 A line are detected in emission and their radial velocity variations are used to derive masses of 61 and 66 Mo, under the assumption that binary interaction effects on this atomic transition are negligible. We propose that this binary system is the product of quasi-chemically homogeneous evolution with little or no mass transfer. Thus, both of these binary stars may be candidates for gamma-ray burst progenitors or even pair instability supernovae. Analysis of the photospheric absorption lines belonging to the third-light object in the system confirm that it consists of an O-type star in a 96.56d eccentric orbit (e=0.82) around an unseen companion. The 5:1 period ratio and high eccentricities of the two binaries suggest that they may constitute a hierarchical quadruple system.