The H-alpha Emission Line Variations of HR 6819

TL;DR

This study reinterprets HR 6819 as a binary system with a Be star and a low-mass remnant, based on H-alpha emission line analysis, challenging previous black hole hypothesis.

Contribution

The paper presents an alternative model for HR 6819, showing it is a binary with a Be star and a stripped remnant, based on H-alpha emission line variations.

Findings

H-alpha emission line shows reflex motion consistent with a binary companion.

Emission line profile varies with orbital phase.

HR 6819 is likely a binary, not a triple system with a black hole.

Abstract

The star system HR 6819 was recently proposed by Rivinius et al. as the site of the nearest example of a stellar mass black hole. Their spectra show evidence of two components: a B3 III star in a 40 d orbit and a stationary B-emission line star. Based upon the orbital mass function and the lack of evidence of a spectral component with reflex orbital motion, Rivinius et al. suggested that HR 6819 is a triple system consisting of an inner B3 III star plus black hole binary orbited by a distant Be star. Here we present an alternative model based upon an examination of the H-alpha emission line in their spectra. We show that the emission line displays the small reflex motion expected for the companion in the inner orbit and shows variations in profile shape related to orbital phase. This indicates that HR 6819 is a binary system consisting of a massive Be star and a low mass companion that is the stripped down remnant of a former mass donor star in a mass transfer binary.