BG Gem - a Poorly-Studied Binary with a Possible Black Hole Component

TL;DR

This paper presents high-resolution spectroscopy and photometry of BG Gem, an eclipsing binary system possibly containing a black hole, revealing a rapidly rotating K-type secondary and providing new constraints on the system's properties.

Contribution

It offers new high-resolution spectral data and analysis, improving understanding of BG Gem's components and their characteristics, especially the secondary star.

Findings

Secondary star is a rapidly rotating K-type star at v sin i = 18 km/s

Spectroscopic and photometric data constrain the system's parameters

Primary component's nature remains uncertain, possibly a black hole

Abstract

BG Gem is an eclipsing binary with a 91.6-day orbital period. The more massive primary component does not seem to show absorption lines in the spectrum, while the less massive secondary is thought to be a K-type star, possibly a supergiant. These results were obtained with optical low-resolution spectroscopy and photometry. The primary was suggested to be a black hole, although with a low confidence. We present a high-resolution optical spectrum of the system along with new BVR-photometry. Analysis of the spectrum shows that the K-type star rotates rapidly at v sin i = 18 km/s compared to most evolved stars of this temperature range. We also discuss constraints on the secondary's luminosity using spectroscopic criteria and on the entire system parameters using both the spectrum and photometry.