An Optical Study of BG Geminorum: An Ellipsoidal Binary with an Unseen Primar Star

Details An Optical Study of BG Geminorum: An Ellipsoidal Binary with an Unseen Primar Star An Optical Study of BG Geminorum: An Ellipsoidal Binary with an Unseen Primar Star

1999-11-10

arxiv.org/abs/astro-ph/9911179v1

Astronomy and Astrophysics

Astrophysics

27 pages, includes 8 figures and 5 tables, accepted to AJ

Scientific paper

10.1086/301230

We describe optical photometric and spectroscopic observations of the bright variable BG Geminorum. Optical photometry shows a pronounced ellipsoidal variation of the K0 I secondary, with amplitudes of ~0.5 mag at VRI and a period of 91.645 days. A deep primary eclipse is visible for wavelengths < 4400A; a shallower secondary eclipse is present at longer wavelengths. Eclipse timings and the radial velocity curve of the K0 secondary star indicate an interacting binary where a lobe-filling secondary, M_2 ~ 0.5 Msun, transfers material into a extended disk around a massive primary, M_1 ~ 4.5 Msun. The primary star is either an early B-type star or a black hole. If it did contain a black hole, BG Gem would be the longest period black hole binary known by a factor of 10, as well as the only eclipsing black hole binary system.

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