Eta Geminorum: An Eclipsing Semiregular Variable Star Orbited by a Companion Surrounded by an Extended Disc

TL;DR

This study presents 11 years of spectroscopic data on eta Gem, confirming its eclipsing binary nature with a long period and proposing a large circum-companion disk as the cause of observed dimmings.

Contribution

The paper provides an improved orbital solution and evidence for a circum-companion disk causing eclipses, expanding understanding of long-period eclipsing semiregular variables.

Findings

Confirmed eta Gem as an eclipsing binary with a 2979-day period.

Detected deep, long-duration eclipses inconsistent with a stellar companion.

Suggested the presence of a large, extended disk around the companion.

Abstract

We report 11 yr of spectroscopic monitoring of the M-type asymptotic giant branch star eta Gem, a semiregular variable and a known spectroscopic binary with a period of 8.2 yr. We combine our radial velocities with others from the literature to provide an improved spectroscopic orbital solution giving a period of 2979 days, which we then use to predict past times of eclipse. We examine archival photometry from amateur variable star observers, and other sources, and find many instances of dimmings that occurred at the right time. This confirms previous indications that the system is eclipsing, and it now ranks among those with the longest known periods. No secondary eclipses are seen. The $\sim$0.4 mag eclipses lasting about 5 months are much too deep to be produced by a stellar companion. We propose instead that the companion is surrounded by a large disk that is at least 1.5 au in diameter, but is likely larger. We predict the center of the next eclipse will occur on New Year's day, 2029.