Phase-Resolved Spectroscopic Analysis of Candidate Extreme-Mass-Ratio Contact Binary Stars

Contact binary stars are systems in which two stars orbit each other so closely that they share a common atmospheric envelope. Approximately once per decade in our galaxy, one of these contact binary stars merges and explodes. These mergers are thought to be most likely for contact binary star systems with extreme mass ratios, which trigger tidal instability. Therefore, identifying systems with extreme mass ratios is one way to predict future mergers. However, photometric data alone are often insufficient to provide a precise measurement of the mass ratio. Light from a tertiary star — either physically related to or along the line of sight of — a binary can produce a light curve that mimics an extreme-mass-ratio system. This possibility of counterfeits, along with the fact that many of the reported mass ratios are lower than the commonly quoted lower limit of q = M2/M1 ~ 0.1 requires that the authenticity of these extreme mass ratio systems be examined.

We present medium-resolution echelle spectroscopy of 23 close binary systems, considered candidate extreme-q systems on the basis of their light curves, to measure their mass ratios. Using orbital inclinations measured from the best-fitting model light curves, we also measure the total masses of the systems. This enables us to test current theoretical predictions that lower limits on mass ratio are a function of orbital period and total mass (theory presented by Molnar et al. at this meeting).

At least three of the targets turn out to be detached binaries. At least two others exhibit strong third light, indicating that their mass ratios are not as extreme as previous photometric studies suggested. One of these systems is V1187 Her, which was thought to be the most extreme mass ratio contact binary star known. However, our primary result is that most of the systems do indeed have extreme mass ratios. We present their spectroscopic mass ratios, total masses, and third light contributions. We also discuss whether these extreme mass ratios pose challenges to theoretical limits.