Detection of the lowest mass ratio contact binary in the universe: TYC 3801-1529-1

TL;DR

This study reports the discovery and analysis of TYC 3801-1529-1, the contact binary with the lowest known mass ratio, providing insights into its structure, evolution, and stability, and challenging existing theoretical models.

Contribution

It is the first detailed analysis of TYC 3801-1529-1, the lowest mass-ratio contact binary, combining multi-band light curves, radial velocity data, and eclipse timing analysis.

Findings

Mass ratio q=0.0356, the lowest known in contact binaries.

Secular increase in orbital period at 7.96e-7 days per year.

System is dynamically unstable and may merge in the future.

Abstract

This paper presents the first analysis of the contact binary TYC 3801-1529-1. We observed four sets of multiple bands complete light curves and one set of radial velocity curve of the primary component. Based on a simultaneous investigation of our observed and TESS light curves and the radial velocity curve, we found that TYC 3801-1529-1 is an extremely low-mass-ratio, medium contact binary with $q=0.0356$, with the contribution of the third light at a level of about 10\%. Its mass ratio is lower than V1187 Her, making TYC 3801-1529-1 the lowest mass-ratio contact binary ever found in the universe. The light curves observed in 2022 are asymmetric, which is aptly explained by a hot spot on the primary component. A 16-year eclipse timings analysis indicates a secular increase orbital period with a rate of dp/dt$=7.96(\pm0.35)\times10^{-7}$ d yr$^{-1}$. We studied the stability of this target and identified that not only the value of $J_{spin}/J_{orb}$, but also the mass ratio surpass the unstable boundary. Hence, TYC 3801-1529-1 presents a challenge to theoretical research and ought to be considered a progenitor of a contact binary merger.