Near-Contact Binaries on the Path to Contact Binaries

Abstract: A comprehensive evolution study was conducted on a carefully selected sample of near-contact binaries (NCBs) with more massive components filling the Roche lobes, utilizing the best-known basic parameters and indications of ongoing mass transfer. The results and discussion highlight that several NCBs with total masses exceeding 2 solar masses survive only a short time after mass exchange as contact binaries (CBs), with both components eventually merging to form a rapidly rotating giant, akin to FK~Com. Less massive NCBs transition into typical CBs and remain in this phase for up to 2 Gyr before ending their binary evolution as systems with extremely low mass ratios, susceptible to Darwin instability. However, this does not fully explain the existence of low-mass CBs with masses in the range of 1-1.5 solar masses. It is noted that there exists a population of low-mass binaries, nearly filling their Roche lobes. Their overall properties suggest that they could be progenitors of low-mass CBs.