Near-Contact Binaries on the Path to Contact Binaries
K. Stȩpień
TL;DR
The paper tackles the evolution of near-contact binaries (NCBs) and their connection to contact binaries (CBs, W UMa-type) by analyzing a carefully selected SD1-type sample with measured period changes, deriving mass-transfer rates, and reconstructing ZAMS progenitors via a three-phase evolutionary model that includes magnetic braking and angular-momentum loss. It finds that NCBs with total masses above about $2\,M_\odot$ tend to merge after mass exchange into FK Com-type giants or become short-lived massive CBs, while lower-mass NCBs survive as long-lived W UMa binaries for up to about $2\,\text{Gyr}$; a population of low-mass NCBs is needed to explain observed low-mass CBs. The results challenge the thermal-relaxation-oscillation (TRO) framework for many SD1 systems and highlight the role of initial mass ratio and AML in determining fate, guiding observational searches for elusive low-mass NCBs. Overall, the work clarifies how CBs may form from NCBs and where to look for the progenitors of low-mass CBs, contributing to a coherent picture of close-binary evolution.
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.