Angular momentum and mass evolution of contact binaries

TL;DR

This paper explores the evolution of contact binary star systems, proposing a new scenario based on observational data and theoretical models that explain their transformation and ultimate coalescence.

Contribution

It introduces an alternative evolutionary scenario for contact binaries grounded in observational correlations and theoretical angular momentum loss models.

Findings

Good agreement between theoretical tracks and observed data.

Evolutionary paths lead to coalescence into a single star.

Different mass/energy transfer rates produce distinct evolutionary outcomes.

Abstract

Various scenarios of contact binary evolution have been proposed in the past, giving hints of (sometimes contradictory) evolutionary sequence connecting A-type and W-type systems. As the components of close detached binaries approach each other and contact binaries are formed, following evolutionary paths transform them into systems of two categories: A-type and W-type. The systems evolve in a similar way but under slightly different circumstances. The mass/energy transfer rate is different, leading to quite different evolutionary results. An alternative scenario of evolution in contact is presented and discussed, based on the observational data of over a hundred low-temperature contact binaries. It results from the observed correlations among contact binary physical and orbital parameters. Theoretical tracks are computed assuming angular momentum loss from a system via stellar wind, accompanied by mass transfer from an advanced evolutionary secondary to the main sequence primary. Good agreement is seen between the tracks and the observed graphs. Independently of details of the evolution in contact and a relation between A-type and W-type systems, the ultimate fate of contact binaries involves the coalescence of both components into a single fast rotating star.