On the origin of W UMa type Contact binaries - a new method for computation of initial masses

TL;DR

This paper introduces a new method to estimate initial masses of W UMa contact binaries, revealing different initial mass ranges for subtypes and evidence of non-conservative mass transfer.

Contribution

It presents a novel approach to determine initial stellar masses in W UMa binaries and distinguishes subtype origins based on initial secondary masses.

Findings

Initial secondary masses range from 1.3 to 2.6 solar masses.

A- and W-subtypes have different initial secondary mass ranges.

Mass transfer is predominantly non-conservative, with only 34% transferred to the primary.

Abstract

W UMa type binaries have two defining characteristics. These are (i) the effective temperatures of both components are very similar, and (ii) the secondary (currently less massive) component is overluminous for its current mass. We consider the latter to be an indication of its mass before the mass-transfer event. For these stars we define a mass difference ($\delta M$) between the mass determined from its luminosity and the present mass determined from fitting the binary orbit. We compare the observed values of the mass difference to stellar models with mass loss. The range of initial secondary masses that we find for observed W UMa type binaries is 1.3-2.6 M$_{\odot}$. We discover that the A- and the W-subtype contact binaries have different ranges of initial secondary masses. Binary systems with an initial mass higher than $1.8 \pm 0.1$ M$_{\odot}$ become A-subtype while systems with initial masses lower than this become W-subtype. Only 6 per cent of systems violate this behavior. We also obtain the initial masses of the primaries using the following constraint for the reciprocal of initial mass ratio: $0 < 1/q_i < 1$. The range of initial masses we find for the primaries is 0.2-1.5 M$_{\odot}$, except for two systems. Finally in comparing our models to observed systems we find evidence that the mass transfer process is not conservative. We find that only 34 per cent of the mass from the secondary is transferred to the primary. The remainder is lost from the system.