Carbon deficiencies in the primaries of some classical Algols

TL;DR

This study measures carbon line strengths in Algol-type binary stars and finds they are systematically carbon-poor, likely due to mass transfer from evolved secondary stars affecting surface abundances.

Contribution

It provides the first quantitative analysis of carbon deficiencies in the primaries of classical Algols and links these deficiencies to mass transfer and mixing processes.

Findings

Algol primaries are systematically carbon-poor compared to standard stars.

Carbon deficiency correlates with mass transfer rates and orbital period changes.

Surface mixing influences the observed abundance anomalies.

Abstract

The equivalent widths of C II $\lambda$ 4267 \AA line were measured for the mass-gaining primary stars of the 18 Algol-type binary systems. The comparison of the EWs of the gainers with those of the single standard stars having the same effective temperature and luminosity class clearly indicates that they are systematically smaller than those of the standard stars. The primary components of the classical Algols, located in the main-sequence band of the HR diagram, appear to be C poor stars. We estimate $ [N_{C} /N_{tot}] $ relative to the Sun as -1.91 for GT Cep, -1.88 for AU Mon and -1.41 for TU Mon, indicating poorer C abundance. An average differential carbon abundance has been estimated to be -0.82 dex relative to the Sun and -0.54 dex relative to the main-sequence standard stars. This result is taken to be an indication of the transferring material from the evolved less-massive secondary components to the gainers such that the CNO cycle processed material changed the original abundance of the gainers. There appear to be relationships between the EWs of C II $\lambda$ 4267 \AA line and the rates orbital period increase and mass transfer in some Algols. As the mass transfer rate increases the EW of the C II line decreases, which indicates that accreted material has not been completely mixed yet in the surface layers of the gainers. This result supports the idea of mixing as an efficient process to remove the abundance anomaly built up by accretion. Chemical evolution of the classical Algol-type systems may lead to constrains on the initial masses of the less massive, evolved, mass-losing stars.