Tracing CNO exposed layers in the Algol-type binary system u Her

TL;DR

This study uses spectral disentangling and evolutionary modeling to analyze the chemical composition and history of the binary system u Her, revealing CNO processing and case A mass transfer.

Contribution

It provides the first detailed abundance analysis of u Her's components, confirming CNO processing and tracing the system's evolutionary history with new spectral and photometric data.

Findings

Primary star shows CNO abundance deviations consistent with nucleosynthesis.

System evolved through case A mass transfer.

Secondary star exhibits signs of strong early mixing.

Abstract

The chemical composition of stellar photospheres in mass-transferring binary systems is a precious diagnostic of the nucleosynthesis processes that occur deep within stars, and preserves information on the components history. The binary system u Her belongs to a group of hot Algols with both components being B-stars. We have isolated the individual spectra of the two components by the technique of spectral disentangling of a new series of 43 high-resolution echelle spectra. Augmenting these with an analysis of the Hipparcos photometry of the system yields revised stellar quantities for the components of u Her. For the primary component (the mass-gaining star) we find M_A = 7.88 +/- 0.26 M_sun, R_A = 4.93 +/- 0.15 R_sun and T_eff_A = 21600 +/- 220 K. For the secondary (the mass-losing star) we find M_B = 2.79 +/- 0.12 M_sun, R_B = 4.26 +/- 0.06 R_sun and T_eff_B = 12600 +/- 550 K. A non-LTE analysis of the primary star's atmosphere reveals deviations in the abundances of nitrogen and carbon from the standard cosmic abundance pattern in accord with theoretical expectations for CNO nucleosynthesis processing. From a grid of calculated evolutionary models the best match to the observed properties of the stars in u Her enabled tracing the initial properties and history of this binary system. We confirm that it has evolved according to case A mass transfer. A detailed abundance analysis of the primary star gives C/N = 0.9, which supports the evolutionary calculations and indicates strong mixing in the early evolution of the secondary component, which was originally the more massive of the two. The composition of the secondary component would be a further important constraint on the initial properties of u Her system, but requires spectra of a higher signal to noise ratio.