Chemical abundances of the secondary star in the Neutron star X-ray binary Cygnus X-2

TL;DR

This study analyzes the chemical composition of the secondary star in Cygnus X-2, revealing enhanced alpha-elements and peculiar oxygen abundance, suggesting contamination from a non-spherical supernova explosion.

Contribution

First detailed chemical abundance analysis of the secondary star in Cygnus X-2 considering veiling and supernova contamination scenarios.

Findings

Secondary star has higher than solar metallicity ([Fe/H]=0.27).

Alpha-elements (Mg, Si, S, Ti) are enhanced, indicating supernova contamination.

Oxygen appears under-abundant, while Fe and Ni are enhanced.

Abstract

We present UES@WHT high-resolution spectra of the low-mass X-ray binary (LMXB) Cygnus X-2. We have derived the stellar parameters of the secondary star using $\chi^{2}$ minimisation procedure, and taking into account any possible veiling from the accretion disk. We determine a metallicity higher than solar ([Fe/H]$=0.27\pm0.19$), as seen also in the neutron star X-ray binary Centaurus X-4. The high-quality of the secondary's spectrum allow us to determine the chemical abundances of O, Mg, Si, Ca, S, Ti, Fe and Ni. We found that some $\alpha$-elements (Mg, Si, S, Ti) are enhanced, consistent with a scenario of contamination of the secondary star during the supernova event. Surprisingly oxygen appears to be under-abundant, whereas enhanced abundances of Fe and Ni are measured. Assuming that these abundances come from matter that has been processed in the SN and then captured by the secondary star, we explore different SN explosion scenarios with diverse geometries. A non-spherically symmetric SN explosion, with a low mass cut, seems to reproduce better the observed abundance pattern of the secondary star compared to the spherical case.