What can we learn from the surface chemical composition of the optical companions of Soft X-ray transients?

Abstract

Several evolutionary sequences with low--mass secondaries ($M_d $=1.25, 1.5 and 1.7$ M_\odot $) and black hole accretors ($M_{bh} $=5 and 10$ M_\odot$) are calculated. The angular momentum losses due to magnetic braking and gravitational wave radiation are included. Using full nuclear networks (p--p and CNO cycles) we follow carefully the evolution of the surface composition of the secondary star. We find that the surface chemical composition of the secondary star may give additional information which helps to understand the formation of soft X-ray transients with black holes as accretors. We show that observations of isotope ratios $^{12}C/^{13}C$, $^{14}N/^{15}N$ and $^{16}O/^{17}O $ with comparison to computed sequences allow estimates independent from spectroscopy of the mass of the secondary component. We find that our evolutionary calculations satisfactorily explain the observed $\it q = M_{sg}/M_{bh}$ -- $P_{orb}$ distribution for Soft X-ray transients with orbital periods less than one day. Using our evolutionary calculations we estimate secondary masses and surface chemical abundances (C,N,O) for different systems. We distinguished three different phases in the SXT's evolution. The optical component shows (i) cosmic C, N, O abundances and $^{12}$C/$^{13}$C isotopic ratio; (ii) cosmic C, N, O abundances but modified $^{12}$C/$^{13}$C ratio; and (iii) depletion of C and enhanced of N abundances and strongly modified isotopic ratios of C, N, O elements.