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

¹ Physics Department, Tartu University, Ülikooli 18, 50090 Tartu, Estonia
² Astronomical Observatory, Helsinki University, Box 14, 00014 Helsinki, Finland
³ N. Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland e-mail: sarna@camk.edu.pl

Corresponding author: E. Ergma, ene@physic.ut.ee

Received: 23 January 2001
Accepted: 11 May 2001

Abstract

Several evolutionary sequences with low-mass secondaries (, 1.5 and 1.7 ) and black hole accretors ( and 10 ) 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 ¹²C/¹³C, ¹⁴N/¹⁵N and ¹⁶O/¹⁷O with comparison to computed sequences allow estimates independent of spectroscopy of the mass of the secondary component. We find that our evolutionary calculations satisfactorily explain the observed 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 ¹²C/¹³C isotopic ratio; (ii) cosmic C, N, O abundances but modified ¹²C/¹³C ratio; and (iii) depletion of C and enhanced of N abundances and strongly modified isotopic ratios of C, N, O.