LB 3459 - an O-type subdwarf eclipsing binary system. Non-LTE analysis of the primary.

Abstract

A non-LTE analysis is presented for the primary component of the sdO eclipsing binary LB 3459. The results of the analysis give T_eff = 40,000⁺³⁰⁰⁰_-2000 K, log g = 5.3±O.2 (cgs) and a surprisingly low helium abundance of y = 0.3^+O.2_-0.1% by numbers. LB 3459 appears to be the result of mass exchange during the first giant phase. The primary most likely has a mass of 0.3 M_sun and consists of a hydrogen shell and degenerate helium core. Diffusion is suggested as the cause of helium depletion in the photosphere. The secondary has a mass of 0.04 M_sun and a radius of 0.09 R_sun, which is close to a degenerate configuration of solar composition.