Mass-loss and composition of wind ejecta in type I X-ray bursts

Abstract

X-Ray bursts (XRB) are powerful thermonuclear events on the surface of accreting neutron stars (NS), where nucleosynthesis of intermediate-mass elements occurs. Their predicted and observed luminosities sometimes exceed Eddington's value, thus some of the material may escape by means of a stellar wind. This work seeks to determine the mass-loss and chemical composition of the material ejected through radiation-driven winds and its significance for Galactic abundances. It also reports on the evolution of pysical quantities during the wind phase that could help constrain the mass-radius relation in neutron stars. A non-relativistic radiative wind model was implemented and linked, through a new technique, to a series of XRB hydrodynamic simulations, that include over 300 isotopes. This allows us to construct a quasi-stationary time evolution of the wind during the XRB. The simulations resulted in the first realistic quantification of mass-loss for each isotope synthesized in the XRB. The total mass ejected by the wind was about $6\times10^{19}g$, the average ejected mass per unit time represents 2.6% of the accretion rate, with 0.1% of the envelope mass ejected per burst and ~90% of the ejecta composed by $^{60}$Ni, $^{64}$Zn, $^{68}$Ge and $^{58}$Ni. The ejected material also contained a small fraction ($10^{-4}-10^{-5}$) of some light p-nuclei, but not enough to account for their Galactic abundances. Additionally, the observable magnitudes during the wind phase showed remarkable correlations, some of which involve wind parameters like energy and mass outflows, that are determined by the conditions at the base of the wind envelope. These correlations could be used to link observable magnitudes to the physics of the innermost parts of the envelope, close to its interface with the NS crust. This is a promising result regarding the issue of NS radii determination.