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

Context: X-ray bursts (XRB) are powerful thermonuclear events on the surface of accreting neutron stars (NS), where nucleosynthesis of heavy elements occur. In most cases, the high surface gravity prevents the ejection of material directly by the thermonuclear explosion. However, the predicted and observed luminosities sometimes exceed Eddington’s value, and some of the material may escape by means of a stellar wind.

Aim: The present work has two main motivating issues. The first is to determine the mass-loss and chemical composition of the material ejected through radiation-driven winds and its significance for Galactic abundances, with an interest on some light p-nuclei (^92,94Mo, ^96,98Ru) that are under-produced in every other astrophysical scenario. The second is to study the evolution of observational quantities during the wind phase, which can help constrain the mass-radius relation in neutron stars.

Methods: A non-relativistic radiative wind model was successfully implemented, with modern opacity tables and treatment of the critical point. This radiative wind model was then linked through a new technique to a series of XRB hydrodynamic simulations, that include over 300 isotopes, allowing us to construct a quasi-stationary time evolution of the wind during the XRB.

Results: The results of our simulations make it possible to assess the mass ejected by radiative wind during XRBs and its detailed composition. In the models studied, the average ejected mass per unit time represents 2.6% of the accretion rate, with 90% of the ejecta composed by ⁶⁰Ni, ⁶⁴Zn, ⁶⁸Ge and ⁵⁸Ni. The ejected material also contained a small fraction (10⁻⁴ − 10⁻⁵ ) of some light p-nuclei of interest. Additionally, the observable magnitudes during the wind phase showed remarkable correlations, partly deriving from to the fact that photospheric luminosity stays close to Eddington limit. Some of these correlations involve wind parameters like energy and mass outflows, that are determined by the conditions at the base of the wind envelope.

Conclusions: The simulations resulted in the first realistic quantification of mass-loss for each isotope synthesized in the XRB. The photospheric correlations found 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.