The Nature of the Emission Spectrum of NGC 7793 P13: modeling the atmosphere of the donor star

TL;DR

This study models the atmosphere of the donor star in NGC 7793 P13, showing good spectral agreement with a high mass loss rate influenced by stellar rotation and X-ray irradiation effects.

Contribution

It introduces a spherically symmetric wind atmosphere model for the donor star, aligning well with observations and considering the impact of high rotation and X-ray irradiation.

Findings

Model spectrum matches observed spectrum at high mass loss rate

High stellar rotation likely causes increased mass loss

X-ray irradiation affects the wind acceleration and spectrum

Abstract

We continue to study the ultra-luminous X-ray source NGC 7793 P13 in the optical range. In this work, we are testing the model of a spherically symmetric wind atmosphere of the donor star, previously identified as a B9 Ia supergiant. The model spectrum has shown good agreement with the observed one at a relatively high mass loss rate of $\dot{M} \approx 6\times10^{-6}\,M_{\odot}\,\text{yr}^{-1}$; other parameters turned out to be close to those expected for late B-supergiants. The increased mass loss rate can be explained by the high rotation velocity of the star. In addition, we have qualitatively demonstrated the effect of X-ray irradiation on the observed spectrum and discuss the fundamental possibility of wind acceleration under conditions of powerful irradiation.