Multicomponent radiatively driven stellar winds II. Gayley-Owocki heating in multitemperature winds of OB stars

TL;DR

This paper investigates Gayley-Owocki heating in multicomponent stellar winds of OB stars, revealing its significance in wind temperature structures and the potential for X-ray emission in cooler B stars.

Contribution

The study derives a formula for Gayley-Owocki heating from the Boltzmann equation and explores its effects on temperature differences among wind components, improving understanding of wind heating mechanisms.

Findings

Gayley-Owocki heating significantly affects wind temperature structure in stars cooler than O6.

Temperature differences between ions and other wind components can reach 10^3 K in B stars cooler than B2.

In some cases, passive components fall back, heating ions to ~10^6 K and enabling X-ray emission.

Abstract

We show that the so-called Gayley-Owocki (Doppler) heating is important for the temperature structure of the wind of main sequence stars cooler than the spectral type O6. The formula for Gayley-Owocki heating is derived directly from the Boltzmann equation as a direct consequence of the dependence of the driving force on the velocity gradient. Since Gayley-Owocki heating deposits heat directly to the absorbing ions, we also investigated the possibility that individual components of the radiatively driven stellar wind have different temperatures. This effect is negligible in the wind of O stars, whereas a significant temperature difference takes place in the winds of main sequence B stars for stars cooler than B2. Typical temperature difference between absorbing ions and other flow components for such stars is of the order 10^3 K. However, in the case when passive component falls back onto the star the absorbing component reaches temperatures of order 10^6 K, which allows for emission of X-rays. Moreover, we compare our computed terminal velocities with the observed ones. We found quite good agreement between predicted and observed terminal velocities. The systematic difference coming from the using of the so called "cooking formula" has been removed.