Effect of rotational mixing and metallicity on the hot star wind mass-loss rates

TL;DR

This study investigates how rotational mixing and metallicity influence the mass-loss rates of hot star winds, revealing that at low metallicities, chemical composition changes significantly impact wind properties and the wind momentum-luminosity relationship.

Contribution

It provides new insights into the effects of CNO-processed chemical compositions on stellar wind mass-loss rates across different metallicities using NLTE wind modeling.

Findings

At high metallicity ($Z/Z_\odot \gtrsim 0.1$), composition changes do not significantly alter mass-loss rates.

At low metallicity ($Z/Z_\odot \lesssim 0.1$), rotational mixing greatly affects wind mass-loss rates.

The wind momentum-luminosity relationship becomes unreliable at very low metallicities due to increased scatter.

Abstract

Hot star wind mass-loss rates depend on the abundance of individual elements. This dependence is usually accounted for assuming scaled solar chemical composition. However, this approach may not be justified in evolved rotating stars. The rotational mixing brings CNO-processed material to the stellar surface, increasing the abundance of nitrogen at the expense of carbon and oxygen, which potentially influences the mass-loss rates. We study the influence of the modified chemical composition resulting from the rotational mixing on the wind parameters, particularly the wind mass-loss rates. We use our NLTE wind code to predict the wind structure and compare the calculated wind mass-loss rate for the case of scaled solar chemical composition and the composition affected by the CNO cycle. We show that for a higher mass-fraction of heavier elements $Z/Z_\odot\gtrsim0.1$ the change of chemical composition from the scaled solar to the CNO-processed scaled solar composition does not significantly affect the wind mass-loss rates. The missing line force caused by carbon and oxygen is compensated for by nitrogen line force. However, for a very low-mass fraction of heavier elements $Z/Z_\odot\lesssim0.1$ the rotational mixing significantly affects the wind mass-loss rates. Moreover, the decrease of the mass-loss rate with metallicity is stronger at such low metallicities. We study the relevance of the wind momentum-luminosity relationship for different metallicities and show that for a metallicity $Z/Z_\odot\lesssim0.1$ the relationship displays a large scatter, which depreciates the use of this relationship at the lowest metallicities.