Winds from stripped low-mass Helium stars and Wolf-Rayet stars

TL;DR

This study predicts mass-loss rates for helium stars, including low-mass and Wolf-Rayet types, using Monte Carlo models, revealing lower rates than previous estimates, which impacts their detectability and role in supernovae and black hole formation.

Contribution

It provides new mass-loss predictions for helium stars across a range of masses and metallicities, especially highlighting lower rates for low-mass stars than extrapolated from empirical data.

Findings

Mass-loss rates for low-mass He stars are an order of magnitude smaller than previous extrapolations.

The mass-loss rate dependence on metallicity has an exponent of 0.61, less steep than recent empirical models.

Lower mass-loss rates imply reduced likelihood of low-mass He stars producing certain supernovae and heavy black holes.

Abstract

We present mass-loss predictions from Monte Carlo radiative transfer models for helium (He) stars as a function of stellar mass, down to 2 Msun. Our study includes both massive Wolf-Rayet (WR) stars and low-mass He stars that have lost their envelope through interaction with a companion. For these low-mass He-stars we predict mass-loss rates that are an order of magnitude smaller than by extrapolation of empirical WR mass-loss rates. Our lower mass-loss rates make it harder for these elusive stripped stars to be discovered via line emission, and we should attempt to find them through alternative methods instead. Moreover, lower mass-loss rates will make it less likely that low-mass He stars provide stripped-envelope supernovae (SNe) of type Ibc. We express our mass-loss predictions as a function of L and Z, and not as a function of the He abundance, as we do not consider this physically astute given our earlier work. The exponent of the dM/dt vs. Z dependence is found to be 0.61, which is less steep than relationships derived from recent empirical atmospheric modelling. Our shallower exponent will make it more challenging to produce "heavy" black holes of order 40 Msun, as recently discovered in the gravitational wave event GW 150914, making low metallicity for these types of events even more necessary.