Empirical mass-loss rates and clumping properties of O-type stars in the LMC

TL;DR

This study empirically determines mass-loss rates and clumping properties of 18 O-type stars in the LMC, revealing lower mass-loss than some models and a correlation between clumping and temperature, enhancing understanding of stellar winds at sub-solar metallicity.

Contribution

It provides the most accurate spectroscopic mass-loss and wind structure measurements for massive stars at sub-solar metallicity, incorporating optically thick clumping effects and a novel fitting approach.

Findings

Mass-loss rates are 4-5 times lower than Vink et al. 2001 predictions.

Clumping factors positively correlate with effective temperature.

In the weak-wind regime, mass-loss rates are orders of magnitude below theoretical predictions.

Abstract

We constrain wind parameters of a sample of 18 O-type stars in the LMC, through analysis with stellar atmosphere and wind models including the effects of optically thick clumping. This allows us to determine the most accurate spectroscopic mass-loss and wind structure properties of massive stars at sub-solar metallicity to date and gain insight into the impact of metallicity on massive stellar winds. Combining high signal to noise (S/N) ratio spectroscopy in the UV and optical gives us access to diagnostics of multiple different physical processes in the stellar wind. We produce synthetic spectra using the stellar atmosphere modelling code FASTWIND, and reproduce the observed spectra using a genetic algorithm based fitting technique. We empirically constrain 15 physical parameters associated with the stellar and wind properties, including temperature, surface gravity, surface abundances, rotation, macroturbulence and wind parameters. We find, on average, mass-loss rates a factor of 4-5 lower than those predicted by Vink et al. 2001, in good agreement with predictions from Bjorklund et al. 2021, and the best agreement with those from Krticka et al. 2018. In the 'weak-wind' regime we find mass-loss rates orders of magnitude below any theoretical predictions. We find a positive correlation of clumping factors (fcl) with effective temperature with an average fcl = 14 +- 8 for the full sample. Above 38 kK an average 46 +- 24% of the wind velocity span is covered by clumps and the interclump density is 10-30% of the mean wind. Below an effective temperature of roughly 38 kK there must be additional light leakage for supergiants. For dwarf stars at low temperatures there is a statistical preference for very low clump velocity spans, however it is unclear if this can be physically motivated as there are no clearly observable wind signatures in UV diagnostics.