X-shooting ULLYSES: massive stars at low metallicity

Context. Our limited understanding of the winds of massive stars hampers our ability to predict their evolution. We currently often rely on mass-loss rate prescriptions that show strong features that lack a firm empirical confirmation, such as the bi-stability jump. This bi-stability jump is a pronounced increase in mass loss in the decreasing temperature regime Teff ~28–21 kK, i.e., spectral types B1-B2. Different prescriptions also give different metallicity dependences; these sometimes include a dependence on effective temperature, luminosity or both, that may also differ among prescriptions. Although recent papers have compared empirical results to prescriptions, a large observational sample of B stars for which the wind has been scrutinised over different metallicity ranges is still lacking.

Aims. Through the modelling of both ultraviolet (from the ULLYSES programme) and optical (from the XShootU programme) high-resolution spectra, we determined the stellar and wind parameters, including detailed clumping parameters, of 24 SMC B stars ranging in Teff from 13 to 29 kK. By combining this sample with LMC studies, we compared the wind behaviour of B stars in two different metallicity regimes. We compared our results to common mass-loss rate prescriptions to test features present in these models and their metallicity dependence.

Methods. We have used the model atmosphere code FASTWIND and the genetic algorithm code Kiwi-GA to fit the UV and optical spectra simultaneously. This allows us to determine wind properties including clumping behaviour.

Results. The moderate to strong metallicity trends present in the mass-loss prescriptions (Ṁ ∝ Z0.41–1.4) explored here overestimate the empirical metallicity dependence in the B-star regime, which appears very weak. Similarly to a previous study of B supergiants in the LMC, we do not find a sudden increase in mass-loss rate at approximately spectral type B1, but only a weak temperature trend. As in this LMC study, we show that on average around 40% of the wind mass is located in the wind medium between the clumps. Using other studies from this paper series, we compiled a sample of 80+ O and B stars in the SMC and LMC. From a comparison we find a clear difference in O- and B-type metallicity dependence.

Conclusions. The apparent lack of a bi-stability jump in the B-star regime and a very weak metallicity dependence for the same stars offers new empirical constraints to theoretical models of line-driven winds. As differences between these models are large (reaching orders of magnitude in parts of parameter space) such constraints are much needed. More studies exploring the mass-loss rates of B stars and cooler objects will be helpful to our understanding of hot star winds.