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 T eff ~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 T eff 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 ∝ Z 0.41–1.4 ) 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.