On the Hα behaviour of blue supergiants: rise and fall over the bi-stability jump

Context. The evolutionary state of blue supergiants is still unknown. Stellar wind mass loss is one of the dominant processes determining the evolution of massive stars, and it may provide clues to the evolutionary properties of blue supergiants. As the H alpha line is the most oft-used mass-loss tracer in the OB-star regime, we investigate H alpha line formation as a function of T-eff. Aims. We provide a detailed analysis of the H alpha line for OB supergiant models over an T-eff range between 30 000 and 12 500 K, with the aim of understanding the mass-loss properties of blue supergiants. Methods. We model the H alpha line using the non-LTE code CMFGEN, in the context of the bi-stability jump at T-eff similar to 22 500 K. Results. We find a maximum in the H alpha equivalent width at 22 500 K exactly at the location of the bi-stability jump. The H alpha line-profile behaviour is characterised by two branches of effective temperature: (i) a hot branch between 30 000 and 22 500 K, where H alpha emission becomes stronger with decreasing T-eff; and (ii) a cool branch between 22 500 and 12 500 K, where the H alpha line becomes weaker. Our models show that this non-monotonic H alpha behaviour is related to the optical depth of Ly alpha, finding that at the cool branch the population of the 2nd level of hydrogen is enhanced in comparison to the 3rd level. This is expected to increase line absorption, leading to weaker H alpha flux when T-eff drops from 22 500 K downwards. We also show that for late B supergiants (at T-eff below similar to 15 000 K), the differences in the H alpha line between homogeneous and clumpy winds becomes insignificant. Moreover, we show that, at the bi-stability jump, H alpha changes its character completely, from an optically thin to an optically thick line, implying that macro-clumping should play an important role at temperatures below the bi-stability jump. This would not only have consequences for the character of observed H alpha line profiles, but also for the reported discrepancies between theoretical and empirical mass-loss rates.