On the H α behaviour of blue supergiants: Rise and fall over the bi-stability jump

Abstract

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 Hα line is the most oft-used mass-loss tracer in the OB-star regime, we investigate Hα line formation as a function of Teff. Aims. We provide a detailed analysis of the Hα line for OB supergiant models over an Teff range between 30 000 and 12 500 K, with the aim of understanding the mass-loss properties of blue supergiants. Methods. We model the Hα line using the non-LTE code cmfgen, in the context of the bi-stability jump at Teff ~ 22 500 K. Results. We find a maximum in the Hα equivalent width at 22 500 K exactly at the location of the bi-stability jump. The Hα line-profile behaviour is characterised by two branches of effective temperature: (i) a hot branch between 30 000 and 22 500 K, where Hα emission becomes stronger with decreasing Teff; and (ii) a cool branch between 22 500 and 12 500 K, where the Hα line becomes weaker. Our models show that this non-monotonic Hα behaviour is related to the optical depth of Lyα, 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 Hα flux when Teff drops from 22 500 K downwards. We also show that for late B supergiants (at Teff below ~15 000 K), the differences in the Hα line between homogeneous and clumpy winds becomes insignificant. Moreover, we show that, at the bi-stability jump, Hα 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 Hα line profiles, but also for the reported discrepancies between theoretical and empirical mass-loss rates. © 2014 ESO.