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. However, their mass-loss properties are not well understood. Therefore, in this thesis, we investigate the wind properties of blue supergiants by means of the non-L TE radiative transfer code cmfgen (Hillier & Miller 199B). The thesis describes two self-contained pieces of research which are linked through their connection with the wind properties of blue supergiants. The first involves a detailed analysis of the Ha line formation over a range in effective temperature between 30000 and 10000K. We find a maximum in the Hα equivalent width around 22500K which is always present in sets of models with various stellar and wind parameters. The non-monotonic Ha behaviour is related to the optical depth of the Lyα line, 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 22500K downwards. We also 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. In the second part of the thesis, the physical ingredients that play a role in the line acceleration are explored. Our calculations confirm the bi-stability jump in mass-loss rates predicted by Vink et al. (1999). We also show that at temperatures around 10000 K, a second jump in mass-loss rates is produced if the observed velocity ratios are applied. The jump is caused by Felll/Fell recombination/ionisation as was suggested by Vink et al. (1999). Understanding the behaviour of the second jump my provide valuable science prospects for late B/A supergiants and LBVs, and therefore, a detailed investigation of this jump might be worth having.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:675422 |
| Date | January 2014 |
| Creators | Petrov, Blagovest Vaskov |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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