Metallicity dependence of late-O and B supergiant winds in the Milky Way and Magellanic Clouds.

Blue supergiants (BSGs) are hot, luminous, and evolved massive stars that produce significant chemical, radiative, and mechanical feedback in their surroundings. BSGs possess line-driven winds, whose properties play a crucial role in determining their evolution and fate. Studying the properties of BSGs across multiple metallicity environments provides significant insight into the effect of metallicity on wind properties. In this thesis, we conduct a combined ultraviolet and optical spectroscopic study of late-O and B supergiants in the Milky Way (MW) and the Magellanic Clouds using the model atmosphere code CMFGEN. We present the properties of 20, 16, and 26 OB supergiants between O9 and B8 in the Small Magellanic Cloud (SMC), Large Magellanic Cloud (LMC), and the MW, respectively. We find a strong metallicity dependence in mass-loss rates and in the wind momentum-luminosity relation. We derive a fully empirical mass-loss recipe as an alternative to the numerical recipe commonly used in stellar-evolution models. Our results show that these numerical recipes do not predict the behaviour of mass loss for stars near the terminal-age main sequence and in the Hertzsprung gap. We do not find any evidence of an increase in mass loss or a decrease in the ratio of terminal wind velocity to escape velocity that is associated with the bi-stability jump around 25-21 kK, with both decreasing monotonically with temperature. We investigate the evolutionary history of our sample using single-star evolutionary models. We find that many stars lie in the Hertzsprung gap, showing enhanced helium and nitrogen abundances and slow rotation rates, suggesting that they may be products of binary interaction, such as stellar mergers. We find that the evolutionary mass is generally higher than the spectroscopically derived mass, with 40% of the stars in our samples exhibiting a significant mass discrepancy.