Research Division Seminar
Empirical mass loss rates and clumping behaviour of B-Supergiants

Resumen

Mass-loss of massive stars is an uncertain, but very important part of stellar evolution, as massive stars lose a significant amount of mass even in their main sequence. Currently, stellar evolution models often include mass-loss in the form of a mass-loss prescription based on the Vink et al. 2001 models. This prescription includes a steep increase in mass-loss for stars cooler than 25000K referred to as the bi-stability jump. The existence of this jump has been contentious for a long time, with multiple observational studies searching for the jump. However, it has never been observed when studying a sample of stars crossing this temperature regime, recently this includes studies with a very large sample size such as the IACOB project. Due to a degeneracy between the clumping behaviour of the wind and the mass-loss rate when looking at optical spectral lines only, it is not possible to measure exact mass-loss rates, but instead you obtain a mass-loss rate convolved with the clumping factor. Thanks to the ULLYSES program and the Xshoot-u optical follow up we now have a sample of B-giants in the LMC and SMC for which we have both optical and UV spectroscopy. UV spectroscopy not only gives us access to important wind diagnostics such as terminal wind speed, but also gives us the ability to determine clumping properties independent from mass-loss rates. The increased complexity we need to include in our models to represent the clumping behaviour does increase the number of free parameters significantly. I will show how we use Genetic Algorithms to fit many parameters at the same time and how we used this method to derive mass-loss rates and clumping properties for LMC and SMC B-supergiants covering the bi-stability jump.