Calum Hawcroft
Hot massive stars are known to host unstable, radiation-driven outflowing winds, giving rise to dense clumps of material which severely affect the diagnostic techniques used to derive wind properties of massive stars.
Most of the current diagnostic wind models account for wind inhomogeneities by assuming a one-component medium consisting of optically thin clumps, and maintaining a smooth velocity-field. However, if clumps become optically thick, this can allow for additional leakage of light through porous channels in-between the clumps. These effects of velocity-porosity have recently been fully incorporated into the stellar atmosphere modelling code FASTWIND. Using this new version of FASTWIND, we have derived updated empirical mass-loss rates and clumping properties for hot massive stars.
In this poster, we present quantitative results from a combined Ultraviolet-Optical wind analysis of well studied galactic O-supergiant stars in order to investigate the impact that including the new wind effects in modelling codes will have on the stellar and wind parameters we derive.
We compare our findings with earlier results found from diagnostic models which do not take into account such optically thick clumping or velocity-space porosity. We also compare to standard mass-loss rates usually included in evolution model studies of massive stars and with predictions of clumping derived from theoretical models. Overall, a key end goal of the project is to extend to larger samples of massive stars and to establish new empirical mass-loss recipes for the evolutionary stages we are investigating.
Currently, stellar evolution models use mass loss rate predictions computed theoretically from Vink et al. 2000. Recent spectroscopic studies have shown that these mass loss rates must be reduced by observing the effect of clumping on spectral diagnostics. The strength of lines driven by recombination is proportional to the density-squared of the wind, making them extremely sensitive to the mass loss but also the clumping. Meaning that the line strength, which is normally used to determine the mass loss rate, must be adjusted due to the effects of clumping and the mass loss adjusted accordingly. Meanwhile, resonance line strengths are proportional only to the density, making the emerging profile more sensitive to the velocity profile of the wind. However, even current spectroscopic studies have been limited to the assumption that the clumps are optically thin, leading to issues when fitting resonance lines. In reality the wind opacity varies through the medium due to clumping, clumps can become optically thick while the opacity can be reduced in the medium surrounding the clumps. Opacity changes can have various effects on line profiles depending on the line driving process. For example, lower opacities in the inter-clump medium allows for additional leakage of light, decreasing the amount of absorption visible in resonance lines.
Using new spectroscopic fitting techniques we are able to account for opacity variations in the wind for the first time, allowing us to achieve high quality fits to lines driven by different emission processes simultaneously. As a result, we do not have to artificially adjust other stellar or wind parameters in order to fit the lines. This gives us the opportunity to make new empirical predictions on the mass loss rates. We are also able to obtain the first empirical constraints on the extent of the optically thick wind clumping.
We find that mass loss rates can be of the order of 3 times lower than previously predicted, which is in closer agreement with new theoretical predictions from Bjorklund et al. 2020. This mass loss rate reduction can have a significant influence on the late evolution and end stages of the star's evolution. For a star with a mass 70 times that of our Sun it can be the difference between evolution into a Wolf-Rayet star or a Red Supergiant star.