Contributed Talk - Splinter HotStars
The Why and How of Improvements on Wolf-Rayet Star Atmospheres.
Roel Lefever, Tomer Shenar, Andreas Sander, Luka Poniatowski, Karan Dsilva
The classical Wolf-Rayet (WR) stars, despite being quite rare, have a strong imprint on their surroundings and host galaxies as a whole. As the direct progenitors of stellar-mass black holes, they emit powerful ionizing radiation due to their high surface temperatures. Having typically high mass loss rates, these stars also contribute strongly to the enrichment of their local interstellar media. The main defining feature of WR stars are their radiation-driven stellar winds, which pushes away such high amounts of material that the wind regime obscures the star itself from sight. As a result, we mainly can only observe the wind of a WR star. This causes the modelling of this wind to be highly important; only by a deep understanding of this wind regime can we reproduce their spectra and learn more about their properties. The current modelling of WR-star winds makes use of a preimposed velocity field, the so-called beta-law, to describe the wind structure. However, multiple problems have accumulated over the years, in part due to the use of these beta-laws. To quantify these problems, we computed and analysed WR-star wind models, adopting different wind velocity fields for stellar models with typical parameters. With these models, we demonstrate that the velocity field has a profound impact on the spectral appearance, and hence the inference of stellar parameters, of these stars. We also show that the impact is to such an extent that we could cover almost the entire range of WR spectral subclasses with the same stellar model, with only changing the velocity fields. However, this degeneracy with the velocity fields can be solved by using UV spectral lines: as the absorption troughs of UV P-Cygni lines can constrain the terminal wind velocities well, we can also use these as a handle on the overall wind structure. To overcome these discerpancies altogether, we solve the hydrodynamic equation of motion to arrive at a consistent solution for our wind structure.