Contributed Talk - Splinter HotStars

Friday, 15 September 2023, 14:10   (H 2036)

Massive, hydrogen-deficient stars as sources of strong ionizing flux

Andreas Sander, Varsha Ramachandran, Roel Lefever, Cormac Larkin
Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg

Young massive stars are a major source of ionizing fluxes. During most of their lifetime, massive stars show hot effective temperatures, thus emitting a significant number of photons with energies sufficiently high enough to ionize hydrogen and potentially also other elements. However, estimating the ionizing flux budget of hot stars is not trivial, as simple blackbody estimates can be off by orders of magnitude, especially when trying to determine the crucial ionizing flux above 54 eV that can give rise to the nebular He II emission observed in lower metallicity and high-redshift galaxies. Current Stellar population synthesis models predict a strong contribution from helium-burning massive Wolf-Rayet stars. However, detailed atmosphere modelling predicts that in particular He II ionizing flux is absorbed in stars with dense winds. In my talk, I will give a brief overview about the underlying wind physics and their non-linear dependencies before focussing on a type of star that can overcome this apparent contradiction. The talk will conclude by discussing the occurence of such stars in low-metallicity environments and their relative impact compared to lower-mass ionizing sources.