Massive Stars in the Far and Extreme Ultraviolet

Abstract

From the main sequence to their late evolutionary stages, massive stars spend most of their life as hot stars. Due to their high effective temperatures, the maximum of their emitted flux falls into the regime of ultraviolet (UV) wavelengths. Consequently, these stars emit a significant number of photons with energies sufficiently high enough to ionize hydrogen and potentially also other elements. As simple as these fundamental considerations are, as complex is a realistic estimate of the resulting ionizing fluxes, in particular for energies above 54 eV.

Estimating the ionizing flux budget of hot stars requires accurate models of their spectral energy distributions (SEDs), covering in particular the far and extreme UV region. Modern atmosphere models that incorporate the so-called line-blanketing effect, i.e. taking into account the millions of lines from iron and other elements, yield a complex picture, illustrating that the SED of a hot, massive star often deviates significantly from a blackbody. The ubiquitous presence of stellar winds complicates the picture: the absorption of photons driving the mass outflow leads to flux being shifted to longer wavelengths, strongly affecting the flux budget at the highest energies. On top of all these challenges, models estimating the ionizing fluxes of a whole population face the challenge of approximating massive star formation and evolution, which contain major unsolved puzzles often interwoven with open questions on the stellar scale.