Understanding the three-dimensional structure of the interstellar medium surrounding stars will always be a challenge due to our single vantage point from the Earth. This situation is aided in the cas..
Understanding the three-dimensional structure of the interstellar medium surrounding stars will always be a challenge due to our single vantage point from the Earth. This situation is aided in the case of an interstellar cloud surrounding a central hot star, where the cloud’s properties can be constrained by an analysis of its spectral-line emission. It is known, from observations by the Wisconsin Hα Mapper (WHAM, Haffner et al. 2003), that such a cloud exists around the star α Virginis (Spica), a concentration of ionized hydrogen with a surface brightness in Hα of roughly 5 Rayleighs (Reynolds 1985). Follow-up observations (Reynolds 1988). provided additional line-brightness ratios, with respect to Hα, including an ionized Sulphur line, [S II] at 671.6 nm. Our goal is to use the photoionization modelling software Cloudy (Ferland et al. 2017) to constrain the geometry of this region by simulating the ionization of a model cloud using realistic stellar atmospheres (Aufdenberg et al. 2020), adjusting model parameters to match Reynolds’ observations. We vary the star’s intensity on the cloud, as well as the hydrogen number density and the filling factor, a measure of the cloud’s “clumpiness.” Thus far, we have matched the observed Hα surface brightness and determined an ionizing flux from α Virginis slightly below Reynolds’ 1985 lower-limit estimate. The research is still ongoing, and the current problem we are facing is the [S II] line is unusually high relative to the primary hydrogen line, Hα. Finding a set of parameters which fits this observation is the next goal for this project.
