CLASS STATUS AND MAJOR: Senior in Physics

Over the past few years, a growing debate has ensued over the origins of the ultraviolet emis-sion lines in the spectra of non-Mira, late-type variable stars. One hypothesis suggests that the emission arises from hydrostatic, chromospheric layers (i.e., the classical chromospheric model). Meanwhile, a second model suggests that outward propagating shocks generated from subphotospheric pulsations, similar to Mira-type variable stars, give rise to the emis-sion features (i.e., the hydrodynamic model). Problems exist for both models however. These semiregular and irregular variables display small flux variations in the ultraviolet emission and do not show hydrogen-Balmer emission, unlike the Mira-type variables. It may be dif-ficult to construct a “shocked ” atmospheric model that does not produce these features. On the other hand, velocity measurements of chromospheric lines suggest that the emitting region is traveling at supersonic speeds, making the hydrostatic assumption invalid. In this proposal, we wish to test both models with the aid of high-resolution IUE spectra for two late-type, semiregular variables, g Her and R Lyr. We will construct spherical chromospheric models and select hydrodynamic models from the Bowen grid for these two stars and generate NLTE synthetic spectra for comparisons with the IUE spectra