We present models of the late stages of stellar evolution intended to explain the "UV upturn" phenomenon in elliptical galaxies. Such models are sensitive to values of a number of poorly constrained physical parameters, including metallicity, age, stellar mass loss, helium enrichment, and the distribution of stars on the zero-age horizontal branch (HB). We explore the sensitivity of the results to values of these parameters and reach the following conclusions. Old, metal-rich galaxies, such as giant ellipticals, naturally develop a UV upturn within a reasonable timescale - less than a Hubble time - without the presence of young stars. The most likely stars to dominate the UV flux of such populations are low-mass, core helium-burning (HB and evolved HB) stars. Metal-poor populations produce a higher ratio of UV-to-V flux, owing to opacity effects, but only metal-rich stars develop a UV upturn, in which the flux increases toward shorter UV wavelengths.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science