On the Origin of the Ultraviolet Upturn in Elliptical Galaxies. II. Test of the Horizontal‐Branch Hypothesis

Whether or not metal-rich HB stars are the dominant UV source in giantelliptical galaxies (gEs) is an important question in current astronomicalresearch. We follow up our previous evolutionary population synthesis study withquantitative tests to answer this question affirmatively under the followingthree conditions: (1) Reimers' empirical mass loss formula is proper, (2) themass loss efficiency parameter in metal-rich stars is somewhat larger than thevalue estimated from the metal-poor star studies, and (3) the true value of thehelium enrichment parameter is positive. All three important empirical characteristics of the UV upturn (i.e., thefact that strong UV upturns are restricted to gEs, the positive UVupturn-metallicity correlation, and the narrow range of the temperature of theUV sources) are closely reproduced for reasonable ranges of input parameters. We discuss the major sources of uncertainties in the models, such as theproduction and role of hot horizontal-branch stars in gEs, and the importanceof galactic nucleosynthesis.