THE ULTRAVIOLET-RADIATION IN METAL-RICH ELLIPTIC GALAXIES

By use of a new method that allows for an analysis of stellar evolutio n with a minimum of model dependency, we discuss the conditions for pr ogenitors of UV-bright asymptotic giant branch (AGB) manque stars to f orm in galaxies of various metallicities. UV-bright manque stars in th e so-called slow blue phase have been suggested as the explanation for the observed excess of UV flux shortward of almost-equal-to 3000 angs trom in metal-rich elliptical galaxies relative to metal-poor systems. We find that in systems where the ratio of helium enrichment relative to metallicity enrichment, R = DELTAY/DELTAZ, is close to the solar v alue, the possible UV-bright manque stars will give rise to UV radiati on which is almost independent of metallicity. Only for systems with v alues of R slightly bigger than the solar value will the predicted rel ative number of manque stars be a strongly increasing function of meta llicity. For such values of R, the increasing UV radiation behaves, qu alitatively, as a function of metallicity, as it is observed in the me tal-rich elliptical galaxies. In particular, we comment on a series of assumed ZAHB models published recently that were identified as potent ial progenitors for the ''slow blue'' AGB manque stars. We find that o nly those of the ZAHB models that have R slightly above the solar valu e can have evolved into the horizontal-branch phase within the present age of the universe (with a reasonable amount of mass loss at the red giant branch), and also only these stars show the necessary functiona l number increase with metallicity.