ON THE ULTRAVIOLET-BRIGHT PHASE OF METAL-RICH HORIZONTAL-BRANCH STARS

We consider the origin of the UV-bright phase of metal-rich helium-bur ning stars, the slow blue phase (SBP) that was predicted by various ea rlier works. Based on improved physics including OPAL opacities, which is the same physics that was used in the construction of the new Yale Isochrones, we confirm the existence of the SBP. In addition to our g rid of evolutionary tracks, we provide an analytical understanding of the main characteristics of the SBP phenomenon. The SBP is slow becaus e it is a dow-evolving, helium-shell-burning phase that is analogous t o the early asymptotic giant branch phase. The SBP of a more metal-ric h star is slower than a metal-poor counterpart if they have the same T -eff because a more metal-rich helium-burning star has a smaller mass than a metal-poor one and because lifetime increases as mass decreases . Metal-rich helium-burning stars easily become hot because the lumino sity from the hydrogen-burning shell is extremely sensitive to the mea n molecular weight mu, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive Delta Y/Delta Z, heliu m abundance plays the most important role in governing mu, and thus Do rman et al. found that the SBP occurs only when Y greater than or simi lar to 0.4 when Delta Y/Delta Z greater than or similar to 0. We sugge st that the SBP phenomenon is a major cause of the UV upturn phenomeno n in giant elliptical galaxies, as will be shown in subsequent papers. The new HB tracks can be retrieved from S. Y.'s web site http://sheme sh.gsfc.nasa.gov/astronomy.html.