On the ultraviolet-bright phase of metal-rich horizontal-branch stars

We consider the origin of the UV-bright phase of metal-rich helium-burning stars, the slow blue phase (SBP) that was predicted by various earlier 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 grid of evolutionary tracks, we provide an analytical understanding of the main characteristics of the SBP phenomenon. The SBP is slow because it is a slow-evolving, helium-shell-burning phase that is analogous to the early asymptotic giant branch phase. The SBP of a more metal-rich 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 luminosity from the hydrogen-burning shell is extremely sensitive to the mean molecular weight μ, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive ΔY/ΔZ, helium abundance plays the most important role in governing μ, and thus Dorman et al. found that the SBP occurs only when Y ≳ 0.4 when ΔY/ΔZ ≳ 0. We suggest that the SBP phenomenon is a major cause of the UV upturn phenomenon 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://shemesh.gsfc.nasa.gov/astronomy.html.