On the UV 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, 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 which is analogous to the early AGB phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if their Teff's are the same 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 whereas the luminosity from the helium-burning core is not. 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.