Horizontal-branch stars as an age indicator

Surface temperature distribution of horizontal-branch (HB) stars is very sensitive to age in old stellar systems, which makes it an attractive age indicator. In this paper, we present the recent revision of our model calculations for the HE morphology of globular clusters. The result is more updated version of our earlier models (Lee, Demarque, & Zinn 1994), which suggests that the HE morphology is more sensitive (similar to 40%) to age. We also present our new model calculations on the effect of HE stars in dating old stellar systems using the H beta index. Our results indicate that the effect of HE stars is rather strong, and suggest a possibility for the systematic difference in age between the globular clusters in the Milky Way Galaxy and those in giant elliptical galaxies. Finally, we compare these results with the relative ages estimated from our far-UV dating technique for giant elliptical galaxies.