The Temperature Distribution of Horizontal Branch Stars: Methods and First Results

Abstract

As part of a large project aimed at characterizing the ultraviolet (UV) properties of globular clusters, we present here a theoretical and observational analysis aimed at setting the framework for the determination of horizontal branch (HB) temperature distributions. Indeed this is crucial information needed to understand the physical parameters shaping the HB morphology in globular clusters and to interpret the UV emission from unresolved stellar systems. We found that the use of zero age HB color-T <SUB>eff</SUB> relations is a robust way to derive effective temperatures of individual HB stars. We investigated the most suitable colors for temperature estimates and the effect on the color-T <SUB>eff</SUB> relations of variations of the initial chemical composition and the evolution off the zero age HB. As a test case, we applied our color-T <SUB>eff</SUB> calibrations to the Galactic globular cluster M15. The photometry of M15 has been obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. The HB of M15 turned out to have a multimodal distribution, with a main component peaking at {T<SUB>eff</SUB>} ∼ {8000} K and confined below {T<SUB>eff</SUB>} ∼ {10,000} K. The second component peaks at {T<SUB>eff</SUB>} ∼ {14,000} K and extends up to {T<SUB>eff</SUB>} ∼ {20,000} K. The vast majority ( ~ 95%) of the HB stars in M15 is below 20,000 K, in agreement with the lack of a well populated extreme HB observed in other metal-poor globular clusters. We also verified that the temperatures derived with our analysis are consistent with spectroscopic estimates available in the literature. <P />Based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.