Variation of Hot Horizontal‐Branch Populations with Metallicity

Abstract

We investigate how distributions of blue horizontal-branch (BHB) stars depend on metallicity. As HB distributions probably reflect the effects of red giant branch (RGB) mass loss (see, e.g., R. T. Rood, ApJ, 184, 815 [1973]; F. R. Ferraro et al., ApJ, 500, 311 [1998]), we attempt to study mass-loss efficiency as a function of metallicity for the HB through three projects. Using our stellar evolution code (B. Dorman, ApJS, 81, 221 [1992]), we computed RGB models including mass loss, over a wide range in metallicity. We assume mass loss followed Reimers’s formula (D. Reimers, AA N. L. D’Cruz et al. ApJ, 466, 359 [1996]). Such stars either form helium white dwarfs or undergo a helium flash at high temperatures forming extreme HB (EHB) stars. The hot flashing EHB stars form a “blue hook” at the blue end of the zero-age HB. EHB stars have envelope masses and 0.05 M, radiate copiously in the ultraviolet (UV). To understand how the number of EHB stars varies with metallicity in a stellar population we considered how the zero-age horizontal branch (ZAHB) is populated. We assumed the distribution of ZAHB stars to be driven by a distribution in the mass-loss efficiency (parameterized by ) rather than by a distribution in mass. hR For globular cluster abundances, the range of producing hR EHB stars was comparable to that producing normal HB stars. As metallicity is increased, the range and magnitude of hR corresponding to EHB stars varies slightly, whereas the range of producing mid-HB stars becomes very small, which imhR plies that the HB will be bimodal for a uniform distribution in that goes to sufficiently large values. hR To explore how might vary with metallicity, we obtained hR fiber spectra of BHB stars in the globular cluster q Centauri with the 3.9 m Anglo-Australian Telescope. We were able to obtain Teff, , and [Ca/H] for stars in the 7500–8200 K log g range. We find that these stars contain more lower metallicity stars than q Cen’s giants and RR Lyrae variables. Metal-rich stars may lie preferentially at higher temperatures. We used far-UV (1600 A) and V-band Hubble Space Telescope (HST) images to study the structure of the large population of hot HB stars discovered by the Ultraviolet Imaging Telescope in q Cen (J. H. Whitney et al., AJ, 108, 1350 [1994]). Figure 1 shows the color-magnitude diagram of the data. The higher quality HST data confirm the presence of sub-HB stars at the hot end of the HB. These appear to be the “blue hook” stars predicted by our evolutionary models. EHB stars and their progeny make up 30% of the HB population. The HB is underpopulated for . The location 3.35 (1600 V ) 0.35 0