Horizontal Branch stars as AmFmHgMn stars.pdfVIP

a r X i v : 0 8 0 2 .1 7 0 7 v 1 [ a s t r o - p h ] 1 2 F e b 2 0 0 8 Contrib. Astron. Obs. Skalnate? Pleso 35, 1 – 10, (2005) Horizontal Branch stars as AmFm/HgMn stars G. Michaud 1,2, and J. Richer 1 1 De?partement de Physique, Universite? de Montre?al, C. P. 6128, Montre?al, PQ, H3C 3J7, CANADA (E-mail: michaudg@astro.umontreal.ca; jacques.richer@umontreal.ca) 2 LUTH, Observatoire de Paris, CNRS, Universite? Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, FRANCE Received: November 21, 2007; Accepted: November 30, 2007 Abstract. Recent observations and models for horizontal branch stars are briefly de- scribed and compared to models for AmFm stars. The limitations of those models are emphasized by a comparison to observations and models for HgMn stars. Key words: Stars: abundances –Stars: diffusion – Stars: Population II – Galaxy: globular clusters – Stars: radiative accelerations – Stars: Horizontal Branch 1. Introduction At around 16000 K, the Teff where the horizontal branch (HB) crosses the main–se- quence, HB stars have the same Teff and log g as HgMn stars. Both have very shal- low surface convection zones and they appear to have some abundance anomalies in common (Sargent Searle, 1967, 1968). At slightly cooler Teff, HB stars have a smaller log g and their convection zones become as deep or even deeper than those of AmFm stars. On the main–sequence, stellar evolution models assuming a mixed outer zone have been relatively successful at explaining AmFm stars (Richer et al., 2000; Michaud et al., 2005). Detailed models for HgMn stars are more complex and have involved NLTE calculations of individual species (see Michaud 1981 for a review but also Michaud et al. 1974 and Proffitt et al. 1999). We first very briefly (§ 2) describe stellar evolution calculations done with all effects of atomic diffusion including radiative accelerations, grad. We then show applications to Pop II stars (§3). We will briefly describe recent observations of abundance