Abundance anomalies in hot horizontal branch stars of the galactic globular cluster NGC 1904

Abstract

We present abundance measurements, based on high resolution optical spectroscopic data obtained with the UVES at VLT, for 10 stars in the blue horizontal branch (BHB) of the Galactic globular cluster NGC 1904 (M79). In agreement with previous findings for other clusters, we obtain normal abundances for stars cooler than Teff ∼ 11000 K, and largely anomalous abundances for hotter stars: large He depletions and overabundances of Fe, Ti, Cr, P and Mn. The abundances of Mg, Si and Ca are roughly normal, in the hot stars as well as in the cooler ones. This abundance pattern can be attributed to the onset of diffusion and to radiation pressure in the stable atmospheres of hot HB stars. A possibly related discontinuity in the stellar rotation rate seems also to occur at Teff ∼ 11000 K. © 2006 Springer-Verlag Berlin Heidelberg.