Ultraviolet Radiation from Evolved Stellar Populations -- I. Models

Abstract

This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short wavelength (900-3000 ˚A) observations. Our sequences have been computed for a range of masses which concentrates on models that begin their HB evolution very close to the hot end of the ZAHB. We have calculated tracks for three metal-poor compositions ([Fe=H] =�2:26;�1:48;�0:47 with [O=Fe]>0), for use with globular cluster observations. We have also chosen three metal rich compositions (Z=0:017 =Z, Z = 0:04;0:06) for use in the study of elliptical galaxy populations. For each of the two supermetal-rich compositions, for which the helium abundance is unconstrained by observation, we have computed two sets of sequences: one assuming no additional helium, and a second with a large enhancement (YHB=0:29 and 0.36 forZ = 0:04), and (YHB=0:29 and 0.46 for Z = 0:06). For each set of sequences our lowest ZAHB envelope masses (M0 env) are in the range