O-VI-BETA+C-II FROM STARBURST AND POSTSTARBURST GALAXIES .1. STELLAR LIBRARY AND EVOLUTIONARY SYNTHESIS PROFILES(LY)

Evolutionary synthesis models of a stellar population in the far-ultra violet are presented. The spectra include the lines O VI lambda lambda 1032, 1038, Ly beta, and C II lambda lambda 1036, 1037. They are base d on a stellar library built with observations of O and B stars collec ted with Copernicus and the Hopkins Ultraviolet Telescope (HUT). This library is used as input into an evolutionary synthesis code. The line profile of O VI + Ly beta + C II is computed for different star forma tion histories (instantaneous burst and continuous star formation) and different assumptions about the initial mass function (IMF). The meta llicity is near the solar value. O VI lambda lambda 1032, 1038 is a ve ry sensitive indicator of the presence or absence of O stars. O VI dev elops a P Cygni profile when formed in stellar winds of the most massi ve stars. When these stars are absent, no O VI is formed. In contrast, Ly beta and C II are very sensitive indicators for B stars. If these stars dominate, as is the case in poststarburst galaxies, Ly beta and C II are present as strong absorption features, and they are formed in the photosphere of B stars. An equivalent width of Ly beta + C II lar ger than 1 Angstrom always indicates a population younger than 1 Gyr. Because of the universal strength of O vr in O stars, O VI is not a go od discriminator between instantaneous versus continuous star formatio n for ages in the starburst phase, but the absence of O VI and the pre sence of stellar Ly beta and C II is a good indicator of a short burst duration and for the galaxy being in a poststarburst phase. Applicati on of this technique to starburst or poststarburst galaxies will requi re careful attention to interstellar absorption.