Ovi+ Lyβ + Ciifrom Starburst and Poststarburst Galaxies. I. Stellar Library and Evolutionary Synthesis Profiles

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Abstract
Evolutionary synthesis models of a stellar population in the far-ultraviolet are presented. The spectra include the lines O VI λλ1032, 1038, Lyβ, and C II λλ1036, 1037. They are based on a stellar library built with observations of O and B stars collected 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 + Lyβ + C II is computed for different star formation histories (instantaneous burst and continuous star formation) and different assumptions about the initial mass function (IMF). The metallicity is near the solar value. O VI λλ1032, 1038 is a very sensitive indicator of the presence or absence of O stars. O VI develops a P Cygni profile when formed in stellar winds of the most massive stars. When these stars are absent, no O VI is formed. In contrast, Lyβ and C II are very sensitive indicators for B stars. If these stars dominate, as is the case in poststarburst galaxies, Lyβ and C II are present as strong absorption features, and they are formed in the photosphere of B stars. An equivalent width of Lyβ + C II larger than 1 Å always indicates a population younger than 1 Gyr. Because of the universal strength of O VI in O stars, O VI is not a good discriminator between instantaneous versus continuous star formation for ages in the starburst phase, but the absence of O VI and the presence of stellar Lyβ and C II is a good indicator of a short burst duration and for the galaxy being in a poststarburst phase. Application of this technique to starburst or poststarburst galaxies will require careful attention to interstellar absorption.