ATLAS OF SYNTHETIC ULTRAVIOLET-SPECTRA OF MASSIVE STAR POPULATIONS

An atlas of synthetic ultraviolet spectra of a population of massive s tars is presented. The spectra are based on a stellar library of IUE h igh-dispersion spectra of O and Wolf-Rayet stars, coupled to an evolut ionary synthesis code. Later spectral types are included via low-dispe rsion spectra. Line profiles of N V lambda 1240, Si V lambda 1400, C I V lambda 1550, He II lambda 1640, and N IV lambda 1720 have been compu ted for star-formation histories and initial mass functions typically found in starburst regions. It is found that the lines are sensitive i ndicators for the presence or absence of massive stars. C IV lambda 15 50 is the strongest stellar line in the ultraviolet spectrum of a typi cal starburst. If O stars with zero-age main-sequence masses above 50 M circle dot are present, C IV always shows a P Cygni profile. In the absence of such stars, only a blue-shifted absorption is present. Duri ng later epochs of the starburst, when late-O/early-B stars dominate, an unshifted photospheric absorption appears. Si IV lambda 1400 shows a conspicuous wind profile when luminous O supergiants are present. A strong P Cygni profile is found only for an instantaneous burst observ ed at 3 to 5 Myr, which has a top-heavy IMF. The velocity of the blues hifted absorption is strongly correlated with the age and the upper cu toff mass (or slope) of the IMF. N V lambda 1240 traces the most massi ve stars and behaves rather similar to Si IV lambda 1400. Its usefulne ss as an indicator of very massive stars is limited due to the strong blending effect of the nearby Ly alpha line. Nevertheless, strong N V lambda 1240 emission in a starburst suggests the presence of stars wit h masses in excess of 60 M circle dot. He II lambda 1640 and N IV lamb da 1720 are produced by very hot and luminous O and Wolf-Rayet stars. Both lines can have weak absorption or emission in a typical starburst but are predicted to be observable only under rare circumstances, suc h as in an instantaneous burst at t approximate to 3 Myr. The profiles presented in the atlas can be compared to high-quality ultraviolet ob servations of galaxies with active star formation in order to constrai n the massive star population. The atlas is published in its entirety in computer-readable form in the AAS CD-ROM series, Vol. 5.