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\title{OVI+L\boldmath{$\beta$}+CII FROM STARBURST AND POST-STARBURST 
GALAXIES.\\
~\\
I:~STELLAR LIBRARY AND EVOLUTIONARY SYNTHESIS PROFILES.}

\author{Rosa M. Gonz\'alez Delgado and Claus Leitherer\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218\\
\\
Electronic mail: gonzalez@stsci.edu; leitherer@stsci.edu\\
\and 
Timothy Heckman\/\thanks{Adjunct Astronomer at ST ScI}\\
\\
Department of Physics \& Astronomy\\
\\
The Johns Hopkins University\\
\\
Baltimore, MD 21218\\
\\
Electronic mail: heckman@pha.jhu.edu}

\pub{The Astrophysical Journal}
\recacc{3 March 1997}{20 May 1997}
\maketitle

\abstract{
Evolutionary synthesis models of a stellar population in the far-ultraviolet 
are presented. The spectra include the lines OVI $\lambda\lambda$1032,1038, 
L$\beta$ and CII $\lambda\lambda$1036,1037. They are based on a stellar 
library built with observations of O and B~stars collected with Copernicus and 
the Hopkins Ultraviolet Telescope (HUT). This library is used as input into 
an evolutionary synthesis code. The line profile of OVI+L$\beta$+CII 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-solar.

OVI $\lambda\lambda$1032,1038 is a very sensitive indicator of the presence 
or absence of O~stars. OVI develops a P~Cygni profile when formed in stellar 
winds of the most massive stars. When these stars are absent, no OVI is 
formed. In contrast, L$\beta$ and CII are very sensitive indicators for 
B~stars. If these stars dominate, as is the case in post-starburst galaxies, 
L$\beta$ and CII are present as strong absorption features, and they are 
formed in the photosphere of B~stars. An equivalent width of L$\beta$+CII 
larger than 1~\AA\ always indicates a population with age younger than 1~Gyr. 
Due to the universal strength of OVI in O~stars, OVI is not a good 
discriminator between instantaneous versus continuous star formation for 
ages in the starburst phase, but the absence of OVI and the presence of 
stellar L$\beta$ and CII is a good indicator of a short burst duration, and 
for the galaxy being in a post-starburst phase. Application of this technique 
to starburst or post-starburst galaxies will require careful attention to
interstellar absorption.}