SYNTHETIC ABSORPTION-LINE STRENGTHS IN STAR-FORMING REGIONS, AND THE EFFECTS ON THE HELIUM ABUNDANCE DETERMINATION

Spectral evolutionary calculations have been employed in order to foll ow the behaviour of absorption lines in single-burst star forming regi ons. The model calculations are based on theoretical evolutionary trac ks and model atmosphere stellar spectra at 5 Angstrom resolution and t hey include the pre-main sequence phase. Three different metallicities have been considered; Z = 0.001, 0.008 and 0.020, this range covers e asily the one occupied by blue compact dwarf galaxies. This analysis f ocuses on the post-active burst phase and follows the evolution up to 15 million years after the episode of star formation has ceased, using an instantaneous burst of star formation. The absorption lines includ ed are hydrogen, neutral and once ionized helium. Non-LTE absorption l ine profiles have been implemented to these lines, also at a 5 Angstro m resolution. The equivalent widths and line profiles are studied as a function of lower and upper mass limits and slope of the mass functio n as well as the age. It is shown that the equivalent widths of the Ba lmer lines are increasing during the course of evolution, while the st rength of the neutral and ionized helium lines decreases monotonically . Both the hydrogen and helium lines show high sensitivity to variatio ns in the initial mass function (IMF). The Balmer line equivalent widt hs range from about 1.5 to 6.5 Angstrom, depending on the IMF paramete rs used. As anticipated, these numbers are rather insensitive to the m etallicity. Contrary to what has been claimed, the Balmer lines are pr ominent even if the lower mass limit is as high as 10 M.. It has been proposed that the weak hydrogen lines sometimes seen in starburst gala xies, often interpreted as evidence for a deficiency in low mass stars , could be due to the finite lifetimes of pre-main sequence stars. Thi s analysis shows that it is very much dependent on the slope of the ma ss function. The equivalent width of the HeI lambda 4471 line never ex ceeds 0.35 Angstrom for any choice of the IMF parameters while the equ ivalent width of the HeII lambda 4686 line reaches at the most 0.20 An gstrom using the most top-heavy IMF. As expected, these numbers are on ly slightly dependent on the metallicity in the optical spectral regio n.