Massive star formation and evolution in starburst galaxies: Mid-infrared spectroscopy with the ISO short wavelength spectrometer

We present new Infrared Space Observatory Short Wavelength Spectrometer dat a for a sample of 27 starburst galaxies, and with these data we examine the issues of formation and evolution of the most massive stars in starburst g alaxies. Using starburst models which incorporate time evolution, new stell ar atmosphere models for massive stars, and a starburst model geometry deri ved from observations of the prototypical starburst M82, we model the integ rated mid-infrared line ratio [Ne III](15.6 mu m)/[Ne II](12.8 mu m). This line ratio is sensitive to the hardness of the stellar energy distribution and therefore to the most massive stars present. We conclude from our model s, with consideration of recent determinations of the stellar census in loc al, high-mass star-forming regions, that the [Ne III]/[Ne II] ratios we mea sure are consistent with the formation of massive (similar to 50-100 M.) st ars in most starbursts. In this framework, the low nebular excitation infer red from the measured line ratios can be attributed to aging effects. By in cluding estimates of the ratio of infrared-to-Lyman continuum luminosity fo r the galaxies in our sample, we further find that most starbursts are rela tively short-lived (10(6)-10(7) yr), only a few O star lifetimes. We discus s a possible cause of such short events: the effectiveness of stellar winds and supernovae in destroying the starburst environment.