Massive star populations and the IMF in metal-rich starbursts

We present new spectroscopic observations of Mkn 309, a starburst galaxy wi th one of the largest WR populations known. A highly super solar metallicit y of 12 + log(O/H) similar to 9.3-9.4 is derived. Using additional objects from Guseva et al. (2000) we analyse a sample of five metal-rich ([OM] > 0) WR galaxies with the main goal of constraining the basic properties of the massive star populations (IMF slope, M-up) and the star formation history (age, burst duration) of these objects by quantitative comparisons with evo lutionary synthesis models. The following main results are obtained: - The observations are well explained by extended bursts of star formation with durations Deltat similar to 4-10 Myr seen at ages of 7-15 Myr or a sup erposition of several bursts with age differences of similar to 4-10 Myr in cluding a young (less than or similar to 5 Myr) burst. This naturally expla ins both the observed WR populations (incLuding WN and WC stars) and the pr esence of red supergiants. The burst durations, somewhat longer compared to those derived in other WR galaxies using the same models (Schaerer et al. 1999a), are plausible in view of the physical sizes of the observed regions and the nature and morphology of our objects (nuclear starbursts), and pos e no fundamental physical problem. - The SEDs in the optical range are very well reproduced for all objects, p rovided the stellar light suffers from a smaller extinction than that of th e gas (derived from the Balmer decrement). This confirms earlier findings f rom studies combining UV-optical data of other starburst galaxies. - All the considered observational constraints are compatible with a Salpet er IMF extending to masses M-up greater than or similar to 40 M.. Adopting st conservative approach we derive a lower limit of M-up greater than or si milar to 30 M. for the Salpeter IMF. From more realistic assumptions on the metallicity and SF history we favour a lower limit M-up greater than or si milar to 30-40 M., which is also in agreement with HP equivalent width meas urements of metal-rich H II regions in spiral galaxies indicating an upper mass cut-off of at least similar to 35-50 M. Steep IMF slopes (alpha greate r than or similar to 3.3) are very unlikely. The uncertainties of our results are discussed. We compare our findings to other work on massive star populations and the IMF in similar environments. We stress the importance of direct analysis of stellar populations compare d to other indirect methods based on properties of ionized gas to constrain the IMF in metal-rich starbursts.