The Stellar Population and Star Clusters in the Unusual Local Group Galaxy IC 10

We present analysis of HST U,V,I, and Halpha images of the peculiar LocalGroup irregular galaxy IC 10. The images are used to determine the nature ofthe stellar products in a portion of the recent starburst in this galaxy. Weidentified 13 stellar associations and clusters, two of which are probably old(>350 Myr) and the rest of which are young (4-30 Myr) and presumably formed inthe starburst. We found the following: 1) The slope of the stellar initial massfunction (IMF) for 6.3--18 Msolar stars formed in the starburst lies betweentwo limiting cases: a value of -1.9+/-0.4 under the assumption of coevalityover the past 13 Myr and of -0.9+/-0.3 under the assumption of constant starformation over the past 40 Myr. Thus, most likely, the IMF of the intermediatemass stars is not very unusual. The slope of the IMF for the underlying galaxypopulation under the assumption of constant star formation is -2.6+/-0.3 for4.8-18 Msolar stars. 2) The lower stellar mass limit in the starburst is <6.3Msolar. This constraint is less than some predictions of what lower stellarmass limits might be in starbursts, but higher than others. 3) There are twomodest-sized Halpha shells that could easily have been produced in the past fewMyr by the clusters they encircle. 4) The dominant mode of star formation inthe starburst has been that of a scaled-up OB association. This mode, with afew compact clusters sprinkled in, is similar to the star formation that tookplace in Constellation III in the LMC, as well as that in the Blue CompactDwarfs IZw18 and VIIZw403. The starburst in this part of IC10 has not produceda super star cluster. We suggest that the high WC/WN ratio could be reproducedif there were small, well-synchronized (<1 Myr), but widely scattered, pocketsof secondary star formation 3-4 Myr ago.Comment: To be published in ApJ, September 20, 200