The stellar population and star clusters in the unusual Local Group galaxy IC 101

We present an analysis of Hubble Space Telescope U, V, I, and H alpha images of the peculiar Local Group irregular galaxy IC 10. The images are used to determine the nature of the stellar products in a portion of the recent starburst in this galaxy. We identified 13 stellar associations and clusters, two of which are probably old (greater than or equal to 350 Myr) and the rest of which are young (4-30 Myr) and presumably formed in the starburst. We found the following: (1) The slope of the stellar initial mass function (IMF) for 6.3-18 M-. stars formed in the starburst lies between two limiting cases: a value of - 1.9 +/- 0.4 under the assumption of coevality over the past 13 Myr and of -0.9 +/- 0.3 under the assumption of constant star formation over the past 40 Myr for Z = 0.004 (- 2.1 +/- 0.4 and - 1.0 + 0.04, respectively, for Z = 0.008). Thus, most likely, the IMF of the intermediate-mass stars is not very unusual. The slope of the IMF for the underlying galaxy population under the assumption of constant star formation is -2.6 +/- 0.3 for 4.8-18 M-. stars assuming Z = 0.004 (-2.3 +/- 0.3 for Z = 0.008) and is unusually steep. (2) The lower stellar mass limit in the starburst is <6.3 M-.. This constraint is less than some predictions of what lower stellar mass limits might be in starbursts but higher than others. (3) There are two modest-sized H alpha shells (similar to 50 pc diameter) that easily could have been produced in the past few Myr by the clusters they encircle. (4) The dominant mode of star formation in the starburst has been that of a scaled-up OB association. This mode, with a few compact clusters sprinkled in, is similar to the star formation that took place in Constellation III in the LMC, as well as that in the blue compact dwarfs I Zw 18 and VII Zw 403. The starburst in this part of IC 10 has not produced a superstar cluster. We also compare the high WC/WN ratio to evolutionary models and discuss possible explanations. The high ratio can be reproduced if there were small, well-synchronized (Delta tau less than or equal to 1 Myr), but widely scattered, pockets of secondary star formation 3-4 Myr ago.