From Stars to Superplanets: The Low‐Mass Initial Mass Function in the Young Cluster IC 348

We investigate the low-mass population of the young cluster IC348 down to thedeuterium-burning limit, a fiducial boundary between brown dwarf and planetarymass objects, using a new and innovative method for the spectral classificationof late-type objects. Using photometric indices, constructed from HST/NICMOSnarrow-band imaging, that measure the strength of the 1.9 micron water band, wedetermine the spectral type and reddening for every M-type star in the field,thereby separating cluster members from the interloper population. Due to theefficiency of our spectral classification technique, our study is complete fromapprox 0.7 Msun to 0.015 Msun. The mass function derived for the cluster inthis interval, dN/dlogM \propto M^{0.5}, is similar to that obtained for thePleiades, but appears significantly more abundant in brown dwarfs than the massfunction for companions to nearby sun-like stars. This provides compellingobservational evidence for different formation and evolutionary histories forsubstellar objects formed in isolation vs. as companions. Because ourdetermination of the IMF is complete to very low masses, we can placeinteresting constraints on the role of physical processes such as fragmentationin the star and planet formation process and the fraction of dark matter in theGalactic halo that resides in substellar objects.Comment: 37 pages, 16 figs, 6 tables (Table 4 is a separate LaTeX file) Accepted for publication in Astrophysical Journal (Oct 1, 2000 issue