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

Abstract

We investigate the low-mass population of the young cluster IC 348 down to the deuterium-burning limit, a —ducial boundary between brown dwarf and planetary mass objects, using a new and innovative method for the spectral classi—cation of late-type objects. Using photometric indices, constructed from HST /NICMOS narrowband imaging, that measure the strength of the 1.9 km water band, we determine the spectral type and reddening for every M-type star in the —eld, thereby separating cluster members from the interloper population. Due to the efficiency of our spectral classi—cation technique, our study is complete from D0.7 to The mass function derived for the cluster in this interval, dN/ 0.015 M _ . d log M P M0.5, is similar to that obtained for the Pleiades, but appears signi—cantly more abundant in brown dwarfs than the mass function for companions to nearby Sunlike stars. This provides compelling observational evidence for diUerent formation and evolutionary histories for substellar objects formed in isolation versus as companions. Because our determination of the IMF is complete to very low masses, we can place interesting constraints on the role of physical processes such as fragmentation in the star and planet formation process and the fraction of dark matter in the Galactic halo that resides in sub- stellar objects. Subject headings: stars: late-typestars: low-mass, brown dwarfs ¨ stars: luminosity function, mass functionstars: premain-sequence