A Census of the Young Cluster IC 348

We present a new census of the stellar and substellar members of the youngcluster IC 348. We have obtained images at I and Z for a 42'x28' fieldencompassing the cluster and have combined these measurements with previousoptical and near-infrared photometry. From spectroscopy of candidate clustermembers appearing in these data, we have identified 122 new members, 15 ofwhich have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015M_sun by recent evolutionary models. The latest census for IC 348 now containsa total of 288 members, 23 of which are later than M6 and thus are likely to bebrown dwarfs. From an extinction-limited sample of members (A_V<=4) for a16'x14' field centered on the cluster, we construct an IMF that is unbiased inmass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units wherethe Salpeter slope is 1.35, the mass function for IC 348 rises from high massesdown to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, andthen declines into the substellar regime. In comparison, the similarly-derivedIMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peaknear 0.8 M_sun and steadily declines to lower masses. The distinctive shapes ofthe IMFs in IC 348 and Taurus are reflected in the distributions of spectraltypes, which peak at M5 and K7, respectively. These data provide compelling,model-independent evidence for a significant variation of the IMF withstar-forming conditions.