A census of the young cluster IC 348

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42' x 28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of similar to0.08-0.015 M-circle dot by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A(V) less than or equal to 4) for a 16' x 14' field centered on the cluster, we construct an initial mass function (IMF) that is unbiased in mass and nearly complete for M/M-circle dot greater than or equal to 0.03 (less than or similar toM8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M-circle dot, and then declines into the substellar regime. In comparison, the similarly derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M-circle dot and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.