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'x28' 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 0.08-0.015 M_sun 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<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). 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_sun, 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_sun 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.Comment: 47 pages, 14 figures, 3rd para of 4.5.3 has been added, this is final version in press at ApJ, also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm

We determine and tabulate A [λ] /A K , the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25µm ≤ λ ≤ 8.0µm along two lines of sight: l = 42 • and l = 284 • . The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49 as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8µm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A [λ] − A K )/(A J − A K ) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g. A J /A K , is required to calculate A [λ] /A K from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sightlines, and fit the reddening as a function of magnitude (distance) to determine A J /kpc, A K /kpc, and A J /A K . Our values of A [λ] /A K show a flattening across the 3-8µm wavelength range, roughly consistent with the Lutz et al. (1996) extinction measurements derived for the sightline toward the Galactic center.

We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (∼ 3 Myr) σ Orionis cluster. The populous nature of this cluster makes it a good target for statistically-significant studies of disk emission as a function of mass. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks (like classical T Tauri stars), class I (protostellar) candidates, and stars with "evolved disks"; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. There appears to be a spatial extension of infrared excess sources to the north-east, which may be associated with the young (< 1 Myr) embedded cluster NGC 2024. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ∼10%

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