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We present an analysis of NICMOS photometry and low-resolution grismspectroscopy of low-mass stars and sub-stellar objects in the youngstar-forming region NGC 1333. Our goal is to constrain the ratio of low-massstars to sub- stellar objects down to 20 Mjup in the cluster as well asconstrain the cluster IMF down to 30 Mjup in combination with a previous surveyof NGC 1333 by Wilking et al. Our survey covers 4 fields of 51.2" x 51.2",centered on brown dwarf candidates previously identified in Wilking et al. Weextend previous work based on the use of a water vapor index for spectraltyping to wavelengths accessible with NICMOS on the HST. Spectral types werederived for the 14 brightest objects in our fields, ranging from <M0 - M8,which at the age of the cluster (0.3 Myr) corresponds to a range in mass of>0.25 - 0.02 Msun. In addition to the spectra, we present an analysis of thecolor-magnitude diagram using pre-main sequence evolutionary models of D'Antona& Mazzitelli. Using an extinction-limited sample, we derive the ratio oflow-mass stars to brown dwarfs. Comparisons of the observed ratio to thatexpected from the field IMF of Chabrier indicate that the two results areconsistent. We combine our data with that of Wilking et al. to compute theratio of intermediate-mass stars (0.1 - 1.0 Msun) to low-mass objects (0.03 -0.1 Msun) in the cluster. We also report the discovery of a faint companion tothe previously confirmed brown dwarf ASR 28, as well as a possible outflowsurrounding ASR 16. If the faint companion is confirmed as a cluster member, itwould have a mass of ~ 5 Mjup (mass ratio 0.15) at a projected distance of 350AU, similar to 2MASS 1207-3923 B.Comment: 33 pages, 6 figures, accepted for publication by A

Hubble Space TelescopeNICMOS Observations of NGC 1333: The Ratio of Stars to Substellar Objects