Hubble Space Telescope NICMOS observations of NGC 1333: The ratio of stars to substellar objects

We present an analysis of NICMOS photometry and low-resolution grism spectroscopy of low-mass stars and substellar objects in the young star-forming region NGC 1333. Our goal is to constrain the ratio of low-mass stars to substellar objects down to 20M_Jup in the cluster, as well as constrain the cluster initial mass function (IMF) down to 30M_Jup, in combination with a previous survey of NGC 1333 by Wilking et al. Our survey covers four fields of 51.2" × 51.2", centered on brown dwarf candidates previously identified in Wilking et al. We extend previous work based on the use of a water vapor index for spectral typing to wavelengths accessible with NICMOS on the Hubble Space Telescope. Spectral types were derived for the 14 brightest objects in our fields, ranging from ≤M0 to M8, which, at the age of the cluster (0.3 Myr), correspond to a range in mass of ≥0.25-0.02 M _⊙. In addition to the spectra, we present an analysis of the color-magnitude diagram using pre-main-sequence evolutionary models of D'Antona & Mazzitelli. Using an extinction-limited sample, we derive the ratio of low-mass stars to brown dwarfs. Comparisons of the observed ratio to that expected from the field IMF of Chabrier indicate that the two results are consistent. We combine our data with those of Wilking et al. to compute the ratio of intermediate-mass stars (0.1-1.0 M_⊙) to low-mass objects (0.03-0.1 M_⊙) in the cluster. We also report the discovery of a faint companion to the previously confirmed brown dwarf ASR 28, as well as a possible outflow surrounding ASR 16. If the faint companion is confirmed as a cluster member, it would have a mass of ≈5M_Jup (mass ratio 0.15) at a projected distance of 350 AU, similar to that of 2MASS 1207-3923B.