2008AJ....136.1372O

We present the results of a deep near-infrared (NIR) imaging survey searching for very low mass young stellar objects (YSOs) in the embedded cluster associated with the Perseus molecular cloud. Our observations cover an area of ∼5'x 5' in the NGC 1333-S region at J-, H-, and Ks-bands. The 10σ limiting magnitudes exceed 18 mag in all three bands. Based on NIR color-color diagrams, embedded YSO candidates were identified using NIR excesses. The derived frequency of these YSO candidates with NIR excess emission among all the detected sources is 58⁺¹⁰_–13%. The higher frequency of YSOs with NIR excesses implies that NGC 1333 is an active and young star-forming region. Approximately half of the YSO candidates exhibit extremely low luminosity, indicating very low mass. Combining the reddening-corrected luminosity with theoretical evolutionary models, the low-luminosity YSO candidates are considered to be young substellar-mass objects. Furthermore, some sources could have planetary masses. In addition, we carried out CO molecular-line observations of the same cloud region. The results suggested that the YSO candidates are likely formed on the side of the parent molecular cloud and star-formation activity could be high. The K-band luminosity function of all the detected sources revealed a significant population of extremely low-luminosity sources in NGC 1333. The reddening-corrected J-band luminosity function of the YSO candidates does not clearly decline to the completeness limit and seems to be bimodal. We also argue that the fraction of substellar objects is larger than those in other young clusters and the mass function of the YSO candidates appears to be increasing toward the substellar-mass regime, similar to that of dust clumps. It implies that substellar-mass distributions may depend on the initial conditions of the molecular cloud.