SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC): THE BOTTOM OF THE INITIAL MASS FUNCTION IN NGC 1333*

摘要

SONYC—Substellar Objects in Nearby Young Clusters—is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. Here, we present the first results from SONYC observations of NGC 1333, a ~1?Myr old cluster in the Perseus star-forming complex. We have carried out extremely deep optical and near-infrared imaging in four bands (i', z', J, K) using Subaru Prime Focus Camera and Multi-Object InfraRed Camera and Spectrograph (MOIRCS) instruments at the Subaru telescope. The survey covers 0.25?deg2 and reaches completeness limits of 24.7?mag in the i' band and 20.8?mag in the J band. We select 196 candidates with colors as expected for young, very low mass objects. Follow-up multi-object spectroscopy with MOIRCS is presented for 53 objects. We confirm 19 objects as likely brown dwarfs (BDs) in NGC 1333, seven of them previously known. Nine additional objects are classified as possible stellar cluster members, likely with early to mid M spectral types. The confirmed objects are strongly clustered around the peak in the gas distribution and the core of the cluster of known stellar members. For 11 of them, we confirm the presence of disks based on Spitzer/Infrared Array Camera photometry. The effective temperatures for the BD sample range from 2500?K to 3000?K, which translates to masses of ~0.015-0.1 M ☉, based on model evolutionary tracks. For comparison, the completeness limit of our survey translates to mass limits of 0.004 M ☉ for AV 5?mag or 0.008 M ☉ for AV 10?mag. Compared with other star-forming regions, NGC 1333 shows an overabundance of BDs relative to low-mass stars, by a factor of 2-5. On the other hand, NGC 1333 has a deficit of planetary-mass objects: based on the surveys in σ?Orionis, the Orion Nebula Cluster and Chamaeleon I, the expected number of planetary-mass objects in NGC 1333 is 8-10, but we find none. It is plausible that our survey has detected the minimum mass limit for star formation in this particular cluster, at around 0.012-0.02 M ☉. If confirmed, our findings point to significant regional/environmental differences in the number of BDs and the minimum mass of the initial mass function.