Substellar Objects in Nearby Young Clusters (SONYC). VI. The Planetary-mass Domain of NGC 1333

Within the SONYC (Substellar Objects in Nearby Young Clusters) survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC 1333. Building upon our extensive previous work, we present spectra for 12 of the faintest candidates from our deep multi-band imaging, plus seven random objects in the same fields, using MOIRCS on Subaru. We confirm seven new sources as young very low mass objects (VLMOs), with T _eff of 2400-3100 K and mid-M to early-L spectral types. These objects add to the growing census of VLMOs in NGC 1333, now totaling 58. Three confirmed objects (one found in this study) have masses below 15 M _Jup, according to evolutionary models, thus are likely planemos. We estimate the total planemo population with 5-15 M _Jup in NGC 1333 is <~ 8. The mass spectrum in this cluster is well approximated by dN/dM vprop M ^-α, with a single value of α = 0.6 ± 0.1 for M < 0.6 M _⊙, consistent with other nearby star-forming regions, and requires α <~ 0.6 in the planemo domain. Our results in NGC 1333, as well as findings in several other clusters by ourselves and others, confirm that the star formation process extends into the planetary-mass domain, at least down to 6 M _Jup. However, given that planemos are 20-50 times less numerous than stars, their contribution to the object number and mass budget in young clusters is negligible. Our findings disagree strongly with the recent claim from a microlensing study that free-floating planetary-mass objects are twice as common as stars—if the microlensing result is confirmed, those isolated Jupiter-mass objects must have a different origin from brown dwarfs and planemos observed in young clusters.