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 extensiveprevious work, we present spectra for 12 of the faintest candidates fromour deep multi-band imaging, plus seven random objects in the samefields, using MOIRCS on Subaru. We confirm seven new sources as youngvery low mass objects (VLMOs), with T eff of 2400-3100 K andmid-M to early-L spectral types. These objects add to the growing censusof VLMOs in NGC 1333, now totaling 58. Three confirmed objects (onefound in this study) have masses below 15 M Jup, according toevolutionary models, thus are likely planemos. We estimate the totalplanemo 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 lsim 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.