The JWST/NIRISS Deep Spectroscopic Survey for Young Brown Dwarfs and Free-Floating Planets

Abstract: The discovery and characterization of free-floating planetary-mass objects (FFPMOs) is fundamental to our understanding of star and planet formation. Here we report results from an extremely deep spectroscopic survey of the young star cluster NGC1333 using NIRISS WFSS on the James Webb Space Telescope. The survey is photometrically complete to K~21, and includes useful spectra for objects as faint as K~20.5. The observations cover 19 known brown dwarfs, for most of which we confirm spectral types using NIRISS spectra. We discover six new candidates with L-dwarf spectral types that are plausible planetary-mass members of NGC1333, with estimated masses between 5-15 MJup. One, at ~5 MJup, shows clear infrared excess emission and is a good candidate to be the lowest mass object known to have a disk. We do not find any objects later than mid-L spectral type (M < ~4 MJup). The paucity of Jupiter-mass objects, despite the survey's unprecedented sensitivity, suggests that our observations reach the lowest mass objects formed like stars in NGC1333. Our findings put the fraction of FFPMOs in NGC1333 at ~10% of the number of cluster members, significantly more than expected from the typical log-normal stellar mass function. We also search for wide binaries in our images and report a young brown dwarf with a planetary-mass companion.

• The paper demonstrates the spectral confirmation of 19 known brown dwarfs and the discovery of 6 new free-floating planetary-mass candidates using JWST/NIRISS.
• The study achieves deep photometric completeness down to K ~ 21, establishing critical limits for observing substellar objects in NGC1333.
• The findings challenge conventional star formation models by revealing a significant fraction of free-floating planets alongside a scarcity of Jupiter-mass objects.

The JWST/NIRISS Deep Spectroscopic Survey for Young Brown Dwarfs and Free-Floating Planets

The paper, "The JWST/NIRISS Deep Spectroscopic Survey for Young Brown Dwarfs and Free-Floating Planets," offers a detailed account of an observational study using the James Webb Space Telescope (JWST) with its Near-Infrared Imager and Slitless Spectrograph (NIRISS) in slitless mode. The primary focus of this research is the identification and spectral characterization of free-floating planetary-mass objects (FFPMOs) and young brown dwarfs in the star-forming region NGC1333.

Key Observations and Findings

The spectroscopic survey involved observing the young star cluster NGC1333 and achieving a photometric completeness down to a magnitude of $K \sim 21$. The study provided spectral confirmation for 19 known brown dwarfs and identified six new candidates with spectral types that align with planetary-mass members, estimated to be between 5–15 Jupiter masses (M$_{\mathrm{Jup}}$). One of these candidates exhibits clear infrared excess, suggesting the presence of a circumstellar disk, marking it as potentially the lowest-mass object with an observed disk.

Notably, the survey does not detect any objects with spectral types later than mid-L, leading to an observed paucity of Jupiter-mass objects, despite the sensitivity of the survey to such low-mass objects. This suggests reaching the current limits of star-formation processes in NGC1333.

Implications of Survey Results

The implications of this survey are manifold, spanning both the practical aspects of observational astronomy with JWST and theoretical considerations about star and planet formation. The survey highlights a higher fraction of FFPMOs in NGC1333, around 10% of cluster members, which is more significant than expectations from a typical log-normal stellar mass function. Such findings challenge existing paradigms of star formation near the deuterium-burning limit.

These results suggest that while NGC1333 hosts a rich population of what are likely the lowest-mass objects formed like stars, it may harbor fewer ejected giant planets. This stands in contrast with other regions, like the Orion Nebula Cluster, where a higher fraction of FFPMOs is hypothesized, potentially indicating stronger environmental dependencies in planet ejection processes.

Prospective Future Developments

With the surveillance capabilities demonstrated by the JWST/NIRISS survey, there is vast potential for further exploration and understanding of substellar populations. Continued observation and improved spectra acquisition techniques could refine our comprehension of FFPMO characteristics and densities across various star-forming regions. This ongoing research could revolutionize our understanding of the mass function at the very low-end and inform the theoretical models of both stellar and planetary formation.

Moreover, the findings recommend collaborations to address the lack of T-dwarf FFPMOs and further investigate the processes behind star formation boundaries and planetary ejection. An integrated approach that combines numerical simulations with empirical data will be critical for unraveling the complex mechanisms that govern these celestial entities.

Conclusion

This deep spectroscopic survey carried out with JWST/NIRISS is pivotal in advancing our understanding of young brown dwarfs and FFPMOs. By expanding the observational frontier to the lowest mass objects and unraveling their environmental and formation complexities, it sets the stage for future breakthroughs in planetary science and star formation theory. The study emphasizes the imperativeness of utilizing cutting-edge technology like JWST for astronomical observations to gain insights into the fundamental processes of the universe.