Ross 458 C: gas giant or brown dwarf?

Ross 458 C is a widely separated planetary mass companion at a distance of 1100 au from its host binary, Ross 458 AB. It is a member of a class of very low-mass companions at distances of hundreds to thousands of astronomical units from their host stars. We aim to constrain Ross 458 C’s formation history by fitting its near-IR spectrum with models to constrain its composition. If its composition is similar to its host star, we infer that it likely formed through turbulent fragmentation of the same molecular cloud that formed the host. If its composition is enhanced in heavy elements relative to the host, this lends evidence to formation in the disk and subsequent migration to its current separation. Here, we present high-resolution (R ∼ 2700) emission spectra of Ross 458 C obtained with JWST NIRSpec’s fixed slit in the F070LP, F100LP, and F170LP filters from 0.8 to 3.1 μm. We fit these spectra using both grids of forward models (Sonora Bobcat, Sonora Elf Owl, and ExoREM) and atmospheric retrievals (POSEIDON). We also constrain the composition of Ross 458 AB by fitting an archival SpeX spectrum with PHOENIX forward models. The forward model grids prefer an enhanced atmospheric metallicity for Ross 458 C relative to the host, but our retrievals return a metallicity consistent with the host within 1σ. Our results offer new insights into the formation history of Ross 458 C, as well as the efficacy of fitting forward model grids versus retrievals to derive atmospheric properties of directly imaged companions.