JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b Reveals Clouds and Possible Metal Enrichment

Abstract

Characterizing the coldest directly imaged companions through direct spectroscopy has only recently become possible with the James Webb Space Telescope. We present moderate-resolution (R ∼ 2700) spectroscopic observations of the directly imaged planetary-mass companion (PMC), GJ 504 b, using the JWST/NIRSpec. The coldest imaged PMC of the pre-JWST era GJ 504 b is too faint for ground-based spectroscopy, with only photometric observations possible. Leveraging advanced postprocessing techniques with a forward-modeling framework, we detect the companion at high signal-to-noise (S/N > 300). We also present the first successful point-spread function (PSF) subtraction with angular differential imaging (ADI) in the NIRSpec point cloud, detecting GJ 504 b at S/N > 10 and reaching contrast limits <10−4. The extracted 2.9–5.3 μm spectra show strong signatures of several molecular species, including H2O, 12C16O, CH4, CO2, NH3, H2S, 13C16O, and 12C18O. Atmospheric modeling of the spectra using petitRADTRANS yields an effective temperature = 564 ± 4 K, surface gravity (Formula presented) logg = 4.87 (Formula presented) −0.12+0.13, metallicity [M/H] = 0.67 (Formula presented) −0.12+0.13, C/O ratio = 0.64 (Formula presented) −0.02+0.02, interstellar 12C/13C and 16O/18O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2 (Formula presented) −6.0+8.4 MJup, which is in agreement with the mass range (19–27 MJup) obtained from ATMO evolutionary models, implying an age of 2.5–4.0 Gyr. Lastly, we compare the abundances of GJ 504 b to its primary, obtaining a stellar abundance of sulfur (S), superstellar carbon (C), and, possibly, oxygen (O). The observed metal enrichment tentatively supports a planetlike formation, but does not entirely exclude stellar abundances for GJ 504 b.