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

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 about 2,700) spectroscopic observations of the directly imaged planetary-mass companion (PMC), GJ 504 b, using the JWST /NIRSpec. As 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 post-processing techniques with a forward modeling framework, we detect the companion at high signal-to-noise (S/N > 300). We also present the first successful 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 mu m spectra show strong signatures of several molecular species, including H 2 O, 12 C 16 O, CH 4, CO 2, NH 3, H 2 S, 13 C 16 O, and 12 C 18 O. Atmospheric modeling of the spectra using petitRADTRANS, yields an effective temperature = 564 plus or minus 4 K, surface gravity = 4.87+0.13-0.12, metallicity [M/H] = 0.67+0.13-0.12, C/O ratio = 0.64+0.02-0.02, interstellar 12 C/ 13 C and 16 O/ 18 O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2+8.4-6.0 M Jup, which is in agreement with the mass range (19--27 M Jup) 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), super-stellar carbon (C), and possibly, oxygen (O). The observed metal enrichment tentatively supports planet-like formation, but does not entirely exclude stellar abundances for GJ 504 b.