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 $\sim$ 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 $\mathtt{\text{petitRADTRANS}}$, yields an effective temperature = 564$\pm$4 K, surface gravity $\log g$ = 4.87${}_{-0.12}^{+0.13}$, metallicity [M/H] = 0.67${}_{-0.12}^{+0.13}$, 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${}_{-6.0}^{+8.4}$ $M_{\mathrm{J}\mathrm{u}\mathrm{p}}$, which is in agreement with the mass range (19–27 $M_{\mathrm{J}\mathrm{u}\mathrm{p}}$) 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 ©, and possibly, oxygen (O). The observed metal enrichment tentatively supports planet-like formation, but does not entirely exclude stellar abundances for GJ 504 b.