Kavli Affiliate: Sara Seager
| Summary:
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 $μm$ 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 textttpetitRADTRANS, yields an effective temperature = 564$pm$4 K, surface gravity $logg$ = 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$^+8.4_-6.0$ $M_mathrmJup$, which is in agreement with the mass range (19–27 $M_mathrmJup$) 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.
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