Kavli Affiliate: Sara Seager
| Summary:
It is unclear how directly imaged substellar companions with masses near the deuterium burning limit form, because these objects are rare and their bulk properties are not diagnostic of their formation. In this paper we revisit this problem using JWST/NIRCam coronagraphic images of the 29 Cygni (=HIP 99770) system that reveal the recently-discovered super-Jovian companion 29 Cyg b at wavelengths covering 4-5$μ$m for the first time. This object has an uncertain mass that straddles the deuterium burning limit ($M_rm bsimeq15pm5,M_rm J$) and a low mass ratio with its early-type host star ($M_rm b/M_starsim0.01$). Absorption from CO$_2$ and CO is apparent at 4.3 and 4.6$μ$m in our images. The strength of the CO$_2$ feature relative to CO provides strong evidence, based on empirical comparison with literature observations at these wavelengths and atmospheric modeling, that the companion is enriched in heavier elements compared to the roughly solar abundances of the host ($Z_rm b/Z_star=3pm2$). In addition, we measure the stellar inclination angle with CHARA/PAVO interferometry: the system is consistent with spin-orbit alignment at the $2,σ$ level, with $Δi=12pm6^circ$. This ensemble of evidence is suggestive of formation within the protoplanetary disk and rapid accretion of metal-rich material, versus disk fragmentation or capture like higher mass ratio companions. 29 Cyg b shows that planet formation around early-type stars can occur on scales at or exceeding the deuterium burning limit, in agreement with the recently revised planetary mass/metallicity trend that predicts $Z_rm pl/Z_star=3.3pm0.5$ at high masses from transiting planet densities (Chachan et al. 2025).
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