ELemental abundances of Planets and brown dwarfs Imaged around Stars (ELPIS): I. Potential Metal Enrichment of the Exoplanet AF Lep b and a Novel Retrieval Approach for Cloudy Self-luminous Atmospheres

Abstract

AF Lep A+b is a remarkable planetary system hosting a gas-giant planet that has the lowest dynamical mass among directly imaged exoplanets. We present an in-depth analysis of the atmospheric composition of the star and planet to probe the planet's formation pathway. Based on new high-resolution spectroscopy of AF Lep A, we measure a uniform set of stellar parameters and elemental abundances (e.g., [Fe/H] = 0.27±0.31-0.27 \pm 0.31 dex). The planet's dynamical mass (2.80.5+0.62.8^{+0.6}_{-0.5} MJup_{\rm Jup}) and orbit are also refined using published radial velocities, relative astrometry, and absolute astrometry. We use petitRADTRANS to perform chemically-consistent atmospheric retrievals for AF Lep b. The radiative-convective equilibrium temperature profiles are incorporated as parameterized priors on the planet's thermal structure, leading to a robust characterization for cloudy self-luminous atmospheres. This novel approach is enabled by constraining the temperature-pressure profiles via the temperature gradient (dlnT/dlnP)(d\ln{T}/d\ln{P}), a departure from previous studies that solely modeled the temperature. Through multiple retrievals performed on different portions of the 0.94.20.9-4.2 μ\mum spectrophotometry, along with different priors on the planet's mass and radius, we infer that AF Lep b likely possesses a metal-enriched atmosphere ([Fe/H] >1.0> 1.0 dex). AF Lep b's potential metal enrichment may be due to planetesimal accretion, giant impacts, and/or core erosion. The first process coincides with the debris disk in the system, which could be dynamically excited by AF Lep b and lead to planetesimal bombardment. Our analysis also determines Teff800T_{\rm eff} \approx 800 K, log(g)3.7\log{(g)} \approx 3.7 dex, and the presence of silicate clouds and dis-equilibrium chemistry in the atmosphere. Straddling the L/T transition, AF Lep b is thus far the coldest exoplanet with suggested evidence of silicate clouds.Comment: AJ, in press. Main text: Pages 1-32, Figures 1-15, Tables 1-6. All figures and tables after References belong to the Appendix (Pages 32-58, Figures 16-20, Table 7). For supplementary materials, please refer to the Zenodo repository https://doi.org/10.5281/zenodo.826746

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