Revisiting the atmosphere of the exoplanet 51 Eridani b with VLT/SPHERE

Abstract

[Full abstract in the paper] We aim to better constrain the atmospheric properties of the directly imaged exoplanet 51~Eri~b by using a retrieval approach on higher signal-to-noise data than previously reported. In this context, we also compare the results of using the atmospheric retrieval code \texttt{petitRADTRANS} vs a self-consistent model to fit atmospheric parameters. We present a higher signal-to-noise $YH$ spectrum of the planet and revised $K1K2$ photometry (M${}_{K1}=15.11\pm 0.04$ mag, M${}_{K2}=17.11\pm 0.38$ mag). The best-fit parameters obtained using an atmospheric retrieval differ from previous results using self-consistent models. In general, we find that our solutions tend towards cloud-free atmospheres (e.g. log $\ddot{I}{„}_{\mathrm{clouds}}=-5.20\pm 1.44$). For our ``nominal'' model with new data, we find a lower metallicity ([Fe/H] $=0.26\pm$0.30 dex) and C/O ratio ($0.38\pm 0.09$), and a slightly higher effective temperature (T${}_{\mathrm{eff}}=807\pm$45 K) than previous studies. The surface gravity (log $g=4.05\pm 0.37$) is in agreement with the reported values in the literature within uncertainties. We estimate the mass of the planet to be between 2 and 4 M${}_{\mathrm{Jup}}$. When comparing with self-consistent models, we encounter a known correlation between the presence of clouds and the shape of the $P-T$ profiles. Our findings support the idea that results from atmospheric retrievals should not be discussed in isolation, but rather along with self-consistent temperature structures obtained using the retrieval's best-fit parameters.

Accepted for publication in A&A. 21 pages, 7 figures in the main text and 9 figures in the Appendix