ARES IV: Probing the Atmospheres of the Two Warm Small Planets HD 106315c and HD 3167c with the HST/WFC3 Camera

We present an atmospheric characterization study of two medium-sized planets bracketing the radius of Neptune: HD 106315c (R_P = 4.98 ± 0.23 R_⊕) and HD 3167c ( ${R}_{{\rm{P}}}={2.740}_{-0.100}^{+0.106}$ R_⊕). We analyze spatially scanned spectroscopic observations obtained with the G141 grism (1.125-1.650 μm) of the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. We use the publicly available Iraclis pipeline and TauREx3 atmospheric retrieval code and detect water vapor in the atmosphere of both planets, with an abundance of ${\mathrm{log}}_{10}[{{\rm{H}}}_{2}{\rm{O}}]=-{2.1}_{-1.3}^{+0.7}$ (∼5.68σ) and ${\mathrm{log}}_{10}[{{\rm{H}}}_{2}{\rm{O}}]=-{4.1}_{-0.9}^{+0.9}$ (∼3.17σ) for HD 106315c and HD 3167c, respectively. The transmission spectrum of HD 106315c also shows possible evidence of ammonia absorption ( ${\mathrm{log}}_{10}[{\mathrm{NH}}_{3}]=-{4.3}_{-2.0}^{+0.7},\sim 1.97\sigma $ , even if it is not significant), while carbon dioxide absorption features may be present in the atmosphere of HD 3167c in the ∼1.1-1.6 μm wavelength range ( ${\mathrm{log}}_{10}[{\mathrm{CO}}_{2}]=-{2.4}_{-1.0}^{+0.7}$ , ∼3.28σ). However, the CO₂ detection appears significant, and it must be considered carefully and put into perspective. Indeed, CO₂ presence is not explained by 1D equilibrium chemistry models, and it could be due to possible systematics. The additional contributions of clouds, CO, and CH₄ are discussed. HD 106315c and HD 3167c will be interesting targets for upcoming telescopes such as the James Webb Space Telescope and the Atmospheric Remote-sensing Infrared Exoplanet Large-survey. ^* ARES: ARIEL Retrieval of Exoplanets School.