GEMS JWST: Hold on to your HATS(-6 b), a sub-solar metallicity giant planet with water, methane and ammonia in its atmosphere

Abstract

HATS-6 b is one of several recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ($\mathrm{T_{eq}}\sim700$ K), Saturn-mass ($M_p\sim0.3~\mathrm{M_J}$), Jupiter-radius ($R_p\sim1~\mathrm{R_J}$) planet that transits its star every $\sim$ 3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of $0.6-5.3$ um. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low metallicity ($\log\mathrm{[M/H]}=-1.99^{+0.2}_{-0.2}$) sub-solar C/O ($\log\mathrm{[C/O]=-0.46^{+0.2}_{-0.2}}$) atmosphere, and find strong evidence for H$_2$O, CH$_4$, and NH$_3$ at volume mixing ratios (in $\log[X]$) of $-4.88_{-0.24}^{+0.25}$, $-5.38_{-0.19}^{+0.18}$, and $-6.03_{-0.19}^{+0.18}$, respectively. We consistently retrieve a significantly lower $\mathrm{T_{eq}}$ than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a non-zero bond albedo. Our planetary interior models retrieve bulk metallicities three orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well-mixed. We find an excess feature around 3 um, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C$_2$H$_4$. Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.