Transmission spectroscopy of the lowest-density gas giant: metals and a potential extended outflow in HAT-P-67b

TL;DR

This study analyzes the atmosphere of the lowest-density gas giant, HAT-P-67b, revealing atomic species, potential atmospheric escape, and variability suggestive of an extended outflow, advancing understanding of ultra-low-density exoplanet atmospheres.

Contribution

First atmospheric analysis of HAT-P-67b, the lowest-density gas giant, detecting atomic species and signs of atmospheric escape with high-resolution spectroscopy.

Findings

Detection of Ca II and Na I with high significance

Evidence of extended planetary outflow from H$ ext{alpha}$ and He I variability

Estimated mass loss rate of $ ext{10}^{13}$ g/s and outflow temperature of $ ext{9900 K}$

Abstract

Extremely low-density exoplanets are tantalizing targets for atmospheric characterization because of their promisingly large signals in transmission spectroscopy. We present the first analysis of the atmosphere of the lowest-density gas giant currently known, HAT-P-67 b. This inflated Saturn-mass exoplanet sits at the boundary between hot and ultrahot gas giants, where thermal dissociation of molecules begins to dominate atmospheric composition. We observed a transit of HAT-P-67 b at high spectral resolution with CARMENES and searched for atomic and molecular species using cross-correlation and likelihood mapping. Furthermore, we explored potential atmospheric escape by targeting H$\alpha$ and the metastable helium line. We detect Ca II and Na I with significances of 13.2$\sigma$ and 4.6$\sigma$, respectively. Unlike in several ultrahot Jupiters, we do not measure a day-to-night wind. The large line depths of Ca II suggest that the upper atmosphere may be more ionized than models predict. We detect strong variability in H$\alpha$ and the helium triplet during the observations. These signals suggest the possible presence of an extended planetary outflow that causes an early ingress and late egress. In the averaged transmission spectrum, we measure redshifted absorption at the $\sim 3.8\%$ and $\sim 4.5\%$ level in the H$\alpha$ and He I triplet lines, respectively. From an isothermal Parker wind model, we derive a mass loss rate of $\dot{M} \sim 10^{13}~\rm{g/s}$ and an outflow temperature of $T \sim 9900~\rm{K}$. However, due to the lack of a longer out-of-transit baseline in our data, additional observations are needed to rule out stellar variability as the source of the H$\alpha$ and He signals.