A Revised Density Estimate for the Largest Known Exoplanet, HAT-P-67 b

Abstract

Low-density ($\ddot{I}<0.1\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$) hot Saturns are expected to quickly ($<100$ Myr) lose their atmospheres due to stellar irradiation, explaining their rarity. HAT-P-67 b seems to be an exception, with $\ddot{I}<0.09\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$ and maintaining its atmosphere to well after 1 Gyr. We present a photometric and spectroscopic follow-up of HAT-P-67 b to determine how it avoided mass loss. HAT-P-67 b orbits a $V=10.1$ evolved F-type star in a 4.81 day orbit. We present new radial velocity observations of the system from the NEID spectrograph on the WIYN 3.5m Telescope from a follow-up campaign robust to stellar activity. We characterize the activity using photometry and activity indicators, revealing a stellar rotation period ($5.40\pm 0.09$ d) near HAT-P-67 b's orbital period. We mitigate the stellar activity using a constrained quasi-periodic Gaussian process through a joint fit of archival ground-based photometry, TESS photometry, and our NEID observations, obtaining a planetary mass of $M_p=0.45\pm 0.15M_{\mathrm{J}}$. Combined with a radius measurement of $R_p=2.140\pm 0.025R_{\mathrm{J}}$, this yields a density of $\ddot{I}{}_p=0.061_{-0.021}^{+0.020}\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, making HAT-P-67 b the second lowest-density hot giant known to date. We find the recent evolution of the host star caused mass loss for HAT-P-67 b to only recently occur. The planet will be tidally disrupted/engulfed in $\sim 150-500$ Myr, shortly after losing its atmosphere. With rapid atmospheric mass loss, a large, helium leading tail, and upcoming observations with the Hubble Space Telescope, HAT-P-67 b is an exceptional target for future studies, for which an updated mass measurement provides important context.

25 pages, 17 figures, 6 tables. Accepted to AJ. v2 updates tidal evolution analysis with correct stellar age