2025/04/18 by Gavin Wang, William O. Balmer, Wang, Gavin +18 · 1 voice
Physics and Astronomy · #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research #Astro and Planetary Science
paper · pdf · doi:10.48550/arxiv.2504.13997
Low-density (ρ< 0.1 \rm~g~cm-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 ρ< 0.09 \rm~g~cm-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±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 Mp = 0.45 ± 0.15~M\rm J. Combined with a radius measurement of Rp=2.140 ± 0.025~R\rm J, this yields a density of ρp = 0.061+0.020-0.021 \rm~g~cm-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 ∼ 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.