Planetary parameters, XUV environments and mass-loss rates for nearby gaseous planets with X-ray detected host-stars

Abstract

We leverage Gaia DR2 parallactic distances to deliver new or revised estimates of planetary parameters and X-ray irradiation for a distance-limited ($\lesssim 100$ pc) sample of 27 gaseous planets (from super-Earths to hot Jupiters) with publicly available Chandra and/or XMM observations, for which we carry out a homogeneous data reduction. For 20 planets with X-ray detected host stars we make use of the photoionization hydrodynamics code ATES to derive updated atmospheric mass outflow rates. The newly derived masses/radii are not consistent with the exoplanet.eu values for five systems; HD 149026b and WASP-38, for mass; and Au Mic b, HAT-P-20 and HAT-P-2 for radii. Notably, the lower mass implies a (Saturn-like) density of $0.86\pm 0.09$ g cm${}^{-3}$ for HD 149026 b. This independent estimate is consistent with the lowest values reported in the literature. Separately, we report on the X-ray detection of GJ 9827, HD 219134 and LHS 1140 for the first time. The inferred stellar X-ray luminosity of LHS 1140 ($1.34_{-0.21}^{+0.19}\times 10^{26}$ erg sec${}^{-1}$) implies that LHS 1140 b is the least irradiated transiting super-Earth known to orbit within the habitable zone of a nearby M-dwarf.

15 pages, 6 figures; accepted for publication in ApJ

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