Prospects for Using H-α Transits to Probe Escaping Atmospheres
Abstract
The recently-observed dearth of super-Earths with radii 1.8 times that of Earth is a member of a rare class of discoveries: observational confirmation of a clear theoretical prediction. Two separate groups predicted this feature using models of photoevaporative atmospheric loss. Since the discovery of the super-Earth radius gap, these models have been used to constrain properties of this planetary population such as the core mass distribution, with exciting results. As the quantitative results of photoevaporation models become more important (and as photoevaporation is compared to core-powered mass loss, a new competing theory for the source of energy driving escape), improved observational constraints on photoevaporation models are sorely needed. I will present new results showing that, though transits in hydrogen's H-α line have thus far provided limited information about escaping atmospheres, for thoughtfully-chosen planetary samples, this line has exciting potential. Most direct observations of photoevaporation in action have been conducted using transits in hydrogen's Lyman-α line. Because this line's center is obscured by ISM absorption, these observations primarily provide information about outflowing gas far from the planet, making mass loss rate calculations model dependent. For most planets currently observed to have escaping gas, the fraction of escaping hydrogen in the n=2 state is too small for significant H-α absorption. I will present new theoretical calculations showing that for planets experiencing a larger XUV flux, recombination cascades can populate the n=2 state at an observable level. I will comment on these results in the context of A-stars such as Kelt 9, flaring M-stars, and young stars, and attempt to convince conference attendees that additional observational campaigns are warranted in the conveniently accessible from the ground H-α line.
- Publication:
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AAS/Division for Extreme Solar Systems Abstracts
- Pub Date:
- August 2019
- Bibcode:
- 2019ESS.....440002M