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Title: The Maximum Mass-loss Efficiency for a Photoionization-driven Isothermal Parker Wind

Observations of present-day mass-loss rates for close-in transiting exoplanets provide a crucial check on models of planetary evolution. One common approach is to model the planetary absorption signal during the transit in lines like Hei10830 with an isothermal Parker wind, but this leads to a degeneracy between the assumed outflow temperatureT0and the mass-loss rateṀthat can span orders of magnitude inṀ. In this study, we re-examine the isothermal Parker wind model using an energy-limited framework. We show that in cases where photoionization is the only heat source, there is a physical upper limit to the efficiency parameterεcorresponding to the maximal amount of heating. This allows us to rule out a subset of winds with high temperatures and large mass-loss rates as they do not generate enough heat to remain self-consistent. To demonstrate the utility of this framework, we consider spectrally unresolved metastable helium observations of HAT-P-11b, WASP-69b, and HAT-P-18b. For the former two planets, we find that only relatively weak (Ṁ1011.5g s−1) outflows can match the metastable helium observations while remaining energetically self-consistent, while for HAT-P-18b all of the Parker wind models matching the helium data are self-consistent. more » Our results are in good agreement with more detailed self-consistent simulations and constraints from high-resolution transit spectra.

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Publication Date:
Journal Name:
The Astrophysical Journal
Page Range or eLocation-ID:
Article No. 96
DOI PREFIX: 10.3847
Sponsoring Org:
National Science Foundation
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