UV driven evaporation of close-in planets: energy-limited, recombination-limited, and photon-limited flows
File(s)
Published version
Author(s)
Owen, James E
Alvarez, Marcelo A
Type
Journal Article
Abstract
We have investigated the evaporation of close-in exoplanets irradiated by ionizing photons. We find that the properties of the flow are controlled by the ratio of the recombination time to the flow timescale. When the recombination timescale is short compared to the flow timescale, the flow is in approximate local ionization equilibrium with a thin ionization front where the photon mean free path is short compared to the flow scale. In this "recombination-limited" flow the mass-loss scales roughly with the square root of the incident flux. When the recombination time is long compared to the flow timescale the ionization front becomes thick and encompasses the entire flow with the mass-loss rate scaling linearly with flux. If the planet's potential is deep, then the flow is approximately "energy-limited"; however, if the planet's potential is shallow, then we identify a new limiting mass-loss regime, which we term "photon-limited." In this scenario, the mass-loss rate is purely limited by the incoming flux of ionizing photons. We have developed a new numerical approach that takes into account the frequency dependence of the incoming ionizing spectrum and performed a large suite of 1D simulations to characterize UV driven mass-loss around low-mass planets. We find that the flow is "recombination-limited" at high fluxes but becomes "energy-limited" at low fluxes; however, the transition is broad occurring over several orders of magnitude in flux. Finally, we point out that the transitions between the different flow types do not occur at a single flux value but depend on the planet's properties, with higher-mass planets becoming "energy-limited" at lower fluxes.
Date Issued
2016-01-01
Date Acceptance
2015-11-06
Citation
Astrophysical Journal, 2016, 816 (1)
ISSN
0004-637X
Publisher
American Astronomical Society
Journal / Book Title
Astrophysical Journal
Volume
816
Issue
1
Copyright Statement
© 2016. The American Astronomical Society. All rights reserved.
Subjects
0201 Astronomical And Space Sciences
0305 Organic Chemistry
0306 Physical Chemistry (Incl. Structural)
Astronomy & Astrophysics
Publication Status
Published
Article Number
34
Date Publish Online
2015-12-29