Abstract
The highly anticipated launch of JWST will open up the possibility of
comprehensively measuring the emission spectra of rocky exoplanets orbiting M
dwarfs to detect and characterize their atmospheres. In preparation for this
opportunity, we present model atmospheres for three M-dwarf planets
particularly amenable to secondary eclipse spectroscopy -- TRAPPIST-1b, GJ
1132b, and LHS 3844b. Using three limiting cases of candidate atmospheric
compositions (pure H2O, pure CO2 and solar abundances) we calculate
temperature-pressure profiles and emission and reflection spectra in
radiative-convective equilibrium, including the effects of a solid surface at
the base of the atmosphere. Our results differ appreciably from simpler
parameterized models of super-Earth atmospheres in terms of the overall
temperatures and the temperature gradient, which has important observational
consequences. We find that the atmospheric radiative transfer is significantly
influenced by the cool M-star irradiation; H2O and CO2 absorption bands in the
near-infrared are strong enough to absorb a sizeable fraction of the incoming
stellar light at low pressures, which leads to temperature inversions in the
upper atmosphere. The non-gray band structure of gaseous opacities in the
infrared is hereby an important factor. Opacity windows are muted at higher
atmospheric temperatures, so we expect temperature inversions to be common only
for sufficiently cool planets. We also find that pure CO2 atmospheres exhibit
lower overall temperatures and stronger reflection spectra compared to models
of the other two compositions. We estimate that for GJ 1132b and LHS 3844b we
should be able to distinguish between different atmospheric compositions with
JWST. The emission lines from the predicted temperature inversions are
currently hard to measure, but high resolution spectroscopy with future ELTs
may be able to detect them.
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