Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b. (arXiv:2011.11553v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Charnay_B/0/1/0/all/0/1">Benjamin Charnay</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blain_D/0/1/0/all/0/1">Doriann Blain</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bezard_B/0/1/0/all/0/1">Bruno Bézard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leconte_J/0/1/0/all/0/1">Jérémy Leconte</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Turbet_M/0/1/0/all/0/1">Martin Turbet</a>
Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune
K2-18b were interpreted as the presence of water vapour with potential water
clouds. 1D modelling studies also predict the formation of water clouds at some
conditions. However, such models cannot predict the cloud cover, driven by
atmospheric dynamics and thermal contrasts, and thus their real impact on
spectra. The main goal of this study is to understand the formation,
distribution and observational consequences of water clouds on K2-18b and other
temperate sub-Neptunes. We simulated the atmospheric dynamics, water cloud
formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We
analysed the impact of atmospheric composition (with metallicity from 1*solar
to 1000*solar), concentration of cloud condensation nuclei and planetary
rotation rate. Assuming that K2-18b has a synchronous rotation, we show that
the atmospheric circulation in the upper atmosphere essentially corresponds to
a symmetric day-to-night circulation. This regime preferentially leads to cloud
formation at the substellar point or at the terminator. Clouds form for
metallicity >100*solar with relatively large particles. For 100-300*solar
metallicity, the cloud fraction at the terminators is small with a limited
impact on transit spectra. For 1000*solar metallicity, very thick clouds form
at the terminator. The cloud distribution appears very sensitive to the
concentration of CCN and to the planetary rotation rate. Fitting HST transit
data with our simulated spectra suggests a metallicity of ~100-300*solar. In
addition, we found that the cloud fraction at the terminator can be highly
variable, leading to a potential variability in transit spectra. This effect
could be common on cloudy exoplanets and could be detectable with multiple
transit observations.
Hubble (HST) spectroscopic transit observations of the temperate sub-Neptune
K2-18b were interpreted as the presence of water vapour with potential water
clouds. 1D modelling studies also predict the formation of water clouds at some
conditions. However, such models cannot predict the cloud cover, driven by
atmospheric dynamics and thermal contrasts, and thus their real impact on
spectra. The main goal of this study is to understand the formation,
distribution and observational consequences of water clouds on K2-18b and other
temperate sub-Neptunes. We simulated the atmospheric dynamics, water cloud
formation and spectra of K2-18b for H2-dominated atmosphere using a 3D GCM. We
analysed the impact of atmospheric composition (with metallicity from 1*solar
to 1000*solar), concentration of cloud condensation nuclei and planetary
rotation rate. Assuming that K2-18b has a synchronous rotation, we show that
the atmospheric circulation in the upper atmosphere essentially corresponds to
a symmetric day-to-night circulation. This regime preferentially leads to cloud
formation at the substellar point or at the terminator. Clouds form for
metallicity >100*solar with relatively large particles. For 100-300*solar
metallicity, the cloud fraction at the terminators is small with a limited
impact on transit spectra. For 1000*solar metallicity, very thick clouds form
at the terminator. The cloud distribution appears very sensitive to the
concentration of CCN and to the planetary rotation rate. Fitting HST transit
data with our simulated spectra suggests a metallicity of ~100-300*solar. In
addition, we found that the cloud fraction at the terminator can be highly
variable, leading to a potential variability in transit spectra. This effect
could be common on cloudy exoplanets and could be detectable with multiple
transit observations.
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