To cool is to keep: residual H/He atmospheres of super-Earths and sub-Neptunes
نویسندگان
چکیده
Super-Earths and sub-Neptunes are commonly thought to have accreted hydrogen/helium envelopes, consisting of a few ten percent their total mass, from the primordial gas disk. Subsequently, hydrodynamic escape driven by core-powered mass-loss and/or photo-evaporation likely stripped much these envelopes lower-mass closer-in planets form super-Earth population. In this work we show that after undergoing mass-loss, some super-Earths can retain small residual H/He envelopes. This retention is possible because, for significantly depleted atmospheres, density at radiative-convective boundary drops sufficiently such thhe cooling time-scale becomes less than time-scale. The envelope therefore able contract, terminating further mass loss. Using analytic calculations numerical simulations, retained as fraction planet's $f_{ret}$, increases with increasing planet $M_{c}$, decreases equilibrium temperature, $T_{eq}$, scaling $f_{ret} \propto M_{c}^{3/2} T_{eq}^{-1/2} \exp{[M_{c}^{3/4} T_{eq}^{-1}]}$. $f_{ret}$ varies $< 10^{-8}$ about $10^{-3}$ typical parameters. To first order, exact amount left-over depends on initial its envelope's opacity. These hydrogen reduce atmosphere's mean molecular weight compared purely secondary atmosphere, signature observable current future facilities. remnant atmospheres may, however, in many cases be vulnerable long-term erosion photo-evaporation. Any plays an important role physical evolution super-Earths, including geology geochemistry.
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ژورنال
عنوان ژورنال: Monthly Notices of the Royal Astronomical Society
سال: 2021
ISSN: ['0035-8711', '1365-8711', '1365-2966']
DOI: https://doi.org/10.1093/mnras/stab895