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A Comprehensive Model of Sub-Neptune Structure, Cooling, and Atmospheric Escape

Presentation #100.03 in the session Plenary 1.

Published onJun 20, 2022
A Comprehensive Model of Sub-Neptune Structure, Cooling, and Atmospheric Escape

The population of planets below 4 Earth radii is split into two populations: sub-Neptunes, which are inferred to have a H/He envelope atop the core, and super-Earths, which are bare rock/iron cores without an envelope. One possible explanation is that some sub-Neptunes undergo strong atmospheric escape that strips away their entire H/He envelope, eventually evolving into a super-Earth. However, the relative roles of stellar XUV-driven escape, long-term “core-powered” mass loss from interior cooling, and post-formation “boil-off” at young ages, is still unclear. As we gain more statistical knowledge about sub-Neptunes we need a comparable theoretical understanding of their interior and atmospheric physics.

We have developed a new python-based one-dimensional interior and evolution model with XUV-driven escape and boil-off. We employ a more sophisticated atmospheric structure model with variable surface gravity encompassing the deep atmosphere to the escaping wind region. We find that the irradiated atmosphere is a far more important component for sub-Neptune radius evolution than previously appreciated. We use the model to provide new assessments of H/He mass fractions as a function of planet radius and mass, finding lower implied H/He mass fractions than previous work, in particular for large radius “super puffs”. With our model the presence of “super-puffs” can be explained, as only a small amount of H/He material is sufficient for a low mass planet to attain a large radius. Moreover, we demonstrate that the early “boil-off” only allows a planet to start photoevaporation/core-powered mass loss below a certain H/He mass fraction. Therefore, we suggest it is important to include the “boil-off” into sub-Neptune evolution as the observed population distribution contains valuable information on such processes. Finally, we provide an update on coupling the evolution code with a state-of-the-art 1D hydrodynamic mass-loss code, which for the first time will be able to treat XUV-driven, core-powered, and boil off mass loss within one model, to assess the relative roles of each process.

Figure caption: A general picture for sub-Neptune evolution under photoevaporation. All planets have a 3.6 Me core orbiting around a sun like star but with different incident bolometric fluxes (orbital separations). The initial envelope mass fraction is determined by our “boil-off” model. The planets under high incident fluxes can not survive the XUV-driven mass loss, and eventually evolve into a super-Earth (the radius of these bare cores is shown in the dotted line).

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