Thursday, August 18, 2016

What *IS* the Radius of hot Neptune Corot-24b?

Identifying the "true" radius of the hot sub-Neptune CoRoT-24b by mass loss modelling


Lammer et al


For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii RT of 3.7±0.4 R⊕ and 4.9±0.5 R⊕, and masses of ≤5.7 M⊕ and 28±11 M⊕, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming RT≈ RPL, where RPL is the planetary radius at the pressure of 100 mbar, we obtained for CoRoT-24b unrealistically high thermally-driven hydrodynamic escape rates. This is due to the planet's high temperature and low gravity, independent of the stellar EUV flux. Such high escape rates could last only for less than 100 Myr, while RPL shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, RPL must be therefore located at ≈1.9−2.2 R⊕ and high altitude hazes/clouds possibly extinct the light at RT. Our analysis constraints also the planet's mass to be 5−5.7 M⊕. For CoRoT-24c, RPL and RT lie too close together to be distinguished in the same way. Similar differences between RPL and RT may be present also for other hot, low-density sub-Neptunes.

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