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arXiv · 1605.03595

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

H. Lammer·N. V. Erkaev·L. Fossati·I. Juvan·P. Odert·P. E. Cubillos·E. Guenther·K. G. Kislyakova·C. P. Johnstone·T. Lueftinger·M. Guedel
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Abstract

For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii R$_{\rm T}$ of 3.7$\pm$0.4 R$_{\oplus}$ and 4.9$\pm$0.5 R$_{\oplus}$, and masses of $\le$5.7 M$_{\oplus}$ and 28$\pm$11 M$_{\oplus}$, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming R$_{\rm T} \approx$ R$_{\rm PL}$, where R$_{\rm PL}$ 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 $<$100 Myr, while R$_{\rm PL}$ shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, R$_{\rm PL}$ must be therefore located at $\approx 1.9-2.2$ R$_{\oplus}$ and high altitude hazes/clouds possibly extinct the light at R$_{\rm T}$. Our analysis constraints also the planet's mass to be 5$-$5.7 M$_{\oplus}$. For CoRoT-24c, R$_{\rm PL}$ and R$_{\rm T}$ lie too close together to be distinguished in the same way. Similar differences between R$_{\rm PL}$ and R$_{\rm T}$ may be present also for other hot, low-density sub-Neptunes.

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H. Lammer, N. V. Erkaev, L. Fossati, I. Juvan, P. Odert, P. E. Cubillos, E. Guenther, K. G. Kislyakova, C. P. Johnstone, T. Lueftinger, M. Guedel. 2016-05-11. Identifying the "true" radius of the hot sub-Neptune CoRoT-24b by mass loss modelling. https://doi.org/10.1093/mnrasl/slw095

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