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

Author(s): Helmut Lammer, N V Erkaev, Luca Fossati, I. Juvan, P Odert, Colin Johnstone, Theresa Rank-Lüftinger, Manuel Güdel
Abstract: For the hot exoplanets CoRoT-24b and CoRoT-24c, observations have provided transit radii R_T of 3.7 ± 0.4R_⊕ and 4.9 ± 0.5R_⊕, and masses of ≤ 5.7M_⊕ and 28 ± 11M_⊕, respectively. We study their upper atmosphere structure and escape applying an hydrodynamic model. Assuming R_T ≈ R_PL, where R_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_PL shrinks till the escape rate becomes less than or equal to the maximum possible EUV-driven escape rate. For CoRoT-24b, R_PL must be therefore located at ≈1.9-2.2R_⊕ and high altitude hazes/clouds possibly extinct the light at RT. Our analysis constraints also the planet's mass to be 5-5.7M_⊕. For CoRoT-24c, R_PL and R_T lie too close together to be distinguished in the same way. Similar differences between R_PL and R_T may be present also for other hot, low-density sub-Neptunes.
Organisation(s): Department of Astrophysics
External organisation(s): Österreichische Akademie der Wissenschaften (ÖAW), Russian Academy of Sciences, Karl-Franzens-Universität Graz
Journal: Monthly Notices of the Royal Astronomical Society: Letters
Volume: 461
Pages: L62 - L66
No. of pages: 5
ISSN: 1745-3925
DOI: https://doi.org/10.1093/mnrasl/slw095
Publication date: 09-2016
Peer reviewed: Yes
Austrian Fields of Science 2012: 103003 Astronomy, 103004 Astrophysics
Keywords
ASJC Scopus subject areas: Astronomy and Astrophysics, Space and Planetary Science
Portal url: https://ucrisportal.univie.ac.at/en/publications/c7555e1a-c588-403a-a8a8-e8bb49047860

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