X-ray irradiation of three planets around Hyades star K2-136

Abstract

We study the X-ray irradiation and likely photoevaporation of the three planets around the star K2-136. These are the Earth-sized K2-136 b, the mini-Neptune K2-136 c, and the super-Earth K2-136 d. XMM-Newton observations of the star indicate an X-ray luminosity of $(1.18\pm 0.1)\times 10^{28}$ erg s${}^{-1}$ in the range 0.15 to 2.4 keV, resulting in an activity of $L_X/L_{\mathrm{bol}}=(1.80\pm 0.68)\times 10^{-5}$. The evaporation past of the planets were modelled using the XUV stellar tracks by Johnstone et al. (2021), the energy limited mass loss formulation (Lecavelier des Etangs et al. 2007, Erkaev et al. 2007), and the thermal evolution formulation by Lopez & Fortney (2014). Our results suggest that planets b and d are most probably purely rocky and have been stripped of their envelopes. On the other hand, planet c likely still has an envelope consisting of 0.8% of the total mass, with its core being relatively large (2.3 $R_{\mathrm{E}}$).

arXiv commentThis paper has been submitted as a proceedings article to Astronomische Nachrichten (AN) as part of the XMM-Newton 2021 Science Workshop

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