K2-370 b: a strongly irradiated sub-Neptune transiting a very active solar-type star

ABSTRACT We report on the detailed characterization of K2-370 b, a transiting sub-Neptune on a 2.14-d orbit around the chromospherically active G-type dwarf HD 284521 ($T_\mathrm{eff} = 5662\pm 44$ K, $\lt \log R^\prime _{\rm HK}\gt =-4.49$). The system parameters are derived based on a global fit to K2, TESS and CHEOPS photometry, and HARPS-N and HARPS radial velocities (RVs). A Gaussian process regression analysis is performed simultaneously to the orbital fit of the RVs of K2-370 to effectively model the strong stellar rotation signal with a period of $13.5\pm 0.05$ d and measure the planetary RV signal with semi-amplitude $K_{\rm b}=5.6\pm 0.7$ m s$^{-1}$. We find that K2-370 b has a radius of $2.67\pm 0.05$ ${\rm R}_{\rm{\oplus }}$ and a mass of $11.1\pm 1.4$ ${\rm M}_{\rm{\oplus }}$. With an estimated equilibrium temperature $T_\mathrm{eq}\sim 1480$ K, K2-370 b is the second-hottest sub-Neptune with a highly precise mass determination around primaries with $T_\mathrm{eff}\gt 5500$ K. The resulting density of $3.2\pm 0.4$ g cm$^{-3}$ implies that K2-370 b either retains a significant ($\sim 2~{{\ \rm per\ cent}}$ by mass) H-rich atmosphere or its interior contains a high ($\sim 40~{{\ \rm per\ cent}}$) water–mass fraction.