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

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 (Teff = 5662 ± 44 K, < log R HK > = -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 ± 0.05 d and measure the planetary RV signal with semi-amplitude Kb = 5.6 ± 0.7 m s-1. We find that K2-370 b has a radius of 2.67 ± 0.05 R⊕ and a mass of 11.1 ± 1.4 M⊕. With an estimated equilibrium temperature Teq ∼ 1480 K, K2-370 b is the second-hottest sub-Neptune with a highly precise mass determination around primaries with Teff > 5500 K. The resulting density of 3.2 ± 0.4 g cm-3 implies that K2-370 b either retains a significant (∼ 2 per cent by mass) H-rich atmosphere or its interior contains a high (∼ 40 per cent) water-mass fraction.