K2-280 b – a low density warm sub-Saturn around a mildly evolved star. Issue 4 (17th July 2020) - Nowak, Grzegorz; Palle, Enric; Gandolfi, Davide; Deeg, Hans J; Hirano, Teruyuki; Barragán, Oscar; Kuzuhara, Masayuki; Dai, Fei; Luque, Rafael; Persson, Carina M; Fridlund, Malcolm; Johnson, Marshall C; et al.
ABSTRACT: We present an independent discovery and detailed characterization of K2-280 b, a transiting low density warm sub-Saturn in a 19.9-d moderately eccentric orbit ( e = $0.35_{-0.04}^{+0.05}$ ) from K2 campaign 7. A joint analysis of high precision HARPS, HARPS-N, and FIES radial velocity measurements and K2 photometric data indicates that K2-280 b has a radius of R b = 7.50 ± 0.44 R⊕ and a mass of M b = 37.1 ± 5.6 M⊕, yielding a mean density of ρb = $0.48 _{ - 0.10 } ^ { + 0.13 }$ ${\rm g\, cm^{-3}}$ . The host star is a mildly evolved G7 star with an effective temperature of T eff = 5500 ± 100 K, a surface gravity of $\log \, g_{\star }$ = 4.21 ± 0.05 (cgs), and an iron abundance of [Fe/H] = ${0.33}\, {± }\, {0.08}$ dex, and with an inferred mass of M ⋆ = 1.03 ± 0.03 M⊙ and a radius of R ⋆ = 1.28 ± 0.07 R⊙ . We discuss the importance of K2-280 b for testing formation scenarios of sub-Saturn planets and the current sample of this intriguing group of planets that are absent in the Solar system.