K2-280 b - a low density warm sub-Saturn around a mildly evolved star
Cochran, William D.; Endl, Michael; Jordán, Andrés; Espinoza, Néstor; Brahm, Rafael; Redfield, Seth; Hirano, Teruyuki; Skarka, Marek; de Leon, Jerome; Livingston, John H.; Dai, Fei; Gandolfi, Davide; Nowak, Grzegorz; Albrecht, Simon; Cabrera, Juan; Eigmüller, Philipp; Erikson, Anders; Fridlund, Malcolm; Fukui, Akihiko; Grziwa, Sascha; Guenther, Eike W.; Hatzes, Artie P.; Korth, Judith; Kuzuhara, Masayuki; Narita, Norio; Nespral, David; Palle, Enric; Pätzold, Martin; Persson, Carina M.; Prieto-Arranz, Jorge; Rauer, Heike; Van Eylen, Vincent; Mathur, Savita; Johnson, Marshall C.; Deeg, Hans J.; Luque, Rafael; Esposito, Massimiliano; Knudstrup, Emil; Lam, Kristine W. F.; Smith, Alexis M. S.; Lund, Mikkel N.; Chaushev, Alexander; Csizmadia, Szilárd; Murgas, Felipe; Carleo, Ilaria; Barragán, Oscar; Georgieva, Iskra; Montañes Rodríguez, Pilar; Alonso Sobrino, Roi; González-Cuesta, Lucía; Hidalgo, Diego; Hjorth, Maria; Kabath, Petr; Niraula, Prajwal; Ribas, Ignasi; Rodler, Florian; Rojas, Felipe; Blay, Pere; Casasayas Barris, Núria; Subjak, Jano
Spain, Italy, Japan, United Kingdom, United States, Sweden, Netherlands, Germany, Denmark, Czech Republic, Chile
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 Rb = 7.50 ± 0.44 R⊕ and a mass of Mb = 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 Teff = 5500 ± 100 K, a surface gravity of $\log \, g_{\star }$ = 4.21 ± 0.05 (cgs), and an iron abundance of [Fe/H] = ${0.33}\, {\pm }\, {0.08}$ 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.