K2-280b -- a low density warm sub-Saturn around a mildly evolved star

Akihiko Fukui; Alexander Chaushev; Alexis M. S. Smith; Anders Erikson; Andr\'es Jord\'an; Artie P. Hatzes; Carina M. Persson; Davide Gandolfi; David Nespral; Diego Hidalgo

K2-280b -- a low density warm sub-Saturn around a mildly evolved star

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arxiv 2007.07939 v1 pith:6RIFMMTE submitted 2020-07-15 astro-ph.EP astro-ph.SR

Grzegorz Nowak , Enric Palle , Davide Gandolfi , Hans J. Deeg , Teruyuki Hirano , Oscar Barragán , Masayuki Kuzuhara , Fei Dai

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Rafael Luque Carina M. Persson Malcolm Fridlund Marshall C. Johnson Judith Korth John H. Livingston Sascha Grziwa Savita Mathur Artie P. Hatzes Jorge Prieto-Arranz David Nespral Diego Hidalgo Maria Hjorth Simon Albrecht Vincent Van Eylen Kristine W. F. Lam William D. Cochran Massimiliano Esposito Szilárd Csizmadia Eike W. Guenther Petr Kabath Pere Blay Rafael Brahm Andrés Jordán Néstor Espinoza Felipe Rojas Núria Casasayas Barris Florian Rodler Roi Alonso Sobrino Juan Cabrera Ilaria Carleo Alexander Chaushev Jerome de Leon Philipp Eigmüller Michael Endl Anders Erikson Akihiko Fukui Iskra Georgieva Lucía González-Cuesta Emil Knudstrup Mikkel N. Lund Pilar Montañes Rodríguez Felipe Murgas Norio Narita Prajwal Niraula Martin Pätzold Heike Rauer Seth Redfield Ignasi Ribas Marek Skarka Alexis M. S. Smith Jano Subjak

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keywords stark2-280bdensitysub-saturnearthevolvedmassmildly

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We present an independent discovery and detailed characterisation of K2-280b, a transiting low density warm sub-Saturn in a 19.9-day 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 K2-280b has a radius of R_b = 7.50 +/- 0.44 R_Earth and a mass of M_b = 37.1 +/- 5.6 M_Earth, yielding a mean density of 0.48_{-0.10}^{+0.13} 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) = 4.21 +/- 0.05 (cgs), and an iron abundance of [Fe/H] = 0.33 +/- 0.08 dex, and with an inferred mass of M_star = 1.03 +/- 0.03 M_sun and a radius of R_star = 1.28 +/- 0.07 R_sun. We discuss the importance of K2-280b 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.

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