Greening of the Brown Dwarf Desert. EPIC 212036875 b -- a 51 M$_\mathrm{J}$ object in a 5 day orbit around an F7 V star
Creators: name: Persson, Carina M.
name: Csizmadia, Szilárd
name: Mustill, Alexander J.
name: Fridlund, Malcolm
name: Hatzes, Artie P.
name: Nowak, Grzegorz
name: Georgieva, Iskra
name: Gandolfi, Davide
name: Davies, Melvyn B.
name: Livingston, John H.
name: Palle, Enric
name: Rodríguez, Pilar Montañes
name: Endl, Michael
name: Hirano, Teruyuki
name: Prieto-Arranz, Jorge
name: Korth, Judith
name: Grziwa, Sascha
name: Esposito, Massimiliano
name: Albrecht, Simon
name: Johnson, Marshall C.
name: Barragán, Oscar
name: Parviainen, Hannu
name: Van Eylen, Vincent
name: Sobrino, Roi Alonso
name: Beck, Paul G.
name: Cabrera, Juan
name: Carleo, Ilaria
name: Cochran, William D.
name: Dai, Fei
name: Deeg, Hans J.
name: de Leon, Jerome P.
name: Eigmüller, Philipp
name: Erikson, Anders
name: Fukui, Akai
name: González-Cuesta, Lucía
name: Guenther, Eike W.
name: Hidalgo, Diego
name: Hjorth, Maria
name: Kabath, Petr
name: Knudstrup, Emil
name: Kusakabe, Nobuhiko
name: Lam, Kristine W. F.
name: Lund, Mikkel N.
name: Luque, Rafael
name: Mathur, Savita
name: Murgas, Felipe
name: Narita, Norio
name: Nespral, David
name: Niraula, Prajwal
name: Olofsson, A. O. Henrik
name: Pätzold, Martin
name: Rauer, Heike
name: Redfield, Seth
name: Ribas, Ignasi
name: Skarka, Marek
name: Smith, Alexis M. S.
name: Subjak, Jan
name: Tamura, Motohide
Publisher: name: arXiv
Year: 2019
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Our aim is to investigate the nature and formation of brown dwarfs by adding a new well-characterised object to the small sample of less than 20 transiting brown dwarfs. One brown dwarf candidate was found by the KESPRINT consortium when searching for exoplanets in the K2 space mission Campaign 16 field. We combined the K2 photometric data with a series of multi-colour photometric observations, imaging and radial velocity measurements to rule out false positive scenarios and to determine the fundamental properties of the system. We report the discovery and characterisation of a transiting brown dwarf in a 5.17 day eccentric orbit around the slightly evolved F7V star EPIC 212036875. We find a stellar mass of 1.15+/-0.08 M$_\odot$, a stellar radius of 1.41+/-0.05 R$_\odot$, and an age of 5.1+/-0.9 Gyr. The mass and radius of the companion brown dwarf are 51+/-2 MJ and 0.83+/-0.03 RJ, respectively, corresponding to a mean density of 108+15-13 g cm-3. EPIC 212036875 b is a rare object that resides in the brown dwarf desert. In the mass-density diagram for planets, brown dwarfs and stars, we find that all giant planets and brown dwarfs follow the same trend from ~0.3 MJ to the turn-over to hydrogen burning stars at ~73 MJ. EPIC 212036875 b falls close to the theoretical model for mature H/He dominated objects in this diagram as determined by interior structure models, as well as the empirical fit. We argue that EPIC 212036875 b formed via gravitational disc instabilities in the outer part of the disc, followed by a quick migration. Orbital tidal circularisation may have started early in its history for a brief period when the brown dwarf's radius was larger. The lack of spin-orbit synchronisation points to a weak stellar dissipation parameter which implies a circularisation timescale of &gt;23 Gyr, or suggests an interaction between the magnetic and tidal forces of the star and the brown dwarf.
description: 14 pages, 12 figures, accepted 13 June 2019 for publication in A&amp;A