* Astronomy

Title: EPIC 219388192 b - an inhabitant of the brown dwarf desert in the Ruprecht 147 open cluster
Author: Grzegorz Nowak, Enric Palle, Davide Gandolfi, Fei Dai, Antonino F. Lanza, Teruyuki Hirano, Oscar Barragán, Akihiko Fukui, Hans Bruntt, Michael Endl, William D. Cochran, Jorge Prieto-Arranz, Amanda Kiilerich, David Nespral, Artie P. Hatzes, Simon Albrecht, Hans Deeg, Joshua N. Winn, Liang Yu, Masayuki Kuzuhara, Sascha Grziwa, Alexis M. S. Smith, Pier G. Prada Moroni, Eike W. Guenther, Vincent Van Eylen, Szilard Csizmadia, Malcolm Fridlund, Juan Cabrera, Philipp Eigmüller, Anders Erikson, Judith Korth, Norio Narita, Martin Pätzold, Heike Rauer, Ignasi Ribas

We report the discovery of EPIC 219388192 b, a transiting brown dwarf in a 5.3-day orbit around a member star of Ruprecht-147, the oldest nearby open cluster association, which was photometrically monitored by K2 during its Campaign 7. We combine the K2 time-series data with ground-based adaptive optics imaging and high resolution spectroscopy to rule out false positive scenarios and determine the main parameters of the system. EPIC 219388192 b has a radius of R_b=0.937±0.042~R_Jup and mass of M_b=36.50±0.09~M_Jup, yielding a mean density of 59.0±8.1~gcm^-3. The host star is nearly a Solar twin with mass M_star=0.99±0.05~solar masses, radius R_star=1.01±0.04~solar radii, effective temperature T_eff=5850±85~K and iron abundance [Fe/H]=0.03±0.08~dex. Its age, spectroscopic distance, and reddening are consistent with those of Ruprecht-147, corroborating its cluster membership. EPIC 219388192 b is the first brown dwarf with precise determinations of mass, radius and age, and serves as benchmark for evolutionary models in the sub-stellar regime.

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