An eccentric companion at the edge of the brown dwarf desert orbiting the 2.4 M_⊙ giant star HIP 67537^⋆

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
e-mail: mjones@eso.org
² Center of Astro-Engineering UC, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
³ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁴ Millennium Institute of Astrophysics, Santiago, Chile
⁵ Computational Engineering and Science Research Centre, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
⁶ School of Physics and Australian Centre for Astrobiology, University of New South Wales, Sydney 2052, Australia
⁷ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁸ Instituto de Física y Astronomía, Universidad de Vaparaíso, Casilla 5030, Valparaíso, Chile

Received: 17 December 2016
Accepted: 7 March 2017

Abstract

We report the discovery of a substellar companion around the giant star HIP 67537. Based on precision radial velocity measurements from CHIRON and FEROS high-resolution spectroscopic data, we derived the following orbital elements for HIP 67537 b: m_b sin i = 11.1^+0.4_-1.1M_jup, a =4.9^+0.14_-0.13 AU and e = 0.59^+0.05_-0.02 . Considering random inclination angles, this object has ≳65% probability to be above the theoretical deuterium-burning limit, thus it is one of the few known objects in the planet to brown-dwarf (BD) transition region. In addition, we analyzed the Hipparcos astrometric data of this star, from which we derived a minimum inclination angle for the companion of ~2 deg. This value corresponds to an upper mass limit of ~0.3 M_⊙, therefore the probability that HIP 67537 b is stellar in nature is ≲7%. The large mass of the host star and the high orbital eccentricity makes HIP 67537 b a very interesting and rare substellar object. This is the second candidate companion in the brown dwarf desert detected in the sample of intermediate-mass stars targeted by the EXoPlanets aRound Evolved StarS (EXPRESS) radial velocity program, which corresponds to a detection fraction of f = ^+2.0_-0.5 %. This value is larger than the fraction observed in solar-type stars, providing new observational evidence of an enhanced formation efficiency of massive substellar companions in massive disks. Finally, we speculate about different formation channels for this object.