A hot Saturn on an eccentric orbit around the giant star K2-132^★

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
e-mail: mjones@eso.org
² Millennium Institute of Astrophysics, Santiago, Chile
³ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Center of Astro-Engineering UC, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁶ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁷ Instituto de Física y Astronomía, Universidad de Vaparaíso, Casilla 5030, Valparaíso, Chile
⁸ Department of Astronomy, Stockholm University, 11419, Stockholm, Sweden

Received: 30 June 2017
Accepted: 18 December 2017

Abstract

Although the majority of radial velocity detected planets have been found orbiting solar-type stars, a fraction of them have been discovered around giant stars. These planetary systems have revealed different orbital properties when compared to solar-type star companions. In particular, radial velocity surveys have shown that there is a lack of giant planets in close-in orbits around giant stars, in contrast to the known population of hot Jupiters orbiting solar-type stars. It has been theorized that the reason for this distinctive feature in the semimajor axis distribution is the result of the stellar evolution and/or that it is due to the effect of a different formation/evolution scenario for planets around intermediate-mass stars. However, in the past few years a handful of transiting short-period planets (P ≲ 10 days) have been found around giant stars, thanks to the high-precision photometric data obtained initially by the Kepler mission, and later by its two-wheel extension K2. These new discoveries have allowed us for the first time to study the orbital properties and physical parameters of these intriguing and elusive substellar companions. In this paper we report on an independent discovery of a transiting planet in field 10 of the K2 mission, also reported recently by Grunblatt et al. (2017, AJ, 154, 254). The host star has recently evolved to the giant phase, and has the following atmospheric parameters: T_eff = 4878 ± 70 K, log g = 3.289 ± 0.004, and [Fe/H] = −0.11 ± 0.05 dex. The main orbital parameters of K2-132 b, obtained with all the available data for the system are: P = 9.1708 ± 0.0025 d, e = 0.290 ± 0.049, M_p = 0.495 ± 0.007 M_J and R_p = 1.089 ± 0.006 R_J. This is the fifth known planet orbiting any giant star with a < 0.1, and the most eccentric one among them, making K2-132 b a very interesting object.