K2-111 b − a short period super-Earth transiting a metal poor, evolved old star

¹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
e-mail: fridlund@strw.leidenuniv.nl
² Department of Earth, Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92, Onsala, Sweden
³ Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴ Dipartimento di Fisica, Universitá di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁵ Landessternwarte Königstuhl, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁷ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan
⁸ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
⁹ Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, 22100 Lund, Sweden
¹⁰ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹³ Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
¹⁴ Department of Astronomy and McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
¹⁵ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁶ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁷ Princeton University, Department of Astrophysical Sciences, 4 Ivy Lane, Princeton, NJ 08544, USA
¹⁸ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁹ Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²⁰ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
²¹ Subaru Telescope, National Astronomical Observatory of Japan, 650 North Aohoku Place, Hilo, HI 96720, USA

Received: 20 March 2017
Accepted: 26 April 2017

Abstract

Context. From a light curve acquired through the K2 space mission, the star K2-111(EPIC 210894022) has been identified as possibly orbited by a transiting planet.

Aims. Our aim is to confirm the planetary nature of the object and derive its fundamental parameters.

Methods. We analyse the light curve variations during the planetary transit using packages developed specifically for exoplanetary transits. Reconnaissance spectroscopy and radial velocity observations have been obtained using three separate telescope and spectrograph combinations. The spectroscopic synthesis package SME has been used to derive the stellar photospheric parameters that were used as input to various stellar evolutionary tracks in order to derive the parameters of the system. The planetary transit was also validated to occur on the assumed host star through adaptive imaging and statistical analysis.

Results. The star is found to be located in the background of the Hyades cluster at a distance at least 4 times further away from Earth than the cluster itself. The spectrum and the space velocities of K2-111 strongly suggest it to be a member of the thick disk population. The co-added high-resolution spectra show that that it is a metal poor ([Fe/H] = − 0.53 ± 0.05  dex) and α-rich somewhat evolved solar-like star of spectral type G3. We find T_eff = 5730 ± 50 K, log g_⋆ = 4.15 ± 0.1 cgs, and derive a radius of R_⋆ = 1.3 ± 0.1 R_⊙ and a mass of M_⋆ = 0.88 ± 0.02 M_⊙. The currently available radial velocity data confirms a super-Earth class planet with a mass of 8.6 ± 3.9 M_⊕ and a radius of 1.9 ± 0.2 R_⊕. A second more massive object with a period longer than about 120 days is indicated by a long-term radial velocity drift.

Conclusions. The radial velocity detection together with the imaging confirms with a high level of significance that the transit signature is caused by a planet orbiting the star K2-111. This planet is also confirmed in the radial velocity data. A second more massive object (planet, brown dwarf, or star) has been detected in the radial velocity signature. With an age of ≳10 Gyr this system is one of the oldest where planets are hitherto detected. Further studies of this planetary system are important since it contains information about the planetary formation process during a very early epoch of the history of our Galaxy.