The K2-ESPRINT Project V: A Short-period Giant Planet Orbiting a Subgiant Star

Creators: Van Eylen, Vincent; Rogers, Leslie

Abstract

We report on the discovery and characterization of the transiting planet K2-39b (EPIC 206247743b). With an orbital period of 4.6 days, it is the shortest-period planet orbiting a subgiant star known to date. Such planets are rare, with only a handful of known cases. The reason for this is poorly understood but may reflect differences in planet occurrence around the relatively high-mass stars that have been surveyed, or may be the result of tidal destruction of such planets. K2-39 (EPIC 206247743) is an evolved star with a spectroscopically derived stellar radius and mass of 3.88^(+0.48)_(-0.42) R⊙ and 1.53^(+0.13)_(-0.12) M⊙, respectively, and a very close-in transiting planet, with a/R⋆ = 3.4. Radial velocity (RV) follow-up using the HARPS, FIES, and PFS instruments leads to a planetary mass of 50.3^(+9.7)_(-9.4) M⊕. In combination with a radius measurement of 8.3 ± 1.1 R⊕, this results in a mean planetary density of 0.50^(+0.29)_(-0.17)g cm^(−3). We furthermore discover a long-term RV trend, which may be caused by a long-period planet or stellar companion. Because K2-39b has a short orbital period, its existence makes it seem unlikely that tidal destruction is wholly responsible for the differences in planet populations around subgiant and main-sequence stars. Future monitoring of the transits of this system may enable the detection of period decay and constrain the tidal dissipation rates of subgiant stars.

Additional Information

© 2016 The American Astronomical Society. Received 2016 March 7; revised 2016 May 25; accepted 2016 May 27; published 2016 October 27. Based on observations made with the NOT telescope under program ID. 50-022/51-503, 50-213(CAT), 52-201 (CAT), 52-108 (OPTICON), 51-211 (CAT), and ESOs 3.6 m telescope at the La Silla Paranal Observatory under program ID 095.C-0718(A). We thank the referee, Alexander Santerne, for helpful comments and suggestions which significantly improved this manuscript. We thank Saul Rappaport for helpful comments during the early stages of this project. We acknowledge kind help by Masayuki Kuzuhara for the analysis of Subaru IRCS data. N.N. acknowledges support from the NAOJ Fellowship, Inoue Science Research Award, and Grant-in-Aid for Scientific Research (A) (No. 25247026) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. I.R. acknowledges support from the Spanish Ministry of Economy and Competitiveness (MINECO) and the Fondo Europeo de Desarrollo Regional (FEDER) through grants ESP2013-48391-C4-1-R and ESP2014-57495-C2-2-R. A.V. is supported by the NSF Graduate Research Fellowship, Grant No. DGE 1144152. This work was performed (in part) under contract with the California Institute of Technology (Caltech)/Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. This article is based on observations obtained with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Further observations made with the 1.55 m Carlos Sánchez Telescope operated on the island of Tenerife by the Instituto de Astrofısica de Canarias in the Spanish Observatorio del Teide. Observations with the HARPS spectrograph at ESO's La Silla observatory (095.C-0718(A)). Data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant agreement no.: DNRF106). The research is supported by the ASTERISK project (ASTERoseismic Investigations with SONG and Kepler) funded by the European Research Council (Grant agreement no.: 267864). We acknowledge A.S.K. for covering travels in relation to this publication. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2013-2016) under grant agreement No. 312430 (OPTICON). This research has made use of the Exoplanet Orbit Database and the Exoplanet Data Explorer at exoplanets.org.

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