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

Van Eylen, VincentAlbrecht, Simon; Gandolfi, Davide; Dai, Fei; Winn, Joshua N.; Hirano, Teriyuki; Narita, Norio; Bruntt, Hans; Prieto-Arranz, Jorge; Béjar, Víctor J. S.; Nowak, Grzegorz; Lund, Mikkel N.; Palle, Enric; Ribas, Ignasi; Sanchis-Ojeda, Roberto; Yu, Liang; Arriagada, Pamela; Butler, R. Paul; Crane, Jeffrey D.; Handberg, Rasmus; Deeg, Hans; Jessen-Hansen, Jens; Johnson, John A.; Nespral, David; Rogers, Leslie; Ryu, Tsuguru; Shectman, Stephen; Shrotriya, Tushar; Slumstrup, Ditte; Takeda, Yoichi; Teske, Johanna; Thompson, Ian; Vanderburg, Andrew; Wittenmyer, Robert

The K2-ESPRINT Project V: A Short-period Giant Planet Orbiting a Subgiant Star,
The Astronomical Journal, Vol. 152, Issue 5


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 3.88-0.42+0.48R⊙ and 1.53-0.12+0.13M, 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 50.3-9.4+9.7 M⊕ . In combination with a radius measurement of 8.3 +/- 1.1 R , this results in a mean planetary density of 0.50-0.17+0.29 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.


Key words: planetary systems / planets and satellites: detection / planets and satellites: dynamical evolution and stability / stars: fundamental parameters / stars: individual: K2-39

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