The K2-ESPRINT Project IV. A Hot Jupiter in a Prograde Orbit with a Possible Stellar Companion

We report on the detection and early characterization of a hot Jupiter in a three day orbit around K2-34 (EPIC 212110888), a metal-rich F-type star located in the K2 Cycle 5 field. Our follow-up campaign involves precise radial velocity (RV) measurements and high-contrast imaging using multiple facilities. The absence of a bright nearby source in our high-contrast data suggests that the transit-like signals are not due to light variations from such a companion star. Our intensive RV measurements show that K2-34b (EPIC 212110888b) has a mass of 1.773+/- 0.086{M}_J, confirming its status as a planet. We also detect the Rossiter-McLaughlin effect for K2-34b and show that the system has a good spin-orbit alignment (λ = -{1}_-9⁺¹⁰ degrees). High-contrast images obtained by the HiCIAO camera on the Subaru 8.2 m telescope reveal a faint companion candidate (CC) ({{Δ }}{m}_H\=\6.19+/- 0.11 mag) at a separation of 0\buildrel{\prime\prime}\over{.} 36. Follow-up observations are needed to confirm that the CC is physically associated with K2-34. K2-34b appears to be an example of a typical “hot Jupiter,” albeit one which can be precisely characterized using a combination of K2 photometry and ground-based follow-up.