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 , 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 ( degrees). High-contrast images obtained by the HiCIAO camera on the Subaru 8.2 m telescope reveal a faint companion candidate (CC) ( mag) at a separation of . 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.