EPIC 246851721 b: A Tropical Jupiter Transiting a Rapidly Rotating Star in a Well-aligned Orbit

Abstract

We report the discovery of EPIC 246851721 b, a “tropical” Jupiter in a 6.18-day orbit around the bright ( V  = 11.439) star EPIC 246851721 (TYC 1283-739-1). We present a detailed analysis of the system using K2 and ground-based photometry, radial velocities, Doppler tomography, and adaptive optics imaging. From our global models, we infer that the host star is a rapidly rotating (  = 74.92 ) F dwarf with  = 6202 K, and . EPIC 246851721 b has a radius of 1.051 ± 0.044 , and a mass of 3.0 . Doppler tomography reveals an aligned spin–orbit geometry, with a projected obliquity of , making EPIC 246851721 the fourth hottest star to host a Jovian planet with P  > 5 days and a known obliquity. Using quasi-periodic signatures in its light curve that appear to be spot modulations, we estimate the star’s rotation period, and thereby infer the true obliquity of the system to be . We argue that this near-zero obliquity is likely to be primordial rather than a result of tidal damping. The host star also has a bound stellar companion, a 0.4  M dwarf at a projected separation of 2100 au, but the companion is likely incapable of emplacing EPIC 246851721 b in its current orbit via high-eccentricity Kozai-Lidov migration.