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

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 (v\sin {I}_\star = 74.92 {km} {{{s}}}^-1) F dwarf with {T}_eff} = 6202 K, {R}_\star =1.586 {R}_⊙ and {M}_\star =1.317 {M}_⊙ . EPIC 246851721 b has a radius of 1.051 ± 0.044 {\text{}}{R}_{{J}}, and a mass of 3.0{}_-1.2^+1.1 {\text{}}{M}_{{J}}. Doppler tomography reveals an aligned spin-orbit geometry, with a projected obliquity of -1\buildrel{\circ}\over{.} {47} _-0.86^+0.87, 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 3\buildrel{\circ}\over{.} {7} _-1.8^+3.7. 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}_⊙ 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.