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 = 74.92 $ km s$^{-1}$) F dwarf with $T_\mathrm{eff}$ = 6202 K, $R_\star = 1.586 \ R_\odot$ and $M_\star= 1.317 \ M_\odot$. EPIC 246851721 b has a radius of $1.051 \pm 0.044 R_J$, and a mass of 3.0$^{+1.1}_{-1.2} M_J$ . Doppler tomography reveals an aligned spin-orbit geometry, with a projected obliquity of $-1.47^{\circ\ +0.87}_{\ -0.86}$, 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 $3.7^{\circ\ +3.7}_{\ -1.8}$. 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 $0.4 \ M_\odot$ 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.