HAT-P-54b: A Hot Jupiter Transiting a 0.64 M_⊙ Star in field 0 of the K2 Mission

We report the discovery of HAT-P-54b, a planet transiting a late K dwarf star in field 0 of the NASA K2 mission. We combine ground-based photometric light curves with radial velocity measurements to determine the physical parameters of the system. HAT-P-54b has a mass of 0.760 ± 0.032 M_J, a radius of 0.944 ± 0.028 R_J, and an orbital period of 3.7998 days. The star has V=13.505 ± 0.060, a mass of 0.645 ± 0.020 M_⊙, a radius of 0.617 ± 0.013 R_⊙, an effective temperature of T_(eff*) = 4390 ± 50, and a subsolar metallicity of [Fe/H] = -0.127 ± 0.080. We also detect a periodic signal with P = 15.6 days and 5.6 mmag amplitude in the light curve, which we interpret as due to the rotation of the star. HAT-P-54b has a radius that is smaller than 92% of the known transiting planets with masses greater than that of Saturn, while HAT-P-54 is one of the lowest-mass stars known to host a hot Jupiter. Follow-up high-precision photometric observations by the K2 mission promise to make this a well-studied planetary system.