HAT-P-54b: A Hot Jupiter Transiting a 0.6 M$_☉$ Star in Field 0 of the K2 Mission

Abstract

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 0.03  2 MJ, a radius of 0.944 ± 0.028 RJ, 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 Teff =  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 lowestmass 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.