KELT-1b: A STRONGLY IRRADIATED, HIGHLY INFLATED, SHORT PERIOD, 27 JUPITER-MASS COMPANION TRANSITING A MID-F STAR

We present the discovery of KELT-1b, the first transiting low-mass companion from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North) transit survey. A joint analysis of the spectroscopic, radial velocity, and photometric data indicates that the V = 10.7 primary is a mildly evolved mid-F star with T-eff = 6516 +/- 49 K, log g = 4.228(-0.021)(+0.014), and [Fe/H] = 0.052 +/- 0.079, with an inferred mass M* = 1.335 +/- 0.063M(circle dot) and radius R* = 1.471(-0.035)(+0.045) R-circle dot. The companion is a low-mass brown dwarf or a super-massive planet with mass M-P = 27.38 +/- 0.93 M-Jup and radius R-P = 1.116(-0.029)(+0.038) R-Jup. The companion is on a very short (similar to 29 hr) period circular orbit, with an ephemeris T-c(BJD(TDB)) = 2455909.29280 +/- 0.00023 and P = 1.217501 +/- 0.000018 days. KELT-1b receives a large amount of stellar insolation, resulting in an estimated equilibrium temperature assuming zero albedo and perfect redistribution of T-eq = 2423(-27)(+34) K. Comparison with standard evolutionary models suggests that the radius of KELT-1b is likely to be significantly inflated. Adaptive optics imaging reveals a candidate stellar companion to KELT-1 with a separation of 588 +/- 1 mas, which is consistent with an M dwarf if it is at the same distance as the primary. Rossiter-McLaughlin measurements during transit imply a projected spin-orbit alignment angle lambda = 2 +/- 16 deg, consistent with a zero obliquity for KELT-1. Finally, the v sin I* = 56 +/- 2 km s(-1) of the primary is consistent at similar to 2 sigma with tidal synchronization. Given the extreme parameters of the KELT-1 system, we expect it to provide an important testbed for theories of the emplacement and evolution of short-period companions, as well as theories of tidal dissipation and irradiated brown dwarf atmospheres.