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 ^+0.014_-0.021, and [Fe/H] = 0.052 ± 0.079, with an inferred mass M_⊙* = 1.335 ± 0.063 M _⊙and radius R_* = 1.471^+0.045 _-0.035 R. 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^+0.038_-0.029R_Jup. The companion is on a very short (∼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⁺³⁴ _-27 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 λ = 2 ± 16 deg, consistent with a zero obliquity for KELT-1. Finally, the vsin I_* = 56 ± 2 km s^-1 of the primary is consistent at ∼2σ 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.