2012ApJ...761..123S

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_–0.035R_☉. 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_–27K. 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 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.