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 , which surveys \sim 40 \% of the northern sky to search for transiting planets around bright stars .
The initial transit signal was robustly identified in the KELT-North survey data , and the low-mass nature of the occultor was confirmed via a combination of follow-up photometry , high-resolution spectroscopy , and radial velocity measurements .
False positives are disfavored by the achromaticity of the primary transits in several bands , a lack of evidence for a secondary eclipse , and insignificant bisector variations .
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 } = 6518 \pm 50 ~ { } { K } , \log { g _ { * } } = 4.229 _ { -0.019 } ^ { +0.012 } and { [ Fe / H ] } = 0.008 \pm 0.073 , with an inferred mass M _ { * } = 1.324 \pm 0.026 ~ { } M _ { \odot } and radius R _ { * } = 1.462 _ { -0.024 } ^ { +0.037 } ~ { } R _ { \odot } .
The companion is a low-mass brown dwarf or a super-massive planet with mass M _ { P } = 27.23 _ { -0.48 } ^ { +0.50 } ~ { } M _ { Jup } , radius R _ { P } = 1.110 _ { -0.022 } ^ { +0.032 } ~ { } R _ { Jup } , surface gravity \log { g _ { P } } = 4.738 _ { -0.023 } ^ { +0.017 } , and a density \rho _ { P } = 24.7 _ { -1.9 } ^ { +1.4 } ~ { } { g~ { } cm ^ { -3 } } .
The companion is on a very short ( \sim 29 hour ) period circular orbit , with an ephemeris T _ { c } ( { { BJD _ { TDB } } } ) = 2455909.292797 \pm 0.00024 and P = 1.2175007 \pm 0.000018 ~ { } { d } , and a semimajor axis of a = 0.02466 \pm 0.00016 AU .
KELT-1b receives a large amount of stellar insolation , with \langle F \rangle = 7.81 _ { -0.33 } ^ { +0.42 } \times 10 ^ { 9 } ~ { } { erg~ { } s ^ { -1 } ~ { } cm ^ { - % 2 } } , implying an equilibrium temperature assuming zero albedo and perfect redistribution of T _ { eq } = 2422 _ { -26 } ^ { +32 } ~ { } { K } .
Upper limits on the secondary eclipse depth in i and z bands indicate that either the companion must have a non-zero albedo , or it must experience some energy redistribution .
Comparison with standard evolutionary models for brown dwarfs 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 \pm 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 \pm 16 degrees , consistent with the orbit pole of KELT-1b being aligned with the spin axis of the primary .
Finally , the { v \sin { I _ { * } } } = 55.4 \pm 2.0 ~ { } { km~ { } s ^ { -1 } } of the primary is consistent at \sim 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 .