KELT-10b: The first transiting exoplanet from the KELT-South survey - a hot sub-Jupiter transiting a V = 10.7 early G-star

We report the discovery of KELT-10b, the first transiting exoplanet discovered using the KELT-South telescope. KELT-10b is a highly inflated sub-Jupiter mass planet transiting a relatively bright V = 10.7 star (TYC 8378-64-1), with T_eff = 5948 ± 74 K, log g = 4.319_-0.030 ^+0.020 and [Fe/H] = 0.09_-0.10 ^+0.11, an inferred mass M* = 1.112_-0.061^+0.055 M_⊙ and radius R* = 1.209_-0.035 ^+0.047 R_⊙. The planet has a radius R_p = 1.399_-0.049 ^+0.069 R_J and mass M_p = 0.679_-0.038 ^+0.039 M_J. The planet has an eccentricity consistent with zero and a semimajor axis a = 0.052 50_{-0.000 97} ^{+0.000 86} au. The best-fitting linear ephemeris is T₀ = 2457 066.720 45 ± 0.000 27 BJD_TDB and P = 4.166 2739 ± 0.000 0063 d. This planet joins a group of highly inflated transiting exoplanets with a larger radius and smaller mass than that of Jupiter. The planet, which boasts deep transits of 1.4 per cent, has a relatively high equilibrium temperature of T_eq = 1377_-23 ⁺²⁸ K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817_-0.054 ^+0.068 × 10⁹ erg s^-1 cm^-2, which places it far above the insolation threshold above which hot Jupiters exhibit increasing amounts of radius inflation. Evolutionary analysis of the host star suggests that KELT-10b may not survive beyond the current subgiant phase, depending on the rate of in-spiral of the planet over the next few Gyr. The planet transits a relatively bright star and exhibits the third largest transit depth of all transiting exoplanets with V < 11 in the Southern hemisphere, making it a promising candidate for future atmospheric characterization studies.