KELT-8b: A highly inflated transiting hot Jupiter and a new technique for extracting high-precision radial velocities from noisy spectra

Abstract

We announce the discovery of a highly inflated transiting hot Jupiter discovered by the KELT-North survey. A global analysis including constraints from isochrones indicates that the V = 10.8 host star (HD 343246) is a mildly evolved, G dwarf with $T_{\mathrm{eff}}=5754_{-55}^{+54}$ K, $\log g=4.078_{-0.054}^{+0.049}$, $[Fe/H]=0.272\pm 0.038$, an inferred mass $M_{\ast }=1.211_{-0.066}^{+0.078}$ M${}_{\odot }$, and radius $R_{\ast }=1.67_{-0.12}^{+0.14}$ R${}_{\odot }$. The planetary companion has mass $M_P=0.867_{-0.061}^{+0.065}$ $M_J$, radius $R_P=1.86_{-0.16}^{+0.18}$ $R_J$, surface gravity $\log g_P=2.793_{-0.075}^{+0.072}$, and density $\ddot{I}{}_P=0.167_{-0.038}^{+0.047}$ g cm${}^{-3}$. The planet is on a roughly circular orbit with semimajor axis $a=0.04571_{-0.00084}^{+0.00096}$ AU and eccentricity $e=0.035_{-0.025}^{+0.050}$. The best-fit linear ephemeris is $T_0=2456883.4803\pm 0.0007$ BJD${}_{\mathrm{TDB}}$ and $P=3.24406\pm 0.00016$ days. This planet is one of the most inflated of all known transiting exoplanets, making it one of the few members of a class of extremely low density, highly-irradiated gas giants. The low stellar $\log g$ and large implied radius are supported by stellar density constraints from follow-up light curves, plus an evolutionary and space motion analysis. We also develop a new technique to extract high precision radial velocities from noisy spectra that reduces the observing time needed to confirm transiting planet candidates. This planet boasts deep transits of a bright star, a large inferred atmospheric scale height, and a high equilibrium temperature of $T_{eq}=1675_{-55}^{+61}$ K, assuming zero albedo and perfect heat redistribution, making it one of the best targets for future atmospheric characterization studies.

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