We describe the detection of water vapor in the atmosphere of the transiting hot Jupiter KELT-2Ab by treating the star-planet system as a spectroscopic binary with high-resolution, ground-based spectroscopy. We resolve the signal of the planets motion with deep combined flux observations of the star and the planet. In total, six epochs of Keck NIRSPEC $L$-band observations were obtained, and the full data set was subjected to a cross correlation analysis with a grid of self-consistent atmospheric models. We measure a radial projection of the Keplerian velocity, $K_P$, of 148 $pm$ 7 km s$^{-1}$, consistent with transit measurements, and detect water vapor at 3.8$sigma$. We combine NIRSPEC $L$-band data with $Spitzer$ IRAC secondary eclipse data to further probe the metallicity and carbon-to-oxygen ratio of KELT-2Abs atmosphere. While the NIRSPEC analysis provides few extra constraints on the $Spitzer$ data, it does provide roughly the same constraints on metallicity and carbon-to-oxygen ratio. This bodes well for future investigations of the atmospheres of non-transiting hot Jupiters.
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