Near-IR Direct Detection of Water Vapor in Tau Boötis b

We use high dynamic range, high-resolution L-band spectroscopy to measure the radial velocity (RV) variations of the hot Jupiter in the τ Boötis planetary system. The detection of an exoplanet by the shift in the stellar spectrum alone provides a measure of the planet's minimum mass, with the true mass degenerate with the unknown orbital inclination. Treating the τ Boo system as a high flux ratio double-lined spectroscopic binary permits the direct measurement of the planet's true mass as well as its atmospheric properties. After removing telluric absorption and cross-correlating with a model planetary spectrum dominated by water opacity, we measure a 6σ detection of the planet at K_p = 111 ± 5 km s^-1, with a 1σ upper limit on the spectroscopic flux ratio of 10^-4. This RV leads to a planetary orbital inclination of i = 45^{+3}_{-4}° and a mass of M_{P} = 5.90^{+0.35}_{-0.20}\,M_Jup. We report the first detection of water vapor in the atmosphere of a non-transiting hot Jupiter, τ Boo b.