Detection of Water Vapor in the Photosphere of Arcturus

We report detections of pure rotation lines of OH and H 2 O in the K1.5 III red giant star Arcturus (α Bootis) using high-resolution, infrared spectra covering the regions 806-822 cm -1 (12.2-12.4 μm) and 884-923 cm -1 (10.8-11.3 μm). Arcturus is the hottest star yet to show water vapor features in its disk-averaged spectrum. We argue that the water vapor lines originate from the photosphere, albeit in the outer layers. We are able to predict the observed strengths of OH and H 2 O lines satisfactorily after lowering the temperature structure of the outermost parts of the photosphere (log τ 500 = -3.8 and beyond) compared to a flux-constant, hydrostatic, standard MARCS model photosphere. Our new model is consistently calculated including chemical equilibrium and radiative transfer from the given temperature structure. Possible reasons for a temperature decrease in the outermost parts of the photosphere and the assumed breakdown of the assumptions made in classical model atmosphere codes are discussed.