  In this talk I presented and discussed our unexpected detection of water
vapor in the disk-averaged spectrum of the K2IIIp red giant Arcturus [for
details, see Ryde et al. (2002)]. Arcturus, or alpha Bootes is, with its
effective temperature of 4300 K, the hottest star yet to show water vapor
features. We argue that the water vapor is photospheric and that its detection
provides us with new insights into the outer parts of the photosphere. We are
not able to model the vater vapor with a standard, one-component, 1D,
radiative-equilibrium, LTE model photosphere, which probably means we are
lacking essential physics in such models. However, we are able to model several
OH lines of different excitation and the water-vapor lines satisfactorily after
lowering the temperature structure of the very outer parts of the photosphere
at log tau_500=-3.8 and beyond compared to a flux-constant, hydrostatic,
standard marcs model photosphere. Our new semi-empirical model is consistently
calculated from the given temperature structure. I will discuss some possible
reasons for a temperature decrease in the outer-most parts of the photosphere
and the assumed break-down of the assumptions made in classical
model-atmosphere codes. In order to understand the outer photospheres of these
objects properly, we will, most likely, need 3D hydrodynamical models of red
giants also taking into account full non-LTE and including time-dependent
effects of, for example, acoustic wave heating sensitive to thermal
instabilities.
