Water in K and M giant stars unveiled by ISO

Based on the spectra obtained with Infrared Space Observatory, ISO, we detected the 6.3 mum bands of water in the late K giant Aldebaran (alpha Tau) and several early M giant stars (between M0 and M3.5), which have been deemed to be too warm for tri-atomic H2O molecule to reside in their photospheres. The water column densities range (0.2 2) x 10(18) molecules cm(-2) in our sample of K and M giant stars and the excitation temperatures are 1500 K or higher. Thus, the water bands are not originating in cool stellar winds either. The presence of water in the K and early M giant stars was quite unexpected from the traditional picture of the atmosphere of the red giant star consisting of the photosphere, hot chromosphere, and cool wind. We confirm that a rather warm molecule forming region should exist as a new component of the atmosphere of red giant stars and that this should be a general phenomenon in late-type stars.