ar X iv : a st ro - p h / 01 09 00 4 v 1 1 S ep 2 00 1 Water in K and M giant stars unveiled by ISO 1 Takashi

Based on the spectra obtained with Infrared Space Observatory, ISO, we detected the 6.3μm bands of water in the late K giant Aldebaran (α 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)× 10 molecules cm in our sample of K and M giant stars and the excitation temperatures are 1500K 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. keywords Infrared: stars – molecular processes – stars: atmospheres – stars: chromospheres – stars: individual: α Tau, β And, α Cet, β Peg, γ Cru – stars: late-type Based on the data archives of ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.