Water in stars: expected and unexpected

T. Tsuji , W. Aoki , & K. Ohnaka 

Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo, Japan

``Water everywhere in the Universe!'' is certainly one of the great discoveries of ISO, and star is not an exception. Although it has been known that water exists in very cool stars such as Miras and OH/IR stars (e.g., H₂O maser has been discovered some 30 years ago in such stars), water has not been expected in plain red giants and supergiants. However, ISO revealed the presence of water in M giants and supergiants as early as M1-2, first by the low resolution mode of SWS (Tsuji et al. A&A, 320, L1,1997; Ap&SS, in press). We now confirmed this finding by the highest resolution grating mode of SWS on Cet (M2III) and KK Per (M2Iab). We also observed M giants stars through M4 to M8 by the high resolution of SWS06 and detected H₂O in all the cases including M4-6 giants (and also in M4 supergiant S Per).

It is somewhat surprising that water is found not only in late M giant stars but also in early and middle M (super)giants, and the problem is what this fact means. Our attempt to understand water in these early and middle M (super)giants by the present photospheric models could never be successful. The excitation temperatures of these H₂O lines appear to be rather high, above 1000K. Also, ISO spectra of late M giants (e.g. SW Vir) show CO₂ band at 4.3 m in absorption while band at 15 m in emission. Again, these CO₂ features cannot be explained by the present photospheric models. The shapes of the observed absorption band and the emission features suggest excitation temperature of several hundred Kelvin, and this fact suggests a presence of an extra CO₂ envelope outside the photosphere.

We have confirmed the ISO detection of H₂O in early and middle M giants by the high resolution infrared spectra observed by KPNO FTS. Despite the disturbing atmospheric absorption fatal to the ground-based observations, some H₂O lines of the hot bands can be confirmed in early through late M giants. With the spectral resolution as high as 10⁵, the radial velocities of the H₂O lines can now be measured with high precision, and it turned out that they are not so different from other photospheric lines. This fact suggests that the H₂O is not expanding and thus the H₂O forming region will be quasi-static.

We conclude that there should be a molecule forming region effectively producing H₂O and CO₂ around M giants including early through late types. This molecule forming region may be not expanding yet and will serve as a ``reservoir'' for supplying matter to the expanding stellar wind observed through early to late M (super)giants. So far, presence of such ``reservoir'' has been suggested only for Mira variables in which matter can be levitated to the outer layer by the shock wave due to stellar pulsation. Now, ISO revealed that the presence of ``reservoir'' is a more general phenomenon in red (super)giants and this fact provides a new key in understanding mass-loss phenomena of red (super)giant stars in general.