Isotopic Abundances of Carbon and Oxygen in Oxygen-Rich Giant Stars

16O/17O and 12C/13C ratios in 23 M giants are determined from high resolution IR spectra observed with the KPNO FTS. The masses of our sample are estimated with the use of the evolutionary tracks by Claret (2004), which could account for only about half of our sample. The resulting rather large variation of 16O/17O in low mass stars is well consistent with the prediction of the evolutionary models, but quite low 16O/17O ratios observed in some higher mass stars cannot be explained with the model prediction. The observed 12C/13C ratios are mostly around 10, in contradiction with the predict 12C/13C ratios of about 20. Thus we confirm a long-standing 12C/13C puzzle, and it appears that this dilemma may not be resolved yet even with extra mixing such as "cool bottom processing" expected only in low mass stars.Comment: 3 pages, 3 figures, to appear in Proceedings of the IAU Symposium No.239 "Convection in Astrophysics" eds. F. Kupka, I. W. Roxburgh & K. L. Cha

Dust in the Photospheric Environment III. A Fundamental Element in the Characterization of Ultracool Dwarfs

Recent photometry of L and T dwarfs revealed that the infrared colors show a large variation at a given Teff and, within the framework of our Unified Cloudy Model (UCM), this result can be interpreted as due to a variation of the critical temperature (Tcr) which is essentially a measure of the thickness of the dust cloud. It appears that the L/T transition takes place at around Teff = 1400K, where Tcr shows a particularly large change. Thus the L/T transition is associated with a drastic change of the thickness of the dust cloud at around Teff = 1400K, but the reason for this change is unknown. Once we allow Tcr to vary at given Teff and log g, the two-color and color-magnitude diagrams can be well explained as the effect of Teff, log g, and Tcr, but not by that of Teff and log g alone. In general, the effects of Teff and Tcr are difficult to discriminate on individual spectra, but this degeneracy of Teff and Tcr can be removed to some extent by the analysis of the SED on an absolute scale. The reanalysis of a selected sample of spectra revealed that the L-T spectral sequence may not necessarily be a sequence of Teff, but may reflect a change of the thickness of the dust cloud, represented by Tcr in our UCM. Also, an odd 'brightening' of the absolute J magnitudes plotted against the L-T spectral types may also be a manifestation that the L-T spectral sequence is not a temperature sequence, since Mbol also shows a similar 'brightening'. Then, the 'J-brightening' may not be due to any atmospheric effect and hence should not be a problem to be solved by model atmospheres including the UCMs. Thus, almost all the available observed data are reasonably well interpreted with the UCMs in which the cloud thickness varies, and the problem now is how to understand why the cloud thickness (or Tcr) changes independently of Teff and log g.Comment: 40 pages, 12 Postscript figures, to be published in The Astrophysical Journal. A short note discussing the effect of methane opacities based on the line list by R. Freedman is added in the revised versio

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 micron 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 2E+17 - 2E+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 (referred to as MOLsphere) 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. ~Comment: 11 pages, 3 figure