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

Near Infrared Spectroscopy of M Dwarfs. I. CO Molecule as an Abundance Indicator of Carbon

Based on the near infrared spectra of 42 M dwarfs, carbon abundances are determined from the ro-vibrational lines of CO 2-0 band. We use Teff values based on the angular diameters if available or apply a logTeff - M3.4 (the absolute magnitude at 3.4 micron based on the WISE W1 flux and the Hipparcos parallax) relation to estimate Teff values of objects for which angular diameters are unknown. Also, we discuss briefly the HR diagram of low mass stars. On the observed spectrum of M dwarf, the continuum is depressed by the numerous weak lines of H2O and only the depressed continuum or the pseudo- continuum can be seen. On the theoretical spectrum of M dwarfs, we find that the pseudo-continuum can be evaluated accurately thanks to the recent H2O line database. Then quantitative analysis of the spectrum of M dwarf can be done by referring to the pseudo-continua both on the observed and theoretical spectra. Since the basic principle of the spectroscopic analysis should be the same whether the true- or pseudo-continuum is referred to, the difficulty related to the continuum in cool stars can in principle be overcome. Then, the numerous CO lines can be excellent abundance indicators of carbon, since almost all the carbon atoms are in stable CO molecules whose abundance remains almost unchanged for the changes of physical condition in the photosphere and, somewhat unexpectedly, carbon abundances in late-type stars can best be determined in M dwarfs rather than in solar type stars. The resulting C/Fe ratios for most M dwarfs are nearly constant at about the solar value based on the classical high carbon abundance rather than on the recently revised lower value. This result implies that the solar carbon abundance is atypical for its metallicity among the stellar objects in the solar neighborhood if the downward revised solar carbon abundance is correct.Comment: 33 pages, 17 figures, 13 tables + 1 table for electronic version only, minor changes in abstract, errors corrected in Tables 5, 8, and 10, corrected typos, accepted by PAS

Photoinduced insulator-metal transition in correlated electrons -- a Floquet analysis with the dynamical mean-field theory

In order to investigate photoinduced insulator-metal transitions observed in correlated electron systems, we propose a new theoretical method, where we combine a Floquet-matrix method for AC-driven systems with the dynamical mean-field theory. The method can treat nonequilibrium steady states exactly beyond the linear-response regime. We have applied the method to the Falicov-Kimball model coupled to AC electric fields, and numerically obtained the spectral function, the nonequilibrium distribution function and the current-voltage characteristic. The results show that intense AC fields indeed drive Mott-like insulating states into photoinduced metallic states in a nonlinear way.Comment: 4 pages, 3 figures, Proceedings of LT2