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Near Infrared Spectroscopy of M Dwarfs. II. H2O Molecule as an Abundance Indicator of Oxygen

Based on the near infrared spectra (R~20000) of M dwarfs, oxygen abundances are determined from the ro-vibrational lines of H2O. Although H2O lines in M dwarfs are badly blended each other and the continuum levels are depressed appreciably by the collective effect of numerous H2O lines themselves, quantitative analysis of H2O lines has been carried out by referring to the pseudo-continua both on the observed and theoretical spectra. For this purpose, the pseudo-continuum on the theoretical spectrum has been evaluated by the use of the recent high-precision H2O line-list. Then, we propose a simple and flexible method of analyzing equivalent widths (EWs) of blended features by the use of a mini curve-of-growth (CG), which is a small portion of the usual CG around the observed EW. The mini CG is generated by using the theoretical EWs evaluated from the synthetic spectrum by exactly the same way as the EWs are measured from the observed spectrum. The observed EW is converted to the abundance by the use of the mini CG, and the process is repeated for all the observed EWs line-by-line or blend-by-blend. In cool M dwarfs, almost all the oxygen atoms left after CO formation are in stable H2O molecules, which suffer little change for the uncertainties due to imperfect modelling of the photospheres. Then the numerous H2O lines are excellent abundance indicators of oxygen. The oxygen abundances are determined to be logAo between -3.5 and -3.0 in 38 M dwarfs. The resulting O/C ratios plotted against logAc appear to be systematically smaller in the carbon-rich M dwarfs, showing the different formation histories of oxygen and carbon in the chemical evolution of the Galactic disk. Also, O/Fe ratios in most M dwarfs are closer to the solar O/Fe ratio based on the classical high oxygen abundance rather than on the recently downward revised low value.