Near-infrared spectroscopy of M dwarfs. II. H2O molecule as an abundance indicator of oxygen

Based on the near-infrared spectra (R approximate to 20000) of M dwarfs, oxygen abundances are determined from the rovibrational lines of H2O. Although H2O lines in M dwarfs are badly blended with each other and the continuum levels are depressed appreciably by the collective effect of the numerous H2O lines themselves, quantitative analysis of H2O lines has been carried out by referring to the pseudo-continua, consistently defined on the observed and theoretical spectra. For this purpose, the pseudo-continuum on the theoretical spectrum has been evaluated accurately by the use of the recent high-precision H2O line-list. Then, we propose a simple and flexible method of analyzing the equivalent widths (EWs) of blended features (i.e., not necessarily limited to single lines) 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 in 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. Thus the numerous H2O lines are excellent abundance indicators of oxygen. The oxygen abundances are determined to be log A(O) (A(O) = N-O/N-H) between -3.5 and -3.0 in 38 M dwarfs, but cannot be determined in four early M dwarfs in which H2O lines are detected only marginally. The resulting log A(O)/A(C) values plotted against log A(C) 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, A(O)/A(Fe) ratios in most M dwarfs are closer to the solar A(O)/A(F)e ratio, based on the classical high oxygen abundance rather than on the recently downward-revised low value.