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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.