The FeH Wing-Ford Band in Spectra of M Stars

We study the FeH Wing-Ford band at 9850 - 10200 Angstrons by means of the fit of synthetic spectra to the observations of M stars, employing recent model atmospheres. On the basis of the spectrum synthesis, we analyze the dependence of the band upon atmospheric parameters. FeH lines are a very sensitive surface gravity indicator, being stronger in dwarfs. They are also sensitive to metallicity (Allard & Hauschildt 1995). The blending with CN lines, which are stronger in giants, does not affect the response of the Wing-Ford band to surface gravity at low resolution (or high velocity dispersions) because CN lines, which are spread all along the spectrum, are smeared out at convolutions of FWHM $\simgreat$ 3 Angstrons. We conclude that the Wing-Ford band is a suitable dwarf/giant indicator for the study of composite stellar populations.