The near-infrared Na I doublet feature in M stars

The Na I near-infrared feature has been used to indicate the dwarf/giant population in composite systems, but its interpretation is still an issue of contention. In order to try to understand the behavior of this controversial feature, we study the spectra of cool stars by means of both observed and synthetic spectra. We conclude that the Na I infrared feature can be used as a dwarf/giant indicator. We propose a modified definition of the Na I index by defining a red continuum at 8234 A and by measuring the equivalent width in the range 8172-8197 Angstrom, avoiding the region at lambda > 8197 Angstrom which contains V I, Zr I, Fe I, and TiO lines.