New infrared spectral indices of luminous cold stars: from early K to
  M-types
  We present infrared spectral indices (1.0-2.3 um) of Galactic late-type
giants and red supergiants (RSGs). We used existing and new spectra obtained at
resolution power R=2000 with SpeX on the IRTF telescope. While a large CO
equivalent width (EW), at 2.29 um ([CO, 2.29]>45 AA) is a typical signature of
RSGs later than spectral type M0, [CO] of K-type RSGs and giants are similar.
In the [CO, 2.29] versus [Mg I, 1.71] diagram, RSGs of all spectral types can
be distinguished from red giants, because the Mg I line weakens with increasing
temperature and decreasing gravity. We find several lines that vary with
luminosity, but not temperature: Si I (1.59 um), Sr (1.033 um), Fe+Cr+Si+CN
(1.16 um), Fe+Ti (1.185 um), Fe+Ti (1.196 um), Ti+Ca (1.28 um), and Mn (1.29
um). Good markers of CN enhancement are the Fe+Si+CN line at 1.087 um and CN
line at 1.093 um. Using these lines, at the resolution of SpeX, it is possible
to separate RSGs and giants. Contaminant O-rich Mira and S-type AGBs are
recognized by strong molecular features due to water vapor features, TiO band
heads, and/or ZrO absorption. Among the 42 candidate RSGs that we observed, all
but one were found to be late-types. 21 have EWs consistent with those of RSGs,
16 with those of O-rich Mira AGBs, and one with an S-type AGB. These infrared
results open new, unexplored, potential for searches at low-resolution of RSGs
in the highly obscured innermost regions of the Milky Way.
