New Infrared Spectral Indices of Luminous Cold Stars: From Early K to M Types

Abstract

We present infrared spectral indices (1.0-2.3 mu m) 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 mu m ([CO, 2.29] greater than or similar to 45 angstrom) is a typical signature of RSGs later than spectral type M0, [CO] <i 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 mu m), Sr (1.033 mu m), Fe+Cr+Si+CN (1.16 mu m), Fe+Ti (1.185 mu m), Fe+Ti (1.196 mu m), Ti+Ca (1.28 mu m), and Mn (1.29 mu m). Good markers of CN enhancement are the Fe+Si+CN line at 1.087 mu m and CN line at 1.093 mu m. 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. Twenty-one 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.