Luminosity and Temperature from Near-Infrared Spectra of Late-Type Giant Stars

We present moderate resolution (lambda / Delta lambda ~ 1380 and lambda / Delta lambda ~ 4830) spectra of 43 K0 to M6 III stars covering 2.19 - 2.34 mu m and measure equivalent widths of the strongest absorption features - Na I, Ca I, and (12) CO(2,0) - present on the spectra. The high resolution Wallace & Hinkle (1996) spectral atlas shows that our moderate resolution measurements of the atomic features have significant contributions from other species, such as Sc, S, Fe, Ti, Si, and V. We also find that our measured equivalent widths are affected by CN absorption present in the continuum bands. In spite of this, the equivalent widths of Na I and Ca I features at moderate resolution still show a strong dependence on effective temperature. The CO equivalent width at moderate resolution is less affected by other lines and continuum placement than the atomic features, because of its relatively greater strength. We compare our data to similar data taken for late-type dwarf stars (Ali et al. 1995) and find that a two dimensional spectral classification can be constructed based on the near-IR spectra. The quantity log[EW(CO)/(EW(Na)+EW(Ca))] is a strong luminosity indicator independent of effective temperature, while the equivalent width of (12) CO(2,0) has a well-defined dependence on effective temperature for a given luminosity. This two dimensional spectral classification is ideal for cool stars obscured by dust in, for example, the central part of the Galactic bulge and regions of star formation.