Moderate-resolution near-infrared spectroscopy of cool stars: A new K-bandlibrary

I present an atlas of near-infrared K-band spectra of 31 late-type giants a nd supergiants and two carbon stars. The spectra were obtained at resolving powers of 830 and 2000, and have a signal-to-noise ratio greater than or s imilar to 100. These data are complemented with results from similar existi ng libraries in both K and H band, and they are used to identify various to ols useful for stellar population studies at moderate resolution. I focus o n several of the most prominent absorption features and (1) investigate the effects of spectral resolution on measurements of their equivalent width ( EW), (2) examine the variations with stellar parameters of the EWs, and (3) construct composite indices as indicators of stellar parameters and of the contribution from excess continuum sources commonly found in star-forming and AGN galaxies. Among the features considered, the (CO)-C-12 (2,0) and (C O)-C-12 (6,3) bandheads together with the Si I 1.59 mu m feature, first pro posed by Oliva, Origlia, and coworkers, constitute the best diagnostic set for stellar spectral classification and for constraining the excess continu um emission. The Ca I 2.26 mu m and Mg I 2.28 mu m features offer alternati ves in the K band to the (CO)-C-12 (6, 3) bandhead and Si I feature.