Infrared spectroscopy of symbiotic stars and the nature of their cool components

Abstract

We present low-resolution 2--4 ..mu..m spectroscopy of a small sample of symbiotic stars, in an effort to determine if the giant components of these systems fill their Roche Lobes. A (2.35)-(2.2) color index measures the strength of the CO absorption band and provides a useful discriminant of luminosity class among single M-type giants which separates normal giants from supergiants at the same spectral type. Although interpretation of symbiotic spectra is complicated somewhat by their binary nature, our results suggest the late-type components in these systems range from normal red giants to bright asymptotic giants. The possible presence of non-Roche Lobe filling, low-luminosity giants in some symbiotic stars cannot be understood within the framework of existing theories for these interesting objects, and thus may provide important information for understanding mass transfer in binary systems.