SPECTRAL IRRADIANCE CALIBRATION IN THE INFRARED .3. THE INFLUENCE OF CO AND SIO

We describe first efforts to establish a network of calibrated infrared spectra of "standard stars" suitable for calibration of at least low-resolution infrared spectrometers using ground-based, airborne, and satellite-borne broadband sensors. The focus of this paper is on the crucial 5-8 mum region, inaccessible from the ground, in K and M giants. In this region the fundamental bands of CO and SiO cause substantial departures from featureless pseudo-continua. These departures are, of course, well-known to stellar atmosphere theorists. However, they are still ignored by many astronomical infrared photometrists and spectroscopists who assume that these bright stars can be represented by blackbodies at their effective temperatures. The purpose of this short paper is to draw the attention of infrared observers in the field to the invalidity of the blackbody assumption in the thermal infrared regime. We hope to achieve this end by observationally demonstrating the importance of these molecular features in cool giant stars in the airborne infrared regime, and illustrating their influence on spectra that are calibrated using blackbody assumptions. To help confirm our identifications of the principal molecular features, we present a synthetic spectrum computed using a model atmosphere with the approximate parameters of alpha Tau. This theoretical synthetic spectrum, which contains line opacity only from CO and SiO, reproduces the observed spectrum of alpha Tau to within a few percent.