SPECTRAL IRRADIANCE CALIBRATION IN THE INFRARED .2. ALPHA-TAU AND THE RECALIBRATION OF THE IRAS LOW RESOLUTION SPECTROMETER

We describe a general technique for assembling continuous calibrated infrared spectra over a wide wavelength range. To demonstrate the method we construct an absolutely calibrated 1-35 mum spectrum of alpha Tau and independently validate the method using new and carefully designed observations. First we investigate the absolute calibration of the IRAS low resolution spectrometer (LRS) database by comparing this observed spectrum of alpha Tau with that assumed in the original LRS calibration scheme. Second, an analysis of asteroidal LRS spectra results in an independent assessment of the calibration problems with the LRS and provides a natural complement to the effort based on alpha Tau. Stellar studies define the short wavelength problems well; the cool asteroids characterize the long wavelengths. Third, we make a direct comparison of LRS stellar spectra with independently calibrated long-wavelength (approximately 20 mum) airborne and ground-based spectra. These three approaches are complementary and have been combined. Neglect of the SiO fundamental band in alpha Tau has led to the presence of a specious "emission" feature in all LRS spectra near 8.5 mum, and to an incorrect spectral slope between 8 and approximately 12 mum. We explain the differences between this new LRS recalibration and that due to Volk & Cohen [AJ, 98, 1918 (1989)] and discuss why the present set of "correction factors" is more accurate. Recognition of the influence of the SiO fundamental on both the LRS calibration and on the continua of cool normal giant stars also leads to an understanding of the apparent plethora of classes into which these normal stars fall when assigned by the AUTOCLASS (Cheeseman et al. 1989) artificial intelligence scheme applied to the IRAS LRS Atlas.