Spectral irradiance calibration in the infrared. XI. Comparison of alpha Bootis and 1 Ceres with a laboratory standard

Infrared spectra of two celestial objects frequently used as flux standards are calibrated against an absolute laboratory flux standard at a spectral resolving power of 100 to 200. The spectrum of the K1.5 III star alpha Boo is measured from 3 to 30 mu m, and that of the C-type asteroid 1 Ceres from 5 to 30 mu m. While these "standard" spectra do not have the apparent prec ision of those based on calculated models, they do not require the assumpti ons involved in theoretical models of stars and asteroids. Specifically, th ey provide a model-independent means of calibrating celestial flux in the s pectral range from 12 to 30 mu m, where accurate absolute photometry is not available. The agreement found between the spectral shapes of alpha Boo an d Ceres based on laboratory standards and those based on observed ratios to alpha CMa (Sirius) and alpha Lyr (Vega), flux-calibrated by theoretical mo deling of these hot stars, strengthens our confidence in the applicability of the stellar models as primary irradiance standards.