Far-Infrared and Millimeter Continuum Studies of K Giants: α Bootis and α Tauri

We have imaged two normal, noncoronal, infrared-bright K giants, α Tau and α Boo, in the 1.4 and 2.8 mm continua using BIMA. These stars have been used as important absolute calibrators for several infrared satellites. Our goals are (1) to establish whether these stars radiate as simple photospheres or possess long-wavelength chromospheres and (2) to make a connection between millimeter-wave and far-infrared (FIR) absolute flux calibrations. To accomplish these goals we also present Infrared Space Observatory Long Wavelength Spectrometer measurements of both these K giants. The FIR and millimeter continuum radiation is produced in the vicinity of the temperature minimum in α Tau and α Boo. We find that current photospheric models predict fluxes in reasonable agreement with those observed for wavelengths that sample the upper photosphere, namely, ≤125 μm in α Tau and α Boo. We clearly detect chromospheric radiation from both stars by 2.8 mm (1.4 mm in the case of α Boo). Only additional observations can determine precisely where beyond 125 μm the purely radiative models fail. Until then, purely radiative models for these stars can only be used with confidence for calibration purposes below 125 μm.