Absolute Physical Calibration in the Infrared

We determine an absolute calibration for the MIPS 24 microns band and recommend adjustments to the published calibrations for 2MASS, IRAC, and IRAS photometry to put them on the same scale. We show that consistent results are obtained by basing the calibration on either an average A0V star spectral energy distribution (SED), or by using the absolutely calibrated SED of the sun in comparison with solar-type stellar photometry (the solar analog method). After the rejection of a small number of stars with anomalous SEDs (or bad measurements), upper limits of ~ 1.5% (rms) are placed on the intrinsic infrared SED variations in both A dwarf and solar-type stars. These types of stars are therefore suitable as general-purpose standard stars in the infrared. We provide absolutely calibrated SEDs for a standard zero magnitude A star and for the sun to allow extending this work to any other infrared photometric system. They allow the recommended calibration to be applied from 1 to 25 microns with an accuracy of ~2 %, and with even higher accuracy at specific wavelengths such as 2.2, 10.6, and 24 microns, near which there are direct measurements. However, we confirm earlier indications that Vega does not behave as a typical A0V star between the visible and the infrared, making it problematic as the defining star for photometric systems. The integration of measurements of the sun with those of solar-type stars also provides an accurate estimate of the solar SED from 1 through 30 microns, which we show agrees with theoretical models.