We have obtained 3.6 and 4.5 um photometry with Spitzer’s Infrared Array Camera (IRAC) for a set of 51 of the stars that were flux calibration standards for Spitzer’s Infrared Spectrograph (IRS). The goal is to help tie the calibration of IRS to the IRAC photometric system. The IRS is already tied to the calibration of the Multiband Infrared Photometer for Spitzer (MIPS). The IRS wavelength range overlaps only slightly with the 4.5 um band and not at all with the 3.6 um band. To relate the IRS and IRAC data, we generated synthetic spectra from spectral templates of stars with the same spectral and luminosity classes as our sample stars and normalized them to the IRS data at 6-7 um. Synthetic photometry from these templates were then compared to the actual IRAC photometry. For the A dwarfs, the synthetic and actual photometry agree to better than 1% in both bands. For the K giants, the IRAC photometry averages about 1.5% higher at 3.6 um and 2% lower at 4.5 um compared to our first attempt with the synthetic photometry. Modifying the synthetic spectra for the K giants with slightly cooler effective temperatures improves the the agreement, but at the cost of larger differences in the absorption bands from SiO and CO. This result implies that a single effective temperature does not fit both the mid-infrared continuum and the absorption bands simultaneously.
This work was sponsored by NASA grant NNXX17AF23G.