Spitzer IRAC Photometry of K Giants Used as Spectro-Photometric Standards for IRS

We used Spitzer's IRAC at 3.6 and 4.5 um to observe 36 K giants which were spectro-photometric standard stars for Spitzer's IRS.  To compare the data from IRAC and the IRS, we generate synthetic photometry from ISO/SWS-based spectral templates to bridge the gap between the end of the spectra and the photometric bands. We applied two methods for selecting templates, both of which confirmed a match of ~1% between the observed and synthetic photometry at 3.6 um.  Choosing the templates based on the optical-NIR photometry and effective temperature of the star produced a match of <1% at 4.5 um, with small but noticeable discrepancies in the 8 um SiO absorption band for some stars.  Choosing the templates to match the 8 um band in the IRS data required somewhat higher temperatures and showed a weak temperature dependence for the 4.5 um photometric ratios.  Thus, we were unable to simultaneously fit both the SiO absorption band at 8 um and the IRAC photometry at 4.5 um with the templates. The discrepancy at 4.5 um was small, less than 2%, but this result points to the continued challenge of using K giants as spectro-photometric standards.