MARCS:: Model stellar atmospheres and their application to the photometric calibration of the Spitzer Space Telescope Infrared Spectrograph (IRS)

We describe state-of-the-art MARCS-code model atmospheres generated for a group of A dwarf, G dwarf, and late-G to mid-K giant standard stars, selected to photometrically calibrate the Spitzer Space Telescope Infrared Spectrograph (IRS) and compare the synthetic spectra to observations of HR 6688, HR 6705, and HR 7891. The general calibration processes and uncertainties are briefly described, and the differences between various templated composite spectra of the standards are addressed. In particular, a contrast between up-to-date model atmospheres and previously published composite and synthetic spectra is illustrated for wavelength ranges around 8 mum (where the SiO Deltav=1 band occurs for the cooler standards) and lambdagreater than or equal to20 mum, where the use of the Engelke function will lead to increasingly large discrepancies as a result of the neglect of gravity in cool stars. At this point, radiometric requirements are being met, absolute flux calibration uncertainties (1 sigma) are similar to20% in the Short-High and Long-High data and similar to15% in the Short-Low and Long Low data, and order-to-order flux uncertainties are similar to10% or less. Iteration between the MARCS model atmosphere inputs and the data processing will improve the S/N ratios and calibration accuracies.