Ground-based Mid-Infrared Spectroscopy of Faint Standard Stars in the North Ecliptic Polar Region for theASTRO-FInfrared Camera

Abstract

Faint standard stars in the mid-infrared region (less than a few hundred millijanskys at 9 μm) are required for the on-orbit calibration of the Infrared Camera (IRC) on board ASTRO-F because of its high sensitivity, as well as the small dynamic range. Mid-infrared low-resolution (λ/Δλ ~ 250) spectra of six faint K and M giant stars (70-180 mJy at 9 μm, K1.5-M0 III), which are located in the north ecliptic polar region and are thus suitable for calibrators for the IRC, are obtained by using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the Subaru telescope to test the faint-infrared standard-star network recently constructed for space-borne instruments. The spectra are directly calibrated relative to Vega (A0 V). The present results indicate that the spectra of the faint standard K-M0 giant stars show good agreement with those predicted in the standard network within 10% accuracy on average. The overall spectral shapes are also in agreement with the predictions. In particular, the observed SiO absorption feature in the 8 μm region matches well with the prediction for an M0 III star. The present observations directly connect a primary standard calibrator (Vega) to faint K giant calibrators of the network in the 10 μm region and validate several faint (~0.1 Jy) members of the standard-star network for the first time. Absolute calibrations are provided for five of the medium-band filters of the COMICS instrument in the 10 μm window.