Mid-infrared Photometric Analysis of Main Belt Asteroids: A Technique for Color-Color Differentiation from Background Astrophysical Sources

Creators: Bhattacharya, B.; Noriega-Crespo, A.; Penprase, B. E.; Meadows, V. S.; Salvato, M.; Aussel, H.; Frayer, D. T.; Ilbert, O.; Le Floc'h, E.; Looper, D.; Surace, J.; Capak, P.; Giorgini, J. D.; Granvik, M.; Grillmair, C.; Hagen, A.; Helou, G.; Reach, W. T.; Rebull, L. M.; Sanders, D. B.; Scoville, N.; Sheth, K.; Yan, L.

Abstract

The Spitzer Space Telescope routinely detects asteroids in astrophysical observations near the ecliptic plane. For the galactic or extragalactic astronomer, these solar system bodies can introduce appreciable uncertainty into the source identification process. We discuss an infrared color discrimination tool that may be used to distinguish between solar system objects and extrasolar sources. We employ four Spitzer Legacy data sets, the First Look Survey-Ecliptic Plane Component (FLS-EPC), SCOSMOS, SWIRE, and GOODS. We use the Standard Thermal Model to derive FLS-EPC main belt asteroid (MBA) diameters of 1-4 km for the numbered asteroids in our sample and note that several of our solar system sources may have fainter absolute magnitude values than previously thought. A number of the MBAs are detected at flux densities as low as a few tens of μJy at 3.6 μm. As the FLS-EPC provides the only 3.6-24.0 μm observations of individual asteroids to date, we are able to use this data set to carry out a detailed study of asteroid color in comparison to astrophysical sources observed by SCOSMOS, SWIRE, and GOODS. Both SCOSMOS and SWIRE have identified a significant number of asteroids in their data, and we investigate the effectiveness of using relative color to distinguish between asteroids and background objects. We find a notable difference in color in the IRAC 3.6-8.0 mm and MIPS 24 μm bands between the majority of MBAs, stars, galaxies, and active galactic nuclei, though this variation is less significant when comparing fluxes in individual bands. We find median colors for the FLS-EPC asteroids to be [F(5.8/3.6), F(8.0/4.5), F(24/8)] = (4.9 ± 1.8, 8.9 ± 7.4, 6.4 ± 2.3). Finally, we consider the utility of this technique for other mid-infrared observations that are sensitive to near-Earth objects, MBAs, and trans-Neptunian objects. We consider the potential of using color to differentiate between solar system and background sources for several space-based observatories, including Warm Spitzer, Herschel, and WISE.

Additional Information

© 2010. The American Astronomical Society. All rights reserved. Received 2009 May 12; accepted 2010 June 16; published 2010 August 6. The authors acknowledge the late S. Tyler, whose enthusiastic presence is missed by the FLS-EPC team. During his years at the Spitzer Science Center, Mr. Tyler assisted in data analysis for this project. The authors also thank the anonymous referee for helpful comments. B.B. thanks Dr. Tom Jarrett for providing valuable feedback, and the FLS-EPC team acknowledges Professor Jim Elliot for developing the original concept of the solar system First Look Survey. This work is based upon observations made with the Spitzer Space Telescope, operated by JPL, Caltech, and funded under NASA contract 1407. Additional work is funded by the NASA Herschel Science Center, operated by JPL, Caltech.

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