TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 50 (Nos. 6, 7), pp. 507-511, 1998

IRTS observation of the mid-infrared spectrum of the zodiacal emission

T. Ootsubo1, T. Onaka1, I. Yamamura 1, T. Tanabé 2, T. L. Roellig3, K.-W. Chan3, and T. Matsumoto4

1Department of Astronomy, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2Institute of Astronomy, Faculty of Science, University of Tokyo, Osawa, Tokyo 181-8588, Japan
3NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, U.S.A.
4Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan

(Received October 8, 1997; Revised February 6, 1998; Accepted February 21, 1998)

Abstract: We present the mid-infrared spectrum (3-12 mm) of the zodiacal emission obtained by the Infrared Telescope in Space (IRTS), the first Japanese cryogenically cooled orbital infrared telescope. The Near-Infrared Spectrometer (NIRS) on board IRTS provided the spectrum of 3-4mm, while that of 4.5-11.7mm has been observed by the Mid-Infrared Spectrometer (MIRS). In this paper we present the data reduction and results of the observations by MIRS. Spectra of the background emission at high galactic latitudes (|b| > 30°) have been extracted from the MIRS observations by excluding point sources. The observed sky brightness has a clear dependence on the ecliptic latitude, indicating that the zodiacal emission dominates in the mid-infrared sky brightness. On the other hand, the spectral shape does not show any appreciable dependence on the ecliptic latitude for b= 0°-75°. The spectrum combining the NIRS and MIRS observations can be fitted by a grey body radiation at 250 K, but excess emission is seen in the 3-6mm range. Alternatively, the spectrum of the zodiacal emission can be reproduced fairly well by a grey body at 280K with an excess around 10mm . In this case the excess may be attributed to a silicate emission band. Other than these excesses, no spectral features above the 10% level are seen in the MIRS spectrum.

Corresponding author E-mail: ootsubo@astron.s.u-tokyo.ac.jp

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