IRTS OBSERVATION OF THE MIDINFRARED SPECTRUM OF THE ZODIACAL EMISSION

We present the mid-infrared spectrum (3-12 mu m) of the zodiacal emiss ion obtained by the Infrared Telescope in Space (IRTS), the first Japa nese cryogenically cooled orbital infrared telescope. The Near-Infrare d Spectrometer (NIRS) on board IRTS provided the spectrum of 3-4 mu m, while that of 4.5-11.7 mu m has been observed by the Mid-Infrared Spe ctrometer (MIRS), In this paper we present the data reduction and resu lts of the observations by MIRS. Spectra of the background emission at high galactic latitudes (\b\ > 30 degrees) have been extracted from t he MIRS observations by excluding point sources. The observed sky brig htness has a clear dependence on the ecliptic latitude, indicating tha t the zodiacal emission dominates in the mid-infrared sky brightness, On the other hand, the spectral shape does not show any appreciable de pendence on the ecliptic latitude for beta = 0 degrees-75 degrees. 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 3-6 m u m range. Alternatively, the spectrum of the zodiacal emission can be reproduced fairly well by a grey body at 280 K with an excess around 10 mu m. In this case the excess may be attributed to a silicate emiss ion band. Other than these excesses, no spectral features above the 10 % level are seen in the MIRS spectrum.