T. Onaka 1, M. Mizutani 1, D. Tomono 1, H. Shibai 2, T. Nakagawa 3, & Y. Doi 4
1 Department of Astronomy, School of Science, University of Tokyo, Japan
2 Department of Physics, School of Science, Nagoya University, Japan
3 Institute of Space and Astronautical Science, Japan
4 Department of Earth Science and Astronomy, College of Arts and Sciences, University of Tokyo, Japan
The latest observations by IRTS and ISO/PHT-S clearly show that the so-called unidentified infrared (UIR) emission bands, whose major features are at 3.3, 6.2, 7.7, 8.6, and 11.2 microns, do exist in the diffuse galactic emission as predicted by Puget et al (1985) and that the UIR band emitters are well mixed with the sub-micron size dust grains (Tanaka et al. 1996; Onaka et al. 1996; Mattila et al. 1996). In order to further study their properties in relation to the large dust grains in various physical conditions, we have made raster-scan observations of diffuse emission in several galactic objects with PHT-S and LWS full grating scan. The objects include an active star-forming region (Carina nebula), a HII region (W1), and reflection nebulae in rho Oph cloud. The far-infrared continuum spectra obtained by LWS enable to estimate the temperature of the large dust grains and the strength of the incident radiation.
The derived radiation field strength in Carina nebula is estimated to span from 300 to 30000 Go, where Go is the radiation strength in the solar neighbourhood. The PHT-S spectra in Carina nebula indicate that the band intensity ratio of 6.2 to 7.7 microns and 11.2 to 7.7 microns are fairly constant over the estimated radiation strength range. The UIR band strength relative to the far-infrared intensity (FIR) decreases slowly with Go in Carina nebula. In the reflection nebula in rho Oph cloud, the band ratios are also indicated to be constant. The UIR to FIR ratio in the reflection nebula seems to be constant for Go<100, but starts deceasing with Go at large radiation strength, being compatible with the results in Carina nebula. A similar trend has been reported by Boulanger et al (1998) for CAM-CVF observations and indicated also by the IRTS observations, suggesting that the trend is universal in general interstellar medium. The constancy in the UIR/FIR ratio for Go<100 is compatible with the interpretation that the UIR band emission comes from very small grains with fluctuating temperatures or infrared fluorescence. The decrease at higher Go may indicate the decrease in abundance of UIR emitters under strong radiation field, but other possibilities cannot be ruled out. These trends together with the constancy in the band ratio provide important information on the nature of the UIR emitters.