Distribution of the 3.3 micrometer Emission Band in the Galaxy

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Development of a cryogenic infrared camera to probe the 3.3 micron emission over the Galactic plane is underway. A Ritchey-Chretien optics system of the primary (25 cm in diameter) and secondary mirrors has been proved to have good optical quality. It has been installed in a cryostat with an InSb 256X256 array, and we have obtained infrared images with them. The electronics was tuned so that optimum operation is achieved under fairly strong background radiation. Observation of the Galactic plane with this system will be started this year.Infrared imaging and spectroscopy of individual star forming regions in the Galaxy has been made to investigate the evolutionary stages when the star forming region radiates the 3.3 micron emission. In S140, its infrared source IRS1 has conspicuous ice absorption at 3.1 microns, and polarization changes across the absorption band. However, it has no 3.3 micron emission, which might mean that this object is too young to emit the 3.3 micron emission. We also searched a IRAS sample of young stellar candidates, and detected the 3.3 micron emission in 17 out of 45 objects. We identify these objects as slightly younger stars than the Herbig Ae/Be stars. Many of such stars seem to show the 3.3 micron emission. We also plan to search for features of young stellar objects with the ice absorption and with the 3.3 micron emission in near infrared images.