3.3 AND 11.3 MICRON IMAGES OF HD-44179 - EVIDENCE FOR AN OPTICALLY THICK POLYCYCLIC AROMATIC HYDROCARBON DISK

Images of HD 44179 (the Red Rectangle) obtained in the 3.3 and 11.3 mu m emission bands show two different spatial distributions. The 3.3 mum band image is centrally peaked and slightly extended N-S while the 11 .3 mum image shows a N-S bipolar shape with no central peak. If the 3. 3 mum band image shows the intrinsic emission of the 11.3 mum band, th en the data suggest absorption of the 11.3 mum emission near the cente r of HD 44179 by a disk with an optical depth of about 1, making HD 44 179 the first object in which the infrared emission bands have been ob served to be optically thick. Since there is no evidence of absorption of the 3.3 mum emission band by the disk, the absorption cross sectio n of the 3.3 mum band must be substantially less than for the 11.3 mum band. Since the 3.3 and 11.3 mum bands are thought to arise from diff erent size PAHs, the similar N-S extents of the two images implies tha t the ratio of small to large PAHs does not change substantially with distance from the center.