The Organic Refractory Material in the Diffuse Interstellar Medium: Mid‐Infrared Spectroscopic Constraints
Abstract
This is an analysis of the 4000È1000 cm~1 (2.5È10 km) region of the spectrum of diuse interstellar medium (DISM) dust compared with the spectra of 13 materials produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chon-dritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the diuse interstellar medium. Spectral analysis of the DISM allows us to place signiÐ-cant constraints on the applicability of proposed candidate materials. The spectra of candidate materials are evaluated using four spectral characteristics based on the interstellar data: (i) comparisons of the proÐle and subpeak positions of the 2940 cm~1 (3.4 km) aliphatic CH stretching-mode band, (ii) the ratio of the optical depth (O.D.) of the aliphatic CH stretch to the O.D. of the OH stretch near 3200 cm~1 (3.1 km), (iii) the ratio of the O.D. of the aliphatic CH stretch to the O.D. of the carbonyl band near 1700 cm~1 (5.9 km), and (iv) the ratio of the O.D. of the aliphatic CH stretch feature to the O.D. of the CH deformation modes near 1470 cm~1 (6.8 km) and 1370 cm~1 (7.25 km). We conclude that the organic refractory material in the diuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic