Abstract:
This is an analysis of the 4000 to 1000 cm^-1 (2.5 to 10 micron) region of the spectrum of diffuse interstellar medium (DISM) dust compared with the spectra of thirteen chemical entities produced in the laboratory which serve as analogs to the interstellar material. The organic signatures of extragalactic dust, carbonaceous chondritic material, and E. coli bacteria are also presented because these have been discussed in the literature as relevant to the diffuse interstellar medium. Spectral analysis of the DISM allows us to place significant 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 profile and sub-peak positions of the 2940 cm^-1 (3.4 micron) 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 micron), 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 micron), 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 micron) and 1370 cm^-1 (7.25 micron).

We conclude that the organic refractory material in the diffuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains H₃C-(CH₂)_n - with n much greater than 4 or 5 are not major constituents of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birthsite for the carrier of the 3.4 micron band in the outflow region of evolved carbon stars. The organic material extracted from the Murchison carbonaceous meteorite and the spectrum of E. coli bacteria reveal spectral features in the 5-10 micron region that are absent in the DISM. Although the presence of unaltered circumstellar components in the Murchison meteorite has been established through several lines of evidence, it is unclear whether or not the aliphatic component which gives rise to the 3.4 micron band is in that category. Considering the complete 2-10 micron wavelength region, there is no spectral evidence for a biological origin of the 3.4 micron interstellar absorption band. The similarity of the aliphatic CH stretch region of dust from our own galaxy compared with that of distant galaxies suggests that the organic component of the ISM is widespread and may be an important universal reservoir of prebiotic organic carbon

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Table X The approximate number of aromatic and aliphatic carbon atoms and the numbers of different types of CH bonds comprising the structure shown in Figure 16.

a) Fragment showing the basic structural and molecular character of a carbonaceous, interstellar dust grain in the diffuse interstellar medium. The molecular details have been deduced from the spectroscopic constraints as discussed in sections 5 and 6. The specific geometries of the aromatic plates and aliphatic components simply represent what is likely. The structure is somewhat splayed out to reveal the molecular structural details; we envision the actual structure somewhat more closed in. For the interconnected species (not the free floating entities), the relative numbers of aromatic and aliphatic carbon-hydrogen bonds, as well as their sub-classification within type, are all consistent with the observed spectrum described here. The latter includes the aliphatic -CH₃ to -CH₂-ratio, and the relative numbers of aromatic solo, duo, trio, and quartet hydrogens deduced from the interstellar IR emission bands as described in Hony et al. 2001. The approximate volume of this fragment is on the order of 10-19 cm³ ). Thus, a typical 0.1 micron DISM carbonaceous dust grain would comprise approximately 104 of these fragments.
b) The encircled regions (not shown in the interactive version) are expanded showing greater detail.

We especially wish to thank Cristina Dalle Ore who spent many hours assembling all the spectral comparison figures and tables presented here. Her help has been invaluable in preparing this manuscript. We are also very grateful to Jason Dworkin and Max Bernstein for their invaluable work, insight, and patience in preparing Figure 16. We are grateful to Dale Cruikshank and Vito Mennella for a careful review of the manuscript. We express thanks to J. Trent, H. Kagawa, and B. Khare for providing us with the IR spectrum of freeze dried E. coli. We also are especially thankful to all our colleagues who sent us electronic versions of their data, and for the helpful discussions we have had throughout the development of this paper with many of them, especially Vito Mennella, Marla Moore, Reginald Hudson and Max Bernstein. We specifically thank the following colleagues for assistance in obtaining the data necessary for these comparisons: M. Bernstein, A. Boogert, J. Chiar, L. Colangeli, C. Dudley, W. Duley, D. Furton, J. M. Greenberg, Th. Henning, R. Hudson, G. Lee-Dadswell, A. Li, V. Mennella, M. Moore, M. Palumbo, V. Pirronello, S. Sandford, M. Schnaiter, S. Seahra, A. Scott, G. Strazzulla, A. G. G. M. Tielens, T. Wdowiak, D. C. B. Whittet, A. Witt, G. Wright, S. Wada, and D. Wooden. We gratefully acknowledge NASA's support for this work with grants from NASA's Exobiology (344-38-12-09 (YJP); 344-38-12-04(LJA)), and Long Term Space Astrophysics (399-20-61; LJA) programs.

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