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The galactic distribution of aliphatic hydrocarbons in the diffuse interstellar mediumThe infrared absorption feature near 2950(exp -1) (3.4 micron), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups m(exp -3) and about 2 to 5 -CH2- groups m(exp -3). These densities are consistent with the strengths of the 2955 and 2925 cm(exp -1) (3.4 micron) band being described by the relations A(sub nu)/tau(sub 2955 cm(exp -1)) = 270 +/- 40 and A(sub nu)/tau(sub 2925 cm(exp -1)) = 250 +/- 40 in the local diffuse ISM.
Document ID
19960009749
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Sandford, Scott A. (NASA Ames Research Center Moffett Field, CA, United States)
Pendleton, Yvonne J. (NASA Ames Research Center Moffett Field, CA, United States)
Allamandola, Louis J. (NASA Ames Research Center Moffett Field, CA, United States)
Date Acquired
September 6, 2013
Publication Date
February 20, 1995
Publication Information
Publisher: The American Astronomical Society
Subject Category
Astrophysics
Report/Patent Number
NASA-CR-200003
NIPS-96-07110
NAS 1.26:200003
Accession Number
96N16915
Funding Number(s)
PROJECT: RTOP 185-52-12-09
PROJECT: RTOP 452-33-93-03
PROJECT: RTOP 188-44-21-04
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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