, Volume 97, Issue 2, pp 369-378

Three components of 3–4 μm absorption bands

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Abstract

We present 2–4 μm spectra of six infrared sources for which the extinction is not purely interstellar, but is dominated by circumstellar or molecular cloud dust. In all cases the absorption bands differ from the interstellar case, though a component of the interstellar absorption is often present. We also present an improved absorption spectrum for the galactic centre source IRS 7, correcting spurious features in our previous spectrum.

Three independent components can be identified: (i) The interstellar component, probably of organic origin, and itself not necessarily invariant; (ii) The symmetrical water ice feature at 3.06 μm, found most commonly in molecular clouds; (iii) A component at 3.53 μm possibly identified with solid formaldehyde grains, and seen only in molecular clouds because of its weakness.

Other absorption components appear to be unrelated to those in the 3–4 μm region, most notably the 10 μm absorption found in oxygen-rich giants and the interstellar medium, and presumable inorganic in nature.

Our observations include the first detection of water ice absorption in a source in the ρ Oph dark cloud. Biological materials provide the best fit to the interstellar case, but do not presently account for the distinct 3.53 μm component. We stress the need for further laboratory experiments using simpler organic materials.