Interstellar extinction curve

Stephens, J. R., 1980. Visible and ultraviolet (800-130 nm) extinction of vapor-condensed silicate, carbon, and silicon carbide smokes and the interstellar extinction curve, Astrophys. J., 23H, 450-461. 

In general, the photoabsorption cross section of individual and multishell fullerenes reproduce the behaviour of the interstellar extinction curve in the near UV. The theoretical spectra show a prominent absorption band around 5.7 eV which fits well... 

Among optical DIBs, the 4,430 A band is the strongest. This band is remarkably broad with a width (FWHM) of order a few tens of A. Krelowski and Walker (1987) assign the two broad DIBs 4,430 and 6,177 A to the same family and Krelowski et al. (1989) and McIntosh and Webster (1993) note that the carrier of this family appears to prefer denser interstellar gas than other carriers. There is also evidence of a positive correlation between the 4,430 A band and the strongest feature in the interstellar extinction curve, the UV bump at 2,175 A (Webster 1992 Nandy and Thompson 1975). It is therefore plausible that these two bands are produced by the same type of molecule. 

Fig. 2.3 (a) The visible-band EEL spectra of unexposed (dashed line) and hydrogenated (solid line) Cm films shown in Fig. 2.2, spectra labeled (a) and (d), compared with the extinction curve of the variable component (open circles) detailed in Webster (1997). (b) The far-UV to near-IR EEL spectrum of the hydrogenated Cm film (solid line) in Fig. 2.2 (d) compared to the mean interstellar extinction curve (open circles). The vertical scales of the EEL spectra and extinction curves in Fig. 2a and b are incommensurable, and the EEL spectra are arbitrarily scaled to give reasonable qualitative agreement for comparison... 

The basic information on the nature of the dust has been obtained from analysis of interstellar extinction from the near-infrared to the far-ultraviolet spectral region, using ground-based telescopes and the first space-borne ultraviolet telescopes. The derived interstellar extinction curve is in most parts rather smooth and shows only one broad and strong absorption feature centered around 220 nm (cf. Fitzpatrick Massa 2007). This feature is explained by carbonaceous dust grains (Stecher Donn 1965) with a wide distribution of sizes. The true nature of the carbonaceous dust material remains still somewhat unclear, but seems to be some kind of amorphous carbon (cf. Draine 2003, 2004, for a detailed discussion). 

This motivated a number of attempts, starting around 1970 with the models published by Hoyle Wickramasinghe (1969), Wickramasinghe (1970), and Wickramasinghe Nandy (1970) to reproduce the interstellar extinction curve with mixtures of silicate and carbon grains, and, occasionally, additional components. These models provided already successful fits to the observed extinction curve. This established silicate and carbon dust as the primary dust components of interstellar dust. In most of these studies it was assumed that interstellar dust is stardust, i.e. dust born in stellar ejecta. 

Figs. 6a und b. a) Typical observed interstellar extinction curve from the IR through the optical range to the UV. b) Schematic representation of the optical part of the extinction curve, defining the quantities A v, Ab and R... 

A new model of interstellar grains (Duley 1987 Jones, Duley and Williams 1987), proposed to account for the variety of observed interstellar extinction curves, implies that grains are a chemically active component of the interstellar medium and that shocks occur in this medium about once every million years or so. The consequences for interstellar chemistry are that for a substantial fraction of the time, diffuse cloud material is far from a chemical steady state, and that elemental abundances in the gas are time dependent. 

Besides contributing to specific bands in the IR, dust grains are also responsible for very broad absorption, culminating in a peak at 217 nm, and continuously rising beyond into the far UV, observed until the Lyman limit at 91.2 nm in the interstellar extinction curve of our Galaxy. Other galaxies have variants of this extinction... 

Fig. 3.11 Interstellar dust particles cause the extinction of starlight by the selective scattering of certain light wavelengths. Far IR is on the left, far UV on the right. Satellite data suggest that the extinction curve consists of three components ...

The comparison of the computed cross sections of fullerenes and buckyonions with observations of the UV bump for Ry = 3.1 allow an estimate of the number of these molecules in the diffuse interstellar medium. Let us describe the extinction curve as a + a2x + a37Tx) where 7Tx) is the theoretical cross section computed for each fullerene or buckyonion. Here we assume that indeed the extinction at the energy of the bump is the result of the fullerene plus silicate contributions. We obtain via a least squared fit the relative contribution of the two components (see Fig. 1.6b). The coefficients of this lineal component do not depend significantly on the particular fullerene under consideration taking typical values of a, 1.6 and a2 = 0.07 with a relative error of 20%. 

The cross section obtained for single fullerenes and buckyonions reproduce the behaviour of the interstellar medium UV extinction curve. A power-law size distribution n(R) R m with in = 3.5 1.0 for these molecules can explain the position and widths observed for the 2,175 A bump and, partly, the rise in the extinction curve at higher energies. We infer ISM densities of 0.2 and 0.1 ppm for small fullerenes and buckyonions (very similar to the densities measured in meteorites). If as expected the cosmic carbon abundance is close to the solar atmosphere value, individual fullerenes may lock up 20-25% of the total carbon in the diffuse interstellar space. 

It has been proposed that fullerane C60I I 6 can be taken as a model of the interstellar hydrogenated carbon dust (Cataldo 2003a, b), but it is also possible that this molecule is a contributor to the interstellar light extinction curve together with other dust. 

UV extinction curve is highly variable in both magnitude and shape in different directions in our galaxy and in the Magellanic Clouds. Hence, it appears that there must be at least three independent components of the interstellar dust, whose relative abundances vary in different ways with local conditions in the interstellar medium. 

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