Interstellar dust 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 interstellar extinction has a great effect on distance determination for stars. The B/V index derived in Chapter 2 will be distorted by the presence of interstellar dust, with an amount of radiation in the blue part of the spectrum removed. The difference between the observed colour index and the colour index on which it should have based its temperature is called the colour excess. We defined m to be the measured apparent magnitude, which must now be corrected by an amount Av and added to the distance modulus equation ... 

Kratschmer, W., and D. R. Huffman, 1979. Infrared extinction of heavy ion irradiated and amorphous olivine, with applications to interstellar dust, Astrophys. Space Sci., 61, 195-203. 

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. 

This difficulty for CS 776 can be assessed quantitatively. Le Bertre (1990) derives a mass loss rate from IRC-20131 of 4.5 x 10 7 MQ a-1. Scaled to the outflow velocity of the material of 26 km s-1 (Zuckerman Dyck 1989) instead of the assumed value of 15 km s 1 and using Le Bertre s distance of 1.3 kpc instead of their estimate of 1.43 kpc, the re-computed mass loss rate from Claussen et al. (1987) is 6.6 x 10 Mq a-1 in reasonable agreement with the rate estimated by Le Bertre (1990). Because the separation of the A star companion from the carbon star CS 776 is 1.81", the projected separation between the two stars is 3.6 x 1016 cm. Therefore, according to equation (3), the column density of circumstellar hydrogen between us and the A star companion to CS 776 is 1.5 x 1019 cm-2. However, the total extinction toward this companion is Av = 1.71 mag (Le Bertre 1990) which, for a standard interstellar dust to gas ratio, corresponds to a hydrogen column density of 3x 1021 cm-2 (Spitzer 1978). This column density is consistent with the expected concentration of interstellar matter within the plane of the Milky Way. Thus, towards the companion to CS 776, there appears to be about 100 times more interstellar than circumstellar matter. Therefore, unless the diffuse bands are extremely strong in the circumstellar matter around CS 776, it seems quite likely that the bulk of the diffuse bands in the spectrum result from interstellar matter. 

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. 

Kim S.-H., Martin P. G., and Hendry P. D. (1994) The size distribution of interstellar dust particles as determined from extinction. Artro/ihyr. J. 422, 164-175. 

The possibility of observing the outburst of Supernovae at the remote Galaxy regions, invisible within optical band because of interstellar extinction using thermal IR-radiation of interstellar dust heated by Supernova outburst is discussed in this paper. The investigation of this phenomenon by means of cooled IR-telescopes will allow the Supernova outburst parameters to be determined and the characteristics of the interstellar dust distribution to be studied. The similar effects can be detected in the vicinities of the well-known remnants of Supernovae, in particular, near Tycho Brahe and Cassiopeia A Supernovae. 

Interstellar dust is a ubiquitous component of our galaxy and most other galaxies. Extinction by interstellar dust is often called reddening since the dust is far more efficient at absorbing and scattering blue light than red or infrared light. We see this same phenomenon in terres-... 

Observations of the reflection spectrum of interstellar dust have also yielded some insights concerning the properties of interstellar dust particles in particular, it has been found that, at least in some reflection nebulae, the particles are very efficient scatterers of far-UV radiation. Except in the vicinity of the 220-nm extinction peak, it appears that most of the interstellar extinction is due to scattering rather than pure absorption. 

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. 

The extinction towards a star at 450 nm is 0.24 and is attributed to an interstellar cloud containing dust particles with an extinction coefficient of 0.0032 pc-1. Calculate the diameter of the intervening molecular cloud, expressing your answer in light-years. 

The extinction for dust particles is found to follow the empirical relation sx = 0.008 k 4/3 mag pc-1 (Equation 5.3 - a truly terrible astronomical unit for a scientist ). Calculate the extinction per metre at a wavelength of 501 nm. The light from a star at 501 nm is 0.25 of that expected for a star with that surface temperature and the extinction is attributed to dust in an interstellar cloud. Calculate the diameter of the cloud along the line of sight. 

The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that the M type stars with CS have higher intrinsic luminosities in the K band than those without CS. The M stars used in the study are obtained from the Two Micron Sky Survey catalogue (IRC) which is an unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog. 

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). 

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