Interstellar dust scattering

Electromagnetic stellar radiation which has passed through regions containing interstellar dust particles. The values of the absorbed or scattered wavelengths make it possible to draw conclusions on the chemical composition or the physical properties of the particles. 

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 ...

Extracting information about interstellar dust from analysis of scattered light, as opposed to transmitted light, is fraught with many difficulties. 

Spectropolarimetric monitoring is being carried out on the Anglo-Australian telescope by Cropper et al. (1987). The initial continuum polarisation was about 0.8%, but this subsequently decreased. However, the polarisation in the lines, particulary in the absorption component of Ha has increased sharply. Since the polarisation is determined by the interstellar dust, the shape of the supernova fireball and the scattering processes in the photosphere, these results are difficult to interpret However, they can provide us with very useful modelling constraints. 

The small particle limit usually applies to interstellar dust (a <, 0.1 pm) for wavelengths from the optical to the IR spectral region. If this limit applies, scattering is inefficient compared to absorption (cf. Fig. A3.1) and can be neglected in most applications. 

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. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Dust Micron-sized particles of silicate in the interstellar medium responsible for short-wavelength scatter - dust particles become covered with water-ice mantels. 

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