Applications: Cosmic Dust Grains

Abstract

Although the importance of cosmic dust has long been recognized in astronomy, only a partial understanding of the nature of dust grains has been achieved. In fact, most of the information on the interstellar matter that forms the so-called interstellar medium (IST) comes from spectroscopical analysis and from the study of the extinction of the starlight that propagates through the IST. We can summarize the available information as follows:

• In the range from the visible to the near infrared, the average galactic extinction curve depends more or less linearly on λ^-1, with the noticeable exception of the 2175 Å bump. This kind of dependence, which is easily discerned in the curve reported, e.g., by Aiello et al. [128], is consistent with an interstellar medium that includes solid grains whose overall size is of the order of the wavelength [129].

• Since the galactic extinction depends on the polarization, the dust grains should be intrinsically nonspherical. Moreover, the detail of the polarization dependence sets to 0.1–0.2μm the upper limit for the size of the grains [129].

• The spectroscopic analysis is consistent with a chemical composition of the grains that includes carbon, both in its amorphous and graphitic form, silicates and (possibly organic) ices [130].