Next Contents


Interstellar dust reveals its presence in astrophysical environments and its (both positive and negative) role in astrophysics mainly through its interaction with electromagnetic radiation (see Li [38] for a recent review):

In order to correct for the effects of interstellar extinction and deredden the reddened starlight, it is essential to understand the absorption and scattering properties of interstellar grains at short wavelengths (particularly in the optical and ultraviolet [UV]). The knowledge of the optical and UV properties of interstellar dust is also essential for interstellar chemistry modeling since the attenuation of UV photons by dust in molecular clouds protects molecules from being photodissociated. The knowledge of the dust emission properties at longer wavelengths are important (i) for interpreting the IR and submillimeter (submm) observations of emission from dust and tracing the physical conditions of the emitting regions, (ii) for understanding the process of star formation for which the dust is not only a building block but also radiates away the gravitational energy of collapsing clouds (in the form of IR emission) and therefore making star formation possible, and (iii) for understanding the heating and cooling of the interstellar medium (ISM) for which interstellar dust is a dominant heating source by providing photoelectrons (in the diffuse ISM) and an important cooling agent in dense regions by radiating in the IR (see Li & Greenberg [45] for a review).

Ideally, if we know the size, shape, geometry and chemical composition (and therefore the dielectric function) of an interstellar grain, we can calculate its absorption and scattering cross sections as a function of wavelength. If we also know the intensity of the illuminating radiation field, we should be able to calculate the equilibrium temperature or temperature distribution of the grain from its absorption cross section and therefore predict its IR emission spectrum.

However, our current knowledge of the grain size, shape, geometry and chemical composition is very limited; the nature of interstellar dust itself is actually mainly derived from its interaction with radiation (see Li [38] for a review):

The inferences from observations for interstellar dust summarized above are quite general and model-independent. But these inferences are not sufficient to quantitatively derive the absorption and emission properties of interstellar grains. For a quantitative investigation, one needs to make prior specific assumptions concerning the grain size, shape, geometry and chemical composition which are still not well constrained by the currently available observational data. To this end, one needs to adopt a specific grain model in which the physical characteristics of interstellar dust are fully specified. While a wide variety of grains models have been proposed to explain the interstellar extinction, scattering, polarization, IR emission and elemental depletion, so far no single model can satisfy all the observational constraints (see Li [39] and Dwek [22] for recent reviews).

In view of this, in this article I will first try to place constraints on the absorption and emission properties of interstellar dust based on general physical arguments; these constraints are essentially model-independent.

In astrophysical literature, the most frequently used quantities describing the dust absorption and emission properties are the mass absorption coefficient (also known as "opacity") kappaabs with a unit of cm2 g-1, and the emissivity epsilonlambda, defined as the energy emitted per unit wavelength per unit time per unit solid angle per unit mass, with a unit of erg s-1 sr-1 cm-1 g-1. The Kirchhoff's law relates epsilonlambda to kappaabs through epsilonlambda = kappaabs(lambda) Blambda(T) if the dust is large enough to attain an equilibrium temperature T when exposed to the radiation field, or epsilonlambda = kappaabs(lambda) integ0infty dT Blambda(T) dP / dT if the dust is so small that it is subject to single-photon heating and experiences "temperature spikes", where Blambda is the Planck function, dP is the probability for the dust to have a temperature in [T, T + dT]. Other often used quantities are the absorption cross section Cabs and absorption efficiency Qabs, with the latter defined as the absorption cross section Cabs divided by the geometrical cross sectional area Cgeo of the grain projected onto a plane perpendicular to the incident electromagnetic radiation beam (Bohren & Huffman [8]). For spherical grains of radii a, Cgeo = pi a2 so that Qabs = Cabs / pi a2. By definition, kappaabs = Cabs / m = Cabs / (Vrho), where m, V and rho are respectively the dust mass, volume, and mass density; for spherical grains, kappaabs = 3Qabs / (4a rho).

In Section 2 I will apply the Kramers-Kronig relation to place a lower limit on beta (the wavelength dependence exponent index of kappaabs) and an upper limit on the absolute value of kappaabs. The state of our knowledge of interstellar grain opacity will be presented in Section 3 (with a focus on beta) and in Section 4 (with a focus on the absolute value of kappaabs), followed by a summary in Section 5.

Next Contents