Determination of dust properties and magnetic fields from polarimetric and photometric observations of stars (Review)

Abstract

Various models of the interstellar extinction and polarization are considered. Special attention is devoted to alignment mechanisms of the dust grains. such as Davies-Greenstein mechanism and suprathernial spinning alignment. and their approximate versions. It is concluded that the grain SIK and the strength and the direction of the interstellar magnetic field may be estimated from the extinction and polarization data. Applications o f the thcorctical results to the observations of the stars in the galactic plane are discussed. 1.

Interstellar extinction and polarization - a spheroidal dust grain approach perspective

“… where C ext,gra (λ) =π r 2 gra Q ext,gra (λ) and is the relative column density of the i th population. It is evident that The values of are obtained by averaging of the cross‐sections over the size distribution and grain orientations (Hong & Greenberg 1980; Voshchinnikov 1989), Here, β is the precession‐cone angle for the angular momentum J which spins around the direction of the magnetic field B , ϕ the spin angle, ω the precession angle (see Fig. 1).…”

“…Therefore, we consider the classical alignment mechanism – so‐called imperfect Davis–Greenstein (DG) orientation (Davis & Greenstein 1951). The imperfect DG mechanism is described by the function f (ξ, β) depending on the alignment parameter ξ and the precession angle β (Hong & Greenberg 1980; Voshchinnikov 1989), The parameter ξ depends on the particle size r V , the imaginary part of the grain magnetic susceptibility χ″=ϰω d / T d , where ω d is the angular velocity of a grain and ϰ= 2.5 × 10 −12 (Davis & Greenstein 1951), gas density n H , the strength of magnetic field B , and temperatures of dust T d and gas T g . By introducing a parameter one can get …”

“…The values of C ext,j and C pol,j (j = C, Si) are obtained by averaging of the cross-sections over the size distribution and grain orientations (Hong & Greenberg 1980;Voshchinnikov 1989),…”

“…In early studies (e.g. Hong & Greenberg 1980; Voshchinnikov 1989), an unphysical model of infinitely long cylinders was applied 1 . More realistic is the spheroidal model of grains with the shape of these axisymmetric particles being characterized by the only parameter – the ratio of the major to minor semi‐axis a / b .…”

Dust in the diffuse interstellar medium

“…However, to account for the widely observed interstellar polarisation, non-spherical dust particles partially aligned by some mechanism need to be considered. This has been done by Hong & Greenberg (1980); Voshchinnikov (1989); Li & Greenberg (1997), who, among others, considered infinite cylinders. More realistic particle shapes such as spheroids have been considered recently.…”