A Theory of Interstellar Polarization.

Abstract

A detailed analysis is given of the polarization of starlight produced by ferromagnetic grains, oriented by an interstellar magnetic field. Lack of information on the optical properties of large elongated grains with a high refractive index prevents a consideration of grains with a circumference very much bigger than 8000 A, the longest wave length at which polarization measures have been made. Grains of about this size, with a mean radius of 1.3 X 10" 6 cm and composed primarily of Fe, Mg, and their compounds, are shown to arise from colhsions between ordinary grains, containing predominantly ZLO, NHz, and C34. It is estimated that one out of every four grains is compound, with a shell of ices surrounding a dense nucleus of this sort. It is quite possible that such nuclei will be ferromagnetic; if so, they will all be highly magnetized single domains. However, a magnetic field of about 2 X 10~ 3 gauss is apparently required to orient these grains sufficiently to explain the observed polarization. Larger grains, which could be oriented by weaker fields, might form in two ways. First, the individual nuclei might stick together, forming long rods; this process will take place at an appreciable rate only if most of the grains occur in concentrations in which the density is one or two orders of magnitude greater than the assumed average cloud density. Second, successive colhsions between compound nuclei and other grains would be expected to produce some ferromagnetic grains with relatively large radii. Since the optical properties of such large grains are unknown, whether or not they could explain the observed polarization is uncertain. The polarization of light from distant stars, found by Hiltner 1 and Hall, 2 is perhaps one of the most unexpected discoveries of modern astrophysics. The observations indicate that light from distant stars is polarized only if the stars are reddened and that the effect is independent of the physical properties of the stars. More recent evidence 3,4 supports these indications. It seems reasonable to infer that the solid particles or grains producing the interstellar extinction are somehow responsible for the polarization. If the grains were elongated and also oriented by some force, the observational results could be explained. A magnetic field provides an evident force which might orient grains. In view of the very long relaxation time for galactic magnetic fields, any field present in the galaxy during its formation would presumably still be present. Moreover, magnetic fields can be produced by turbulence of the interstellar gas, as suggested by Fermi. 5 According to this mechanism, turbulence in an extended, electrically conducting medium will increase the magnetic induction B until equipartition of energy is established between the magnetic energy density, B 2 /8ir, and the density of kinetic energy, %pv 2. An average interstellar density of one E atom per cubic centimeter, and a root-mean-square cloud velocity of 22.5 km/sec, the value adopted by Oort and van de Hulst, 6 yield a value of 1.0 X 10-5 gauss for 5. A more detailed hydrodynamic theory by Batchelor 7 shows that isotropic .* This work was supported in part by a contract with the Office