INTERSTELLAR POLARIZATION AND GALACTIC MAGNETIC FIELD STRENGTHS

The maximum observed ratio of polarization to extinction is used as a criterion to attempt to make a choice among models of interstellar clouds consisting of grains of 1) dirty ice, 2) graphite and 3) graphite flakes with dirty ice mantles. The optical scattering properties for the appropriate nonspherical particles are obtained either from exact theory or from microwave analog measurements. The magnetic properties of the grains and the orientation mechanism are treated with as much detail and accuracy as seems useful and feasible at present. The required magnetic field strength is generally greater than 10 × 10-6 gauss. However, under rather reasonable conditions the magnetic field requirement for the dielectric grains may be reduced to 6 × 10-6 gauss for very high ratio of polarization to extinction and for moderate ratios the field requirement is reduced to less than 4 × 10-6 gauss. On an overall basis the models are satisfactory in the order (1), (3), (2) although the difference between (1) and (3) is not large. The effect of grain and gas temperature is discussed and a possible criterion is presented for distinguishing between the three models based on different interactions with the ambient gas and radiation field. © 1969.