Polarizing efficiency as indicator of the interstellar magnetic fields and grain evolution

N. V. Voshchinnikov, V. B. Il'in, H. K. Das

Published 2015-09-20Version 1

We quantitatively interpret the relation between the polarizing efficiency $P_{\max}/E(B-V)$ and the wavelength of the maximum polarization $\lambda_{\max}$ observed for 16 objects (including 246 stars) separated into two groups: dark clouds and open clusters. The groups are distinguished by the distribution of the parameter $\lambda_{\max}$. We use the model of homogeneous silicate and carbonaceous spheroidal particles having imperfect alignment and the size distribution evolving due to gas accretion and grain coagulation. We assume that polarization is mainly produced by large silicate particles with sizes $r_{V} \ga r_{V,\rm cut}$. We find that the models with the initial size distribution fail to explain the values of $\lambda_{\max} \ga 0.65\,\mkm$ observed for several dark clouds. After an inclusion of evolutionary effects, $\lambda_{\max}$ shifts to longer wavelengths on time-scales $\sim 20 (n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr ($n_\mathrm{H}$ is the hydrogen density in molecular clouds where dust processing occurs). The ratio $P_{\max}/E(B-V)$ strongly goes down when the size of polarizing grains grows. The influence of the variations of the degree and direction of particle orientation on this ratio is of lesser importance. We also find that the aspect ratio of prolate grains does not significantly affect the polarizing efficiency. For oblate particles, the shape effect is stronger but most of them produce too narrow polarization curves.