An analysis of the distribution of background star polarization in dark clouds

The polarization observed for stars behind dark clouds (Bok globules) is often used as a diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds, CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246, in our previous work. With a view to understanding the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence, etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures ( T d and T g ) in the cloud as expected from the Davis–Greenstein grain alignment mechanism. However, the average observed polarization ( p av ) appears to be related to the turbulence Δ V (measured by the 12 CO linewidth) by the mathematical relation p av = 2.95 exp(−0.24Δ V ) . The possible relation between the direction of the polarization vector and other physical parameters is also discussed. For this analysis, in addition to the data on the above eight dark clouds, the data on CB4 are also included for comparison. In order to study the spatial distribution of the degree of polarization and position angles across the different parts of the cloud a simple model is proposed, where the cloud has been assumed to be a simple dichroic polarizing sphere and the light from the background star first passes through the interstellar medium and then through the cloud, before reaching the observer. One finds this simple model can explain to a reasonable extent the observed spatial (radial) dependence of the value of p for two of the clouds (CB25, CB39), but for the rest of the clouds the model fails. However, through this model one can explain why the polarization ( p ) need not always increase with total extinction A v as one moves into the deeper interior of the cloud.