2.1. Methodology and Techniques
2.1.1. Optical dust absorption
The optical polarization observed on Earth of stars located behind dusty molecular clouds is due to dichroic extinction by dust grains aligned by a cloud magnetic field (e.g., Johnson 1982). But the dense cores within a cloud are opaque at optical wavelengths, prohibiting optical polarization observations of background stars, restricting optical polarimetry to the tenuous parts of cloud halos. At optical and near infrared wavelengths, the observed polarization from stars through a cloud halo could be due to various mechanisms. It could be due to a magnetic field via dust absorption, but Goodman (1995) and Goodman et al. (1995) warn that a lack of polarizing grains for optical and near infrared photons in a dark cloud could prevent measurement of the cloud's own magnetic field, when observing background starlight through a cloud. It could be due also to multiple scattering via dust without any effect from a magnetic field (e.g., Bastien and Ménard 1988; 1990) - thus the typical centro-symmetric pattern for the polarization position angle around a bright star gives no information at all about the magnetic field there.
Looking at dust absorption at optical and near infrared
wavelengths, one finds that
the ratio of the polarization amplitude at 2 different
wavelengths varies according to the
Serkowski relation with 2 parameters:
max
~ 0.55 µm which is related to the mean size of
the polarizing grain, and pmax which is directly related to the
extinction along the line of
sight, and thus indirectly related to the distance to the star
being looked at. In dark clouds,
max
~ 0.75 µm (e.g.,
Breger et al. 1981).