1999A&A...349..912H

We have carried out deep CCD polarimetry in the I band in selected fields in three filamentary, molecular clouds: the L 1400 complex, L 204, and MBM 25, at galactic latitudes 4°, 21°, and 31°, respectively. I magnitudes and the degree and direction of linear polarization were measured with satisfactory accuracies for 387 stars down to I=19.5. Typical uncertainties of the degree and angle of polarization are ΔP≃0.5% and Δθ≃10°. We also derived extinctions from star counts in and around the observed fields. In the L 1400 complex we imaged a 48arcmin² field in the H and K bands, providing independent measures of extinction based on the E(H-K) colour excess. Our measurements show that the patterns of polarization are remarkably smooth over the fields studied, particularly in the L 1400 complex. There is no indication of any statistically significant difference in polarization angle and degree between obscured and non-obscured regions. MBM 25, with the lowest levels of extinction and polarization, shows some anomalies in the polarization vectors, but the statistics is here less good. Our method opens the possibility to study small-scale irregularities in the polarization pattern on angular scales down to a few arcseconds. Comparing the extinction A_V and P/A_V in the field observed in the H and K bands we find that P/A_V can attain high values (close to the P=3A_V relation) only for small values of A_V. Modelling the stellar content along the line of sight reveals that we, for magnitudes around I=19, sample stars at very large distances behind the clouds. Our general conclusion is that the interstellar filaments contribute very little to the polarization measured. This is in line with recent results obtained for other cloud regions comprising less detailed spatial scale and a smaller depth in brightness. Using a semi-empirical version of the Davis-Greenstein mechanism we have calculated the polarization for simple models of the three clouds studied, and find that it is considerably smaller than the typical background polarization, a result in consonance with the polarimetric measurements. For MBM 25 we derive a new distance of ∼145pc from Hipparcos parallaxes of two stars surrounded by bright nebulosities, on the assumption that the stars are physically associated with the cloud.