Linear polarization and molecular filamentary clouds

We have carried out deep CCD polarimetry in the I band in selected fields i n three filamentary, molecular clouds: the L1400 complex, L204, and MBM25, at galactic latitudes 4 degrees, 21 degrees, and 31 degrees, respectively. I magnitudes and the degree and direction of linear polarization were measu red with satisfactory accuracies for 387 stars down to I = 19.5. Typical un certainties of the degree and angle of polarization are Delta P approximate to 0.5% and Delta theta approximate to 10 degrees. We also derived extinct ions from star counts in and around the observed fields. Tn the L1400 compl ex we imaged a 48 arcmin(2) field in the H and K bands, providing independe nt measures of extinction based on the E(H - K) colour excess. Our measurements show that the patterns of polarization are remarkably smoo th over the fields studied, particularly in the L1400 complex. There is no indication of tiny statistically significant difference in polarization ang le and degree between obscured and non-obscured regions. MBM25. with the lo west levels of extinction and polarization, shows some anomalies in the pol arization vectors, but the statistics is here less good. Our method opens t he possibility to study small-scale irregularities in the polarization patt ern 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 v alues 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 c louds. Our general conclusion is that the interstellar filaments contribute very little to the polarization measured. This is in line with recent resu lts obtained for other cloud regions comprising less detailed spatial scale and a smaller depth in brightness. Using a semi-empirical version of the D avis-Greenstein mechanism we have calculated the polarization for simple mo dels of the three clouds studied, and find that it is considerably smaller than the typical background polarization, a result in consonance with the p olarimetric measurements. For MBM25 we derive a new distance of similar to 145 pc from Hipparcos para llaxes of two stars surrounded by bright nebulosities, on the assumption th at the stars are physically associated with the cloud.