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Linear polarization and molecular filamentary clouds
We have carried out deep CCD polarimetry in the I band in selectedfields in three filamentary, molecular clouds: the L1400 complex, L204,and MBM25, at galactic latitudes 4degr , 21degr , and 31degr ,respectively. I magnitudes and the degree and direction of linearpolarization were measured with satisfactory accuracies for 387 starsdown to I=19.5. Typical uncertainties of the degree and angle ofpolarization are ΔP{~}0.5% and Δθ{~}10degr . We alsoderived extinctions from star counts in and around the observed fields.In the L1400 complex we imaged a 48 arcmin(2) field in the H and Kbands, providing independent measures of extinction based on the E(H-K)colour excess. Our measurements show that the patterns of polarizationare remarkably smooth over the fields studied, particularly in the L1400complex. There is no indication of any statistically significantdifference in polarization angle and degree between obscured andnon-obscured regions. MBM25, with the lowest levels of extinction andpolarization, shows some anomalies in the polarization vectors, but thestatistics is here less good. Our method opens the possibility to studysmall-scale irregularities in the polarization pattern on angular scalesdown to a few arcseconds. Comparing the extinction A_V and P/A_V in thefield observed in the H and K bands we find that P/A_V can attain highvalues (close to the P=3 A_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 behindthe clouds. Our general conclusion is that the interstellar filamentscontribute very little to the polarization measured. This is in linewith recent results obtained for other cloud regions comprising lessdetailed spatial scale and a smaller depth in brightness. Using asemi-empirical version of the Davis-Greenstein mechanism we havecalculated the polarization for simple models of the three cloudsstudied, and find that it is considerably smaller than the typicalbackground polarization, a result in consonance with the polarimetricmeasurements. For MBM25 we derive a new distance of ~ 145 pc fromHipparcos parallaxes of two stars surrounded by bright nebulosities, onthe assumption that the stars are physically associated with the cloud.Based on observations made with the Nordic Optical Telescope, operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norway,and Sweden, in the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofisica de Canarias.
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