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Astron. Astrophys. 349, 912-926 (1999)

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Linear polarization and molecular filamentary clouds *

P. Harjunpää 1,2, A.A. Kaas 1, P. Carlqvist 3 and G.F. Gahm 1

1 Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
2 Helsinki Observatory, P.O. Box 14, SF-000 14, University of Helsinki, Finland
3 Department of Plasma Physics, Alfvén Laboratory, Royal Institute of Technology, S-100 44 Stockholm, Sweden

Received 25 February 1999 / Accepted 6 July 1999

Abstract

We have carried out deep CCD polarimetry in the I band in selected fields in three filamentary, molecular clouds: the L1400 complex, L204, and MBM25, at galactic latitudes 4o, 21o, and 31o, respectively. I magnitudes and the degree and direction of linear polarization were measured with satisfactory accuracies for 387 stars down to [FORMULA]. Typical uncertainties of the degree and angle of polarization are [FORMULA]0.5% and [FORMULA]10o. We also derived extinctions from star counts in and around the observed fields. In the L1400 complex we imaged a 48 arcmin2 field in the H and K bands, providing independent measures of extinction based on the [FORMULA] colour excess.

Our measurements show that the patterns of polarization are remarkably smooth over the fields studied, particularly in the L1400 complex. There is no indication of any statistically significant difference in polarization angle and degree between obscured and non-obscured regions. MBM25, 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 [FORMULA] and [FORMULA] in the field observed in the H and K bands we find that [FORMULA] can attain high values (close to the [FORMULA] relation) only for small values of [FORMULA].

Modelling the stellar content along the line of sight reveals that we, for magnitudes around [FORMULA], 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 MBM25 we derive a new distance of [FORMULA]145 pc from Hipparcos parallaxes of two stars surrounded by bright nebulosities, on the assumption that the stars are physically associated with the cloud.

Key words: ISM: clouds – ISM: dust, extinction – ISM: magnetic fields – techniques: polarimetric

* Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Send offprint requests to: P. Harjunpää (to Helsinki address)

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© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999
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