THE ASTRONOMICAL JOURNAL     VOLUME 107, NUMBER 4, PAGE 1433         APRIL 1994

     R BAND POLARIMETRY OF CYGNUS OB2: IMPLICATIONS FOR THE MAGNETIC FIELD
                       GEOMETRY AND POLARIZATION MODELS

                              HENRY A. KOBULNICKY

     Department of Astronomy, University of Minnesota, 116 Church St. SE,
                         Minneapolis, Minnesota 55455
                    Electronic mail: chip@ast1.spa.umn.edu

                              LAWRENCE A. MOLNAR

 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242
                 Electronic mail: lam@astro.physics.uiowa.edu

                                TERRY JAY JONES

     Department of Astronomy, University of Minnesota, 116 Church St. SE,
                         Minneapolis, Minnesota 55455
                    Electronic mail: tjj@ast1.spa.umn.edu

                                   ABSTRACT

    We present new R band polarimetry of 132 members of the Cygnus OB2
    association.  From these data we have determined the angular coherence
    length of the polarization position angle and tested two models for
    interstellar polarization toward a region where the Galactic magnetic
    field is expected to be primarily along the line of sight.  The
    polarization magnitudes and position angles in our sample decorrelate
    on size scales of ~0.5 deg, compared to 0.2 deg for the optical
    extinction.  This constrains the variations in the magnetic field
    orientation to sizes less than 8 pc.  The distribution of polarization
    magnitudes and position angles is consistent with a model which
    describes the polarization in terms of preferential extinction in small
    dark clouds threaded by a two-component magnetic field.  One component
    is constant in direction, while the second is random in orientation and
    is characterized by the amplitude of Alfven waves in the interstellar
    medium.  A similar model, in which the magnitude of the random
    component has a Gaussian distribution and an orientation evenly
    distributed through 4 pi steradians, is also consistent with the data.
    A previously proposed geometrical model involving two slabs of
    obscuring material with different magnetic field orientations does not
    seem adequate given these new data.  We explore the predictions of each
    model for the polarization magnitude and position angle as a function
    of the orientation of the constant component and point out several key
    differences that might be exploited to further test their potential for
    describing interstellar polarization.  We find the polarization
    position angles of stars in the vicinity of Cygnus X-3 are
    approximately orthogonal to the position angle of the elliptical radio
    scattering reports by Molnar et al. [BAAS, 21, 1124 (1989)].  If the
    scattering is taking place on the near side of Cyg OB2, the data are
    consistent with mechanisms that require the major axis of the
    scattering ellipse to lie perpendicular to the projected magnetic
    field.