R band polarimetry of Cygnus OB2: Implications for the magnetic field geometry and polarization models

Henry A. Kobulnicky, Lawrence A. Molnar, Terry Jay Jones

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21 Scopus citations


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°, compared to 0.2° 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 Alfvén 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 4π 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 reported 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.

Original languageEnglish (US)
Pages (from-to)1433-1443
Number of pages11
JournalAstronomical Journal
Issue number4
StatePublished - Apr 1994


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