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Session 32 - General ISM.
Oral session, Wednesday, January 07

[32.03] The Polarizing Power of the ISM

H. G. Arce, A. A. Goodman, S. J. Kenyon, M. Sumner (CfA), P. Bastien, N. Manset (U.Montreal)

We present a study of the polarizing power of localized, dense regions of dust (dark clouds) along the line of sight compared to that of the general interstellar medium (ISM), in the Taurus dark cloud complex. Our study uses new polarimetric, optical, and spectral classification data for 36 stars to carefully study the relation between polarization percentage (p) and extinction (A_V) in regions near dark clouds. We find two trends in our p-A_V relation: the percentage of polarization of stars background to cold dark clouds does not increase with extinction; (2) stars background to the warmer surrounding ISM show an increase in p with A_V. We detect a break in the p-A_V relation at an extinction value of approximately 1.3 mag, which we expect corresponds to a set of conditions where the polarizing power of interstellar material in this region drops precipituosly. This breakpoint places important restrictions on the use of polarimetry in studying interstellar magnetic fields.

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