The polarizing power of the interstellar medium in Taurus

We present a study of the polarizing power of dust in cold dense regions (dark clouds) compared with that of dust in the general interstellar medium (ISM), 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 the Taurus dark cloud complex. We find two trends in our p-A(v) study: (1) stars that are background to the warm ISM show an increase in p with A(v), and (2) the percentage nf polarization of stars that are background to cold dark clouds does not increase with extinction. We detect a break in the p-A(v) relation at nrl exttlzctiooz of 1.3 +/- 0.2 mag, which we expect corresponds to a set of conditions where the polarizing power of the dust associated with the Taurus dark clouds drops precipitously. This break point places important restrictions on the use of polarimetry in studying interstellar magnetic fields.