THE POLARIZING POWER OF THE INTERSTELLAR-MEDIUM IN TAURUS

We present a study of the polarizing power of dust in cold dense regio ns (dark clouds) compared with that of dust in the general interstella r medium (ISM), Our study uses new polarimetric, optical, and spectral classification data for 36 stars to carefully study the relation betw een polarization percentage (p) and extinction (A(v)) in the Taurus da rk cloud complex. We find two trends in our p-A(v) study: (1) stars th at 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 co ld 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 o f the dust associated with the Taurus dark clouds drops precipitously. This break point places important restrictions on the use of polarime try in studying interstellar magnetic fields.