Abstract
We present a study of the polarizing power of the dust in cold dense regions (dark clouds) compared to 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 background to the warm ISM show an increase in p with A_V; and (2) the percentage of polarization of stars background to cold dark clouds does not increase with extinction. We detect a break in the p-A_V relation at an extinction 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 breakpoint places important restrictions on the use of polarimetry in studying interstellar magnetic fields.
Highlights
The polarization of background starlight has been used for nearly half a century to probe the magnetic field direction in the interstellar medium (ISM)
Recent studies in the Taurus region (Goodman et al 1992; Gerakines, Whittet, & Lazarian 1995) and other parts of the sky (Creese, Jones, & Kobulnicky 1995; Goodman et al 1995) have uncovered substantial evidence to show that dust inside cold dark clouds has lower polarizing power than dust in the general warm ISM
Since the polarization efficiency of the dust inside dark clouds is very low, most of the polarization observed for lines of sight that pass through these extinction peaks is not due to the dark cloud but is due to dust that is background and foreground to the cloud
Summary
The polarization of background starlight has been used for nearly half a century to probe the magnetic field direction in the interstellar medium (ISM). We chose this orientation for the cuts in order to ensure both (1) a sample of stars with a large dynamic range in extinction and (2) that many of the high-AV values measured would be produced by a single localized dark cloud along the line of sight.
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