Near-infrared imaging polarimetry of embedded young stars in the Taurus-Auriga molecular cloud

We describe near-infrared (JHK) imaging polarimetry of 21 embedded protostars in the Taurus-Auriga molecular cloud. These objects display extended, highly polarized reflection nebulae with V-shaped, unipolar, and bipolar morphologies. Most sources have P-K approximate to 5%-20% in an 8 '' aperture; a few objects have P-K less than or similar to 5%. The polarization increases toward shorter wavelengths and is generally aligned perpendicular to the long axis of the reflection nebula. We develop an analytic scattering model for the near-IR colors and polarizations of embedded protostars. Our Taurus data require visual extinctions, A(V) approximate to 25-60 mag, comparable to those predicted for models of collapsing clouds. The ratio of scattered flux to intrinsic source flux ranges from F-s/F-0 approximate to 0.001 at 1.25 mu m to F-s/F-0 approximate to 0.015 at 3.5 mu m. These results indicate that the observed ratio of scattered light to direct (extincted) light increases from F-s/F-d similar to 0.1 at 3.5 mu m to F-s/F-d similar to 25 at 1.25 mu m. Our data further require intrinsic colors of 0.6 less than or similar to J-H less than or similar to 0.9, 0.3 less than or similar to H-K less than or similar to 0.6, and 0.4 less than or similar to K-L less than or similar to 1.2 for the central sources of Taurus protostars. We adopt the Terebey, Shu, & Cassen solution for an infalling, rotating protostellar cloud and use a two dimensional Monte Carlo radiative transfer code to model the near-IR polarization data for this sample. Our results indicate envelope parameters in agreement with previous estimates from broadband spectral energy distributions and near-IR images. We estimate infall rates, (M) over dot similar to (2-5) x 10(-6) M. yr(-1); centrifugal radii, R-c similar to 10-50 AU; and opening angles of the bipolar cavity, theta(h) approximate to 10 degrees-20 degrees, for a typical object. Standard grain parameters can explain the near-IR colors and polarizations of Taurus protostars. The polarization maps show that Taurus grains have a high maximum polarization at K, P-max,P-K greater than or similar to 80%. The large image sizes of this sample further imply a high K-band albedo, omega(K) approximate to 0.3-0.4. Model polarization maps indicate that the size of the ''polarization disk'' increases with the size of the instrumental point-spread function. Relating the morphology of polarization vectors to disk or envelope properties thus requires some care and a good understanding of the characteristics of the instrument.