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 ≈ 5%-20% in an 8'' aperture; a few objects have P_K ≲ 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 ≈ 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/F₀ ≈ 0.001 at 1.25 μm to F_s/F₀ ≈ 0.015 at 3.5 μm. These results indicate that the observed ratio of scattered light to direct (extincted) light increases from F_s/F_d ~ 0.1 at 3.5 μm to F_s/F_d ~ 25 at 1.25 μm. Our data further require intrinsic colors of 0.6 ≲ J-H ≲ 0.9, 0.3 ≲ H-K ≲ 0.6, and 0.4 ≲ K-L ≲ 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, ~(2-5) × 10⁻⁶ M_☉ yr^-1; centrifugal radii, R_c ~ 10-50 AU; and opening angles of the bipolar cavity, θ_h ≈ 10°-20°, 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,K ≳ 80%. The large image sizes of this sample further imply a high K-band albedo, ω_K ≈ 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.