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 / F 0 ≈ 0.001 at 1.25 μm to F s / F 0 ≈ 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.