Near‐Infrared Imaging Polarimetry of Embedded Young Stars in the Taurus‐Auriga Molecular Cloud

Abstract

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 PK ? 5%-20% in an 8'' aperture; a few objects have PK 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, AV ? 25-60 mag, comparable to those predicted for models of collapsing clouds. The ratio of scattered flux to intrinsic source flux ranges from Fs/F0 ? 0.001 at 1.25 ?m to Fs/F0 ? 0.015 at 3.5 ?m. These results indicate that the observed ratio of scattered light to direct (extincted) light increases from Fs/Fd ~ 0.1 at 3.5 ?m to Fs/Fd ~ 25 at 1.25 ?m. 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, ${u{M}{705F}}$ --> ~(2-5) ? 10 -->?6 M? yr-1; centrifugal radii, Rc ~ 10-50 AU; and opening angles of the bipolar cavity, ?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, Pmax,K 80%. The large image sizes of this sample further imply a high K-band albedo, ?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.