Imaging polarimetry of class I young stellar objects

Abstract

We present near-infrared imaging polarimetry of three class I young stellar objects in the Taurus-Auriga dark cloud. We use Monte Carlo simulations to analyse the flux distributions and polarization patterns of these three sources and five others from an earlier paper. In addition, we present high-resolution polarimetry of HL Tau using the shift and add technique. Most young stellar objects in the sample display sharp, unresolved, peaks in the scattered light distribution. This is most simply explained by a strong concentration of matter in the centre, which we model by applying the rho proportional to r(-1.5) power law throughout the envelope. In terms of the Ulrich/Terebey, Shu and Cassen solution for the late stages of contraction of an initially spherical non-magnetic cloud, this corresponds to r(c) < 10 au. However, this almost spherically symmetric density distribution is inconsistent with observations of flattened, disc-like structures, so we conclude that this solution is not appropriate and different initial conditions apply. The multiple-scattering models with spherical grains do not reproduce some features of the observed polarization patterns, in particular the broad regions of aligned vectors seen in some sources. We interpret this as evidence for elongated aligned grains. The weak wavelength dependence of nebular morphology shows that the dust grains in circumstellar envelopes obey a much shallower extinction law than interstellar grains in the near-infrared, which we describe by the opacity ratio kappa(J/K) = 1.8 +/- 0.3, compared to the interstellar value of 3.25. We place an upper limit on albedo of omega < 0.6 from 1.25 to 2.2 mu m and we find 0.1 < omega < 0.6 at 2.2 mu m. Analysis of the quadrupolar source IRAS 04302+2247 indicates that the scattering function is not highly fonvard-throwing, which we express by the constraint g = [cos(theta)] < 0.4. With the addition of two more observables derived from the observed degrees of linear and circular polarization, we identify five empirical constraints on the properties of circumstellar dust. After calculating these observables for grain mixtures with a wide range of refractive indices and a variety of size distributions, we find that highly metallic spherical grains or highly elongated grains can satisfy these constraints and the size distribution extends to a greater than or similar to 0.351 mu m. Amorphous carbon is the most plausible absorptive component, since graphite appears to be ruled out by its strong dispersion in the near-infrared.