IMAGING POLARIMETRY OF CLASS-I YOUNG STELLAR OBJECTS

We present near-infrared imaging polarimetry of three class I young st ellar objects in the Taurus-Auriga dark cloud. We use Monte Carlo simu lations to analyse the flux distributions and polarization patterns of these three sources and five others from an earlier paper. In additio n, we present high-resolution polarimetry of HL Tau using the shift an d add technique. Most young stellar objects in the sample display shar p, unresolved, peaks in the scattered light distribution. This is most simply explained by a strong concentration of matter in the centre, w hich we model by applying the rho proportional to r(-1.5) power law th roughout 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 al most spherically symmetric density distribution is inconsistent with o bservations of flattened, disc-like structures, so we conclude that th is 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 th e broad regions of aligned vectors seen in some sources. We interpret this as evidence for elongated aligned grains. The weak wavelength dep endence of nebular morphology shows that the dust grains in circumstel lar envelopes obey a much shallower extinction law than interstellar g rains in the near-infrared, which we describe by the opacity ratio kap pa(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 an d we find 0.1 < omega < 0.6 at 2.2 mu m. Analysis of the quadrupolar s ource IRAS 04302+2247 indicates that the scattering function is not hi ghly fonvard-throwing, which we express by the constraint g = [cos(the ta)] < 0.4. With the addition of two more observables derived from the observed degrees of linear and circular polarization, we identify fiv e 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 satisf y these constraints and the size distribution extends to a greater tha n or similar to 0.351 mu m. Amorphous carbon is the most plausible abs orptive component, since graphite appears to be ruled out by its stron g dispersion in the near-infrared.