Near-infrared imaging polarimetry of young stellar objects in ρ Ophiuchi

Abstract

The results of a near-infrared (J H K LP) imaging linear polarimetry survey of 20 young stellar objects in ρ Ophiuchi are presented. The majority of the sources are unresolved, with K band polarizations, PK 20% are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K band core polarizations >6% are only observed in Class I YSOs. A 3-dimensional Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the ρ Oph young stellar objects with extended nebulae. A ρ / r 1.5 power law for the density is applied throughout the envelopes. The large scale centrosymmetric polarisation structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned nonspherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually 610%, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes 61.05 � m.