4.5. Circumstellar Gas (out to ~ 3 × 10¹⁰ km; ~ 0.001 pc)

Here we take a circumstellar gas to mean gas located close to a star, closer than in a young protostellar nebula. Hence we shall take as circumstellar objects those stellar-powered nebula with an envelope extending up to 200 AU or so (~ 3 × 10¹⁰ km; ~ 0.001 pc).

Young Stellar Objects (YSO) are usually low-mass stars, such as T Tauri stars or Herbig Ae/Be stars, usually visible at optical wavelengths and the near infrared, showing variable linear polarization (e.g., Bastien 1996). The time variations may be due to binarity (two stars), spots on a rotating stellar surface, orbiting dust condensations, comet-like activity (e.g., Bastien 1996). The earlier interpretation with a model involving the alignment of grains by a magnetic field is no longer valid, and nothing can be said about the magnetic fields there from these data at optical and near infared wavelengths.

More detailed optical polarization maps show (i) a small pattern of linearly aligned vectors close to the central source, due to multiple scattering by dust in a central disk, (ii) an extended centrosymmetric pattern, suggesting single scattering of light by dust in an envelope, and (iii) two null points in polarization, located at the transition between the linearly aligned vectors (i) and the centrosymmetric vectors (ii).

Fischer et al. (1994) studied theoretical models of dust scattering around young starforming objects without magnetic fields, and found that (i) grains in the inner regions of the dust disk (optically thick) give E-vectors of polarization aligned parallel to the disk plane; (ii) grains in the outer regions of dust disk (optically thin) and in the surrounding envelope/lobe around the disk (optically thin) give E-vectors of polarization with a centro-symmetric polarization pattern around the center of the object.

The apparent polarization disk is much larger than the true, centrifugally supported "planetary range" or "accretion disk" 200 AU (~ 3 × 10¹⁰ km; 0.001 pc). Dust and gas beyond 200 AU may not be centrifugally supported but may be infalling toward the center.