On the interpretation of polarization maps of young stellar objects

Abstract

Polarization maps obtained during the past few years of regions around young stellar objects associated with optical jets and/or bipolar outflows and/or cometary nebulas show, in many cases, a centrosymmetric pattern usually attributed to scattering of radiation from the central sources by dust grains in their vicinity. In other cases, a pattern of aligned polarization vectors is observed close to the central object; dichroic extinction by aligned grains has been proposed for their interpretation. This paper explores different ways to explain such a pattern without invoking aligned grains. It is found that multiple scattering alone with an optically thick disk and optically thin polar lobes can explain most, if not all, polarization maps so far observed. If this interpretation is correct, aligned grains and magnetic fields are no longer required. However, this does not disprove the presence of magnetic fields around young stars; it only shows that they are not efficient at aligning grains in the circumstellar environment of premain-sequence stars.