Scattering line polarization in rotating, optically thick disks

TL;DR

This study models polarized line formation in optically thick, rotating astrophysical disks, highlighting the impact of radiative transfer, scattering, and velocity fields on emergent spectra, and proposing diagnostic tools for disk rotation.

Contribution

It provides a self-consistent radiative transfer model for polarized line formation in rotating disks, emphasizing the effects of optical thickness, scattering, and velocity fields on polarization profiles.

Findings

Polarization degree depends on disk inclination, optical thickness, and rotation.

Stokes U profiles show double-lobed shapes that increase with rotation.

Disk rotation can be diagnosed by the presence of Stokes U signals.

Abstract

To interpret observations of astrophysical disks it is essential to understand the formation process of the emitted light. If the disk is optically thick, scattering dominated and permeated by a Keplerian velocity field, Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to compute the emergent spectrum from a given disk model. We investigate Non-local thermodynamic equilibrium polarized line formation in different simple disk models and aim to demonstrate the importance of both radiative transfer effects and scattering as well as the effects of velocity fields. We self-consistently solve the coupled equations of radiative transfer and statistical equilibrium for a two level atom model by means of Jacobi iteration. We compute scattering polarization, that is Q/I and U/I line profiles. The degree of scattering polarization is significantly influenced by the inclination of the disk with respect to observer, but also by the optical thickness of the disk and the presence of rotation. Stokes U shows double-lobed profiles with amplitude which increases with the disk rotation. Our results suggest that the line profiles, especially the polarized ones, emerging from gaseous disks differ significantly from the profiles predicted by simple approximations. The profiles are diverse in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A clear indicator of disk rotation is the presence of Stokes U, which might prove to be a useful diagnostic tool. We also demonstrate that, for moderate rotational velocities, an approximate treatment can be used, where non-local thermodynamic equilibrium radiative transfer is done in the velocity field-free approximation and Doppler shift is applied in the process of spatial integration over the whole emitting surface.