Nonazimuthal linear polarization in protoplanetary disks

¹ Departamento de Física y Astronomía, Universidad de Valparaíso, 951 Valparaíso, Chile
e-mail: hector.canovas@dfa.uv.cl
² UMI-FCA, CNRS/INSU, France (UMI 3386), and Dept. de Astronomía, Universidad de Chile, Casilla 36-D Santiago, Chile
³ Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Departamento de Astronomía, Universidad de Chile, Casilla 36- D Santiago, Chile
⁵ Millennium Nucleus Protoplanetary Disks in ALMA Early Science, Casilla 36- D Santiago, Chile

Received: 28 August 2015
Accepted: 17 September 2015

Abstract

Several studies discussing imaging polarimetry observations of protoplanetary disks use the so-called radial Stokes parameters Q_φ and U_φ to discuss the results. This approach has the advantage of providing a direct measure of the noise in the polarized images under the assumption that the polarization is only azimuthal, i.e., perpendicular to the direction toward the illuminating source. However, a detailed study of the validity of this assumption is currently missing. We aim to test whether departures from azimuthal polarization can naturally be produced by scattering processes in optically thick protoplanetary disks at near infrared wavelengths. We use the radiative transfer code MCFOST to create a generic model of a transition disk using different grain size distributions and dust masses. From these models we generate synthetic polarized images at 2.2 μm. We find that even for moderate inclinations (e.g., i = 40°), multiple scattering alone can produce significant (up to ~ 4.5% of the Q_φ image, peak-to-peak) nonazimuthal polarization reflected in the U_φ images. We also find that different grain populations can naturally produce radial polarization (i.e., negative values in the Q_φ images). Despite the simplifications of the models, our results suggest that caution is recommended when interpreting polarized images by only analyzing the Q_φ and U_φ images. We find that there can be astrophysical signal in the U_φ images and negative values in the Q_φ images, which indicate departures from azimuthal polarization. If significant signal is detected in the U_φ images, we recommend checking the standard Q and U images to look for departures from azimuthal polarization. On the positive side, signal in the U_φ images once all instrumental and data-reduction artifacts have been corrected for means that there is more information to be extracted regarding the dust population and particle density.