Inclination-induced polarization of scattered millimetre radiation from protoplanetary discs: the case of HL Tau

Abstract

Spatially resolved polarized millimeter/submillimeter emission has been observed in the disk of HL Tau and two other young stellar objects. It is usually interpreted as coming from magnetically aligned grains, but can also be produced by dust scattering, as demonstrated explicitly by Kataoka et al. for face-on disks. We extend their work by including the polarization induced by disk inclination with respect to the line of sight. Using a physically motivated, semi-analytic model, we show that the polarization fraction of the scattered light increases with the inclination angle i, reaching 1/3 for edge-on disks. The inclination-induced polarization can easily dominate that intrinsic to the disk in the face-on view. It provides a natural explanation for the two main features of the polarization pattern observed in the tilted disk of HL Tau (i ∼ 45• ): the polarized intensity concentrating in a region elongated more or less along the major axis, and polarization in this region roughly parallel to the minor axis. This broad agreement provides support to dust scattering as a viable mechanism for producing, at least in part, polarized millimeter radiation. In order to produce polarization at the observed level (∼ 1%), the scattering grains must have grown to a maximum size of tens of microns. However, such grains may be too small to produce the opacity spectral index of β 1 observed in HL Tau and other sources; another population of larger, millimeter/centimeter-sized, grains may be needed to explain the bulk of the unpolarized continuum emission.

Scattering-produced (sub)millimetre polarization in inclined discs: optical depth effects, near–far side asymmetry and dust settling

“…This is the same argument that we used in Yang et al (2016a) of polarization degree with inclination angle in a geometrically and optically thin disk. The difference is that this trend continues all the way to i = 90…”

“…Such a config-uration would produce a polarization pattern that appears inconsistent with the pattern observed in HL Tau (Stephens et al 2014). The apparent inconsistency led Yang et al (2016a) to propose that the disk polarization in HL Tau comes from dust scattering, based on Kataoka et al (2015) theory of scattering-induced millimeter polarization (see also Kataoka et al 2016a). Specifically, they show that dust scattering in a disk inclined to the line of sight can naturally explain why the observed polarization vectors are roughly parallel to the minor axis and the distribution of polarized intensity is elongated along the major axis.…”

Grain Size Constraints on Hl Tau With Polarization Signature

“…The unpolarized light is scattered by other grains because of the high scattering opacity. Due to the ring-like structure of the emission and the inclination of the disk, the distribution of the incoming flux to the scattering dust grains has anisotropies (see also Yang et al 2016). As a result, the total flux has a residual polarization corresponding to the anisotropic radiation field.…”

“…Note that, during the preparation of the manuscript, Yang et al (2016) independently found that the interpretation of the millimeter-wave polarization of HL Tau due to dust scattering.…”

DETECTION OF LINEARLY POLARIZED 6.9 mm CONTINUUM EMISSION FROM THE CLASS 0 YOUNG STELLAR OBJECT NGC 1333 IRAS4A

“…Close to the peak, the observed lower polarization percentage at 0.88 mm, then 6.9 mm (Figure 6), may be because of the more significant polarization canceling effect at 0.88 mm. The high optical depth at 0.88 mm may further lead to additional (de-)polarization due to scattering (Kataoka et al 2015;Yang et al 2015).…”