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

Abstract

Disk polarization at (sub)millimeter wavelengths is being revolutionized by ALMA observationally, but its origin remains uncertain. Dust scattering was recently recognized as a potential contributor to polarization, although its basic properties have yet to be thoroughly explored. Here, we quantify the effects of optical depth on the scattering-induced polarization in inclined disks through a combination of analytical illustration, approximate semi-analytical modeling using formal solution to the radiative transfer equation, and Monte Carlo simulations. We find that the near-side of the disk is significantly brighter in polarized intensity than the far-side, provided that the disk is optically thick and that the scattering grains have yet to settle to the midplane. This asymmetry is the consequence of a simple geometric effect: the near-side of the disk surface is viewed more edge-on than the far-side. It is a robust signature that may be used to distinguish the scattering-induced polarization from that by other mechanisms, such as aligned grains. The asymmetry is weaker for a geometrically thinner dust disk. As such, it opens an exciting new window on dust settling. We find anecdotal evidence from dust continuum imaging of edge-on disks that large grains are not yet settled in the youngest (Class 0) disks, but become more so in older disks. This trend is corroborated by the polarization data in inclined disks showing that younger disks have more pronounced near-far side asymmetry and thus less grain settling. If confirmed, the trend would have far-reaching implications for grain evolution and, ultimately, the formation of planetesimals and planets.

ALMA Observations of Polarization from Dust Scattering in the IM Lup Protoplanetary Disk

“…This is the result of a slight azimuthal curvature in the polarization orientations furthest from the disk center (along the major axis), seen in the top panel of Figure 1. This is consistent with the pure-scattering models of Yang et al (2016aYang et al ( , 2017, as well as with the morphology of the polarization in the 870 μm observations of HLTau reported in Stephens et al (2017). Note, however, that while the slight azimuthal curvature can be explained by pure self-scattering, it could also be due to the superposition of polarized emission both from self-scattering and from dust grains aligned with the dust emission gradient (Tazaki et al 2017).…”

“…Dust settling can be constrained with scattering models (e.g., Kataoka et al 2015;Yang et al 2017). Yang et al (2017) have shown that, in inclined disks such as IMLup, the near side of the disk would be significantly brighter in polarized intensity than the far side if the disk were optically thick (i.e., τ ⊥ > 1) and the grains responsible for the scattering-induced polarization had a significant vertical extent.…”

Highly Ordered and Pinched Magnetic Fields in the Class 0 Protobinary System L1448 IRS 2

“…Depolarization can also occur due to high optical depth (see Figure3 of Yang et al 2017), but this is not likely the case here since the optical depth is expected to be low along this depolarization strip: τ∼0.08 even at the highest contour level assuming T=30 K.…”

Probing the cold magnetised Universe with SPICA-POL (B-BOP)

“…More recently, it has been demonstrated that very large (a > 10 µm) dust grains are able to produce polarisation by scattering thermal dust emission, a process that is called 'self-scattering'. This was first predicted to be observable in protoplanetary disks (Kataoka et al 2015) and then confirmed by numerous ALMA observations (Kataoka et al 2016;Yang et al 2017;Girart et al 2018). B-BOP, with its 100 µm polarised channel, would be able to detect and characterise the spectral dependence of polarisation by scattering due to ∼15 µm dust grains (Kataoka et al 2015), if present.…”