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

Haifeng Yang, Zhi Yun Li, Leslie Looney, Ian Stephens

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Spatially resolved polarized millimetre/submillimetre emission has been observed in the disc 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 discs. We extend theirwork by including the polarization induced by disc inclinationwith respect to the line of sight. Using a physically motivated, semianalytic model, we show that the polarization fraction of the scattered light increases with the inclination angle i, reaching 1/3 for edge-on discs. The inclination-induced polarization can easily dominate that intrinsic to the disc in the face-on view. It provides a natural explanation for the two main features of the polarization pattern observed in the tilted disc 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 millimetre radiation. In order to produce polarization at the observed level (~1 per cent), 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, millimetre/centimetre-sized, grains may be needed to explain the bulk of the unpolarized continuum emission.

Original languageEnglish (US)
Pages (from-to)2794-2805
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Mar 1 2016


  • Dust, extinction
  • Polarization
  • Protoplanetary discs
  • Scattering

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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