Oral Presentation

Modeling of the ALMA HL Tau polarization by mixture of grain alignment and self-scattering

Author(s): Tomohiro Mori (the University of Tokyo), Akimasa Kataoka (NAOJ)

Presenter: Tomohiro Mori (Institute of Astronomy, the University of Tokyo)

Polarization features obtained on the HL Tau disk in Band 3 are (1) elliptical polarization pattern and (2) uniform polarization fraction in the azimuthal directions. In previous studies, the origin of the polarization was concluded to originate from thermal emission of grains aligned by a radiative gradient in the disk (Kataoka et al. 2017 and Stephens et al. 2017). However, Yang et al. 2019a pointed out that the radiative alignment predicts the circular pattern and strong polarization variations in the azimuthal directions, both of which are incompatible with (1) and (2). Alternatively, gas-flow alignment, where aspherical grains are aligned by gas-flow with their long axes parallel to the flow, can produce the elliptical pattern. However, it also predicts strong azimuthal polarization variations, which is incompatible with (2).

To solve the problem, we perform semi-analytical and radiative transfer modeling that include the contributions from scattered emission as well as thermal emission of the aligned grains. As a result, we find that the combination of the gas-flow alignment and scattering model can reproduce the features (1) and (2), whereas the radiative alignment + scattering model cannot reproduce both of the features. This result is surprising because the gas-flow alignment has been believed to occur when the velocity of the gas-flow on grains is supersonic while that in disks is subsonic. This raises a question regarding our understanding of alignment processes.