Evidence of Grain Alignment by Magnetically Enhanced Radiative Torques from Multiwavelength Dust Polarization Modeling of HL Tau
The Astrophysical Journal(2023)
Abstract
Atacama Large Millimeter/Submillimeter Array (ALMA) has revolutionized the
field of dust polarization in protoplanetary disks across multiple wavelengths.
Previous observations and empirical modeling suggested multiple mechanisms of
dust polarization toward HL Tau, including grain alignment and dust scattering.
However, a detailed modeling of dust polarization based on grain alignment
physics is not yet available. Here, using our updated POLARIS code, we perform
numerical modeling of dust polarization arising from both grain alignment by
Magnetically Enhanced Radiative Torque (MRAT) mechanism and self-scattering to
reproduce the HL Tau polarization observed at three wavelengths 0.87, 1.3, and
3.1$\,$mm. Our modeling results show that the observed multi-wavelength
polarization could be reproduced only when large grains contain embedded iron
inclusions and those with slow internal relaxation must have wrong internal
alignment (i.e., the grain's major axis parallel to its angular momentum). The
abundance of iron embedded inside grains in the form of clusters is constrained
to be $\gtrsim 16$%, and the number of iron atoms per cluster is $N_{\rm cl}
\sim 9\times10^2$. Maximum grain sizes probed at wavelengths $\lambda$ = 0.87,
1.3, and 3.1$\,$mm are constrained at $\sim$ 60, 80, and 90$\,\mu$m,
respectively.
MoreTranslated text
Key words
Protoplanetary disks,Polarimetry,Radio astronomy,Circumstellar dust,Magnetic fields
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