Polarization at millimeter wavelengths caused by drifting grains in protoplanetary disks

TL;DR

The study investigates whether mechanical grain alignment driven by gas-dust drift in VSI-active protoplanetary disks can imprint observable millimeter polarization. It combines 3D radiation hydrodynamics (PLUTO) to obtain density and velocity fields, MAD-based Monte Carlo simulations to model grain alignment, and 3D polarized radiative transfer (POLARIS) to synthesize flux and polarization maps, including both dichroic emission/absorption and self-scattering. The results show that large millimeter grains can align via mechanical torques, producing polarization predominantly along the disk major axis in inclined disks, while self-scattering can dominate at shorter wavelengths; polarization orientation can flip when λ is near 2π$a_{ m eff}$, and VSI-induced irregular velocity fields reduce net polarization compared to ideal analytic drift models. These findings suggest millimeter polarization as a promising diagnostic of aligned, drifting grains in VSI-dominated disks, though real disks likely host multiple polarization mechanisms that require integrated modeling and broader observational tests for confirmation.

Abstract

During the evolution of protoplanetary disks, dust grains start to grow, form larger particles, settle to the midplane, and rearrange the disk, mainly by the inward radial drift. Because of this, dust pebbles with an irregular shape usually align mechanically and thus cause polarization signatures in their thermal radiation due to dichroic emission or absorption. The goal of this paper is to evaluate the potential to trace the impact of mechanical grain alignment in protoplanetary disks on the observed degree and orientation of linear polarization at millimeter wavelengths. We combined 3D radiation hydrodynamical simulations to determine the density distribution and the velocity field of gas and dust particles, Monte Carlo dust-gas interaction simulations to calculate the mechanical alignment of dust in a gas flow, and, finally, 3D Monte Carlo polarized radiative transfer simulations to obtain synthetic polarimetric observations. We find that large grains, which contribute the most to the net polarization, are potentially mechanically aligned in the protoplanetary disk under the effect of the vertical shear instability (VSI). Thereby, the drift velocity is parallel to the rotational disk axis. Assuming oblate dust grains that are aligned with their short axis parallel to the direction of the drift velocity, the resulting polarization is usually along the major axis of the disk. This is in contrast to typical drift models that propose either a radial or azimuthal drift velocity component. If hydrodynamical instabilities, such as the VSI, dominate the kinematics in protoplanetary disks, the mechanical alignment of dust is a promising mechanism for grain alignment in these systems. In that case, the resulting millimeter polarization allows us to trace the orientation of aligned millimeter-sized grains.

Figures (14)

• Figure 1: Exemplary BAM1 dust aggregate with an effective radius of $a_{\mathrm{eff}} = 1.2\um$. The aggregate rotates with an angular velocity, $\vec{\omega,}$ that corresponds to its maximum moment of inertia, $I_{\mathrm{m}}$. The rotation and precession of the dust aggregate result from gas-dust interactions, where the gas and dust components move with a relative velocity, $\vec{\varv}_{\mathrm{drift}}$ , through the disk. The angle, $\Theta,$ is defined as the angle between $\vec{\omega}$ and $\vec{\varv}_{\mathrm{drift}}$.
• Figure 2: Left panel: Angular velocity, $\omega,$ as a function of the ratio ${\varv_{\mathrm{drift}} / \varv_{\mathrm{th}}}$. The effective grain radii, $a_{\mathrm{eff}} = 0.1\um$ to 1.4, are color-coded. Solid lines represent the average values of $\omega$, while the shaded areas indicate the range between the minimum and maximum values. Dotted lines show the lower limit, $3\times \omega_{\mathrm{th}}$, for long-term stable alignment, and dashed lines represent the upper limit for rotational disruption, $\omega_{\mathrm{disr}}$. Right panel: Same as the left panel, but for the Rayleigh reduction factor $\mathcal{R}$ (solid lines). Dashed-dotted lines depict the parametric representation of $\mathcal{R}$ for grains with $a_{\mathrm{eff}} \geq 1\um$. See Sect. \ref{['subsec:grain_alignment']} for details.
• Figure 3: Cross-section view of the dust mass density in units of g.cm^-3 of 0.1 to 10-sized grains (green), 100-sized grains (purple), and 1mm-sized grains (orange). The values are azimuthally averaged.
• Figure 4: Ratio of absorption efficiencies $Q_\perp / Q_\parallel$ (top) and absorption efficiency (bottom) of an oblate spheroid as a function of radius of an equal volume sphere. This figure shows an axis ratio of the spheroid of $1 / 1.5$ and a wavelength of radiation of 3mm. The absorption efficiency $Q_\parallel$ is the efficiency along the minor grain axis, that is, the axis of alignment. If $a / \lambda \geq 1$, we assume a spherical particle, thus, $Q_\perp = Q_\parallel$. See Sect. \ref{['subsec:grain_properties']} for details.
• Figure 5: Ratio of the drift velocity and the thermal velocity in the disk midplane of 100 (top) and 1mm-sized (bottom) grains. Dust grains are aligned if $1.78e-2 \leq \varv_\mathrm{drift} / \varv_\mathrm{th} \leq 10$ (see Eq. \ref{['eq:rayleigh_reduction_factor']}), otherwise they are marked in red in the figure. See Sect. \ref{['subsec:orientation_alignment']} for details.