The Effects of Dust Optical Properties on the Scattering-induced Disk Polarization by Millimeter-sized Grains

Abstract

Spatially resolved (sub)millimeter polarization has been detected by ALMA in an increasing number of disks around young stellar objects. The majority of the observations show polarization patterns that are consistent with that expected from scattering by dust grains, especially at the shortest wavelength band for ALMA polarization (λ ≈ 870 µm). The inferred sizes of the grains responsible for the scattering-induced polarization are typically of order 100 µm, which is very different from the millimeter size commonly inferred from the dust opacity index β. In an effort to resolve this discrepancy, we first introduced the so-called "Coplanar Isotropic Radiation Field" approximation, which enables the computation of the (signed) polarization fraction (negative means polarization reversal) analytically.With an oft-adopted dust composition (used, e.g., by DSHARP), we find that models with big dust grains produce very small polarization with reversed orientation (relative to the well-known Rayleigh scattering by small particles), which hasn't been observed. The semi-analytic results are validated through Monte Carlo radiative transfer simulations. In these models, the "correct" (not reversed) polarization orientation and the small β index are mutually exclusive. To resolve this tension, we explore a wide range of dust models, parameterized by their complex refractive index m = n + ik. We find that both the fraction of the polarization and whether it is reversed or not depend on the refractive index in a complex way, and this dependence is mapped out on an n − k plane for a representative dust size distribution (MRN with a max = 3 mm) and wavelength of 870 µm. In particular, 3 mm-sized grains made of refractory organics produce polarization that is reversed, whereas grains of the same size but made of absorptive carbonaceous materials can produce a percent-level polarization that is not reversed; the latter may alleviate the tension between the grain sizes inferred from the scatteringinduced polarization and the opacity index β. We conclude that scattering-induced polarization has the potential to probe not only the sizes of the grains but also their composition.

Self-scattering in protoplanetary disks with dust settling

“…Ohashi & Kataoka 2019;Dent et al 2019;Bacciotti et al 2018;Ohashi et al 2018;Hull et al 2018;Stephens et al 2017Stephens et al , 2014 where the polarisation is thought to come at least partially from scattered re-emission. This is in agreement with the recently published study by Yang & Li (2020); in their model, only pure carbonaceous material can produce a high enough polarisation degree in a size distribution with a maximum grain size of 3 mm to explain at least some of the observations. We find similar results when decreasing the fractional amount of silicate or increasing the grain porosity.…”

“…This contradicts the usual finding of very large grains if deduced from the spectral index at mm wavelengths. Proposed solutions to this contradiction include grain porosity and different compositions (see also Yang & Li 2020;Kataoka et al 2017Kataoka et al , 2015. Keeping the maximum grain size of 1 mm, we calculated the grain-size averaged degree of polarisation of a single scattering event p = − S 12/S 11 for different mass fractions of silicate in the dust composition, and for different grain porosities.…”

Magnetically regulated disk-formation in the inner 100 au region of the Class 0 young stellar object OMC-3/MMS~6 resolved by JVLA and ALMA

“…However, their assumption of β =1 can only be achieved when a max is a lot greater than 1 mm (e.g., Figure 4). This statement is true also for the other commonly adopted dust opacity tables for protoplanetary disks (e.g., Kataoka et al 2015;Woitke et al 2016), unless one assumes a very significant inclusion of amorphous carbon (e.g., Woitke et al 2016;Yang & Li 2020). However, significant inclusion of amorphous carbon will make the values of α close to 3 also at 29-36 GHz frequency.…”

Recent advances in Prussian blue-based photothermal therapy in cancer treatment

“…80 PB NPs with catalase-like activity can decompose hydrogen peroxide (H 2 O 2 ) in tumours to produce oxygen, further relieving tumour hypoxia. 81,82 Zhang group prepared zirconium-porphyrin (PCN)-encapsulated and cancer cell membrane-coated PB NPs (PB@PCN@MEM) (Fig. 5C).…”