Characterization of mid-infrared polarization due to scattering in protoplanetary disks

Abstract

Context. It is generally assumed that magnetic fields play an important role in the formation and evolution of protoplanetary disks. One way of observationally constraining magnetic fields is to measure polarized emission and absorption produced by magnetically aligned elongated dust grains. The fact that radiation also becomes linearly polarized by light scattering at optical to millimeter wavelengths complicates magnetic field studies. Aims. We characterize the linear polarization of mid-infrared radiation due to scattering of the stellar radiation and dust thermal re-emission radiation (self-scattering). Methods. We computed the radial polarization profiles at wavelengths across the N and Q bands for a broad range of circumstellar disk configurations. These simulations served as a basis to analyze the correlations between selected disk parameters and the resulting linear polarization.Results. We find that the thermal re-emission radiation is stronger than the scattered stellar radiation for disks with inner holes smaller than ∼ 10 au within the considered parameter range. The mid-infrared polarization due to scattering shows several clear trends: For scattered stellar radiation only, the linear polarization degree decreases slightly with increasing radial distance, while it increases with radial distance for thermal re-emission radiation only and for a combination of scattered stellar radiation and thermal re-emission radiation. The linear polarization degree decreases with increasing disk flaring and luminosity of the central star. An increasing inner radius shifts the increase of the linear polarization degree further outside, while a larger scale height increases the linear polarization degree for small radial distances and decreases this degree further outside. For longer wavelengths, i.e., toward the Q band in our study, the linear polarization degree converges more slowly. Conclusions. We found several clear trends for polarization due to scattering. These trends are the basis to distinguish polarization due to scattering from polarization due to dichroic emission and absorption.

Self-scattering in protoplanetary disks with dust settling

“…This can be seen in the left-hand panel of Fig. 3 and it was also mentioned for the mid-infrared by Heese et al (2020). For steeper density distributions (i.e.…”

“…In addition, the general shape of the radial profiles is determined by the degree of anisotropy of the local radiation field. An anisotropic radiation field is one of the main requirements for polarisation due to scattering of thermal re-emission radiation (Kataoka et al 2015;Heese et al 2020;Brunngräber & Wolf 2020). In the inner regions of the disk, the radiation field is close to isotropic because of the axisymmetric density and temperature distribution.…”

Polarimetric investigation of selected cloud compositions in exoplanetary atmospheres

“…To study the polarization at various phase angles and wavelengths ranging from 0.3 µm to 1 µm, the publicly available three-dimensional Monte Carlo radiative transfer code POLARIS (Reissl et al 2016) was applied. It is well tested and has been used to investigate a broad range of astrophysical models such as molecular clouds (Reissl et al 2017;Pellegrini et al 2020;Seifried et al 2020), Bok globules (Brauer et al 2016;Zielinski et al 2021), and protoplanetary disks (Heese et al 2020;Brunngräber & Wolf 2020. For the purpose of our study, it has been optimized to handle the radiative transfer in planetary atmospheres (Lietzow et al 2021).…”

Backscattering-Based Discrimination of Microparticles Using an Optofluidic Multiangle Scattering Chip

“…Scattering of light is a powerful characterization method with broad fields of application such as astronomy, 1 , 2 biology, 3 , 4 environmental study, 5 , 6 and particle analysis. 7 − 11 The latter is based on the sensitivity of the scattering phenomena to the size, morphology, and refractive index of the particles.…”