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.
Astronomy and Astrophysics
- Pub Date:
- February 2020
- methods: numerical;
- protoplanetary disks;
- radiative transfer;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Astrophysics of Galaxies