Tracing the large-scale magnetic field morphology in protoplanetary disks using molecular line polarization

TL;DR

This paper models spectral line polarization in protoplanetary disks to identify molecular lines suitable for mapping magnetic field structures, highlighting molecules like HCN as promising tracers.

Contribution

It introduces a comprehensive simulation approach combining PORTAL and LIME to analyze polarized line emission in complex disk models, advancing magnetic field mapping techniques.

Findings

High-density molecules like HCN show strong polarization signals.

Low-density molecules like CO are polarized mainly in outer disk regions.

Simulated polarization maps suggest feasible ALMA observations for magnetic field studies.

Abstract

(abridged) Magnetic fields are fundamental to the accretion dynamics of protoplanetary disks and they likely affect planet formation. Typical methods to study the magnetic field morphology observe the polarization of dust or spectral lines. However, it has recently become clear that dust-polarization in ALMA's spectral regime does not always faithfully trace the magnetic field structure of protoplanetary disks, which leaves spectral line polarization as a promising method for mapping the magnetic field morphologies of such sources. We aim to model the emergent polarization of different molecular lines in the ALMA wavelength regime that are excited in protoplanetary disks. We explore a variety of disk models and molecules to identify those properties that are conducive to the emergence of polarization in spectral lines and may therefore be viably used for magnetic field measurements in protoplanetary disks. We used PORTAL in conjunction with LIME. Together, they allowed us to treat the polarized line radiative transfer of complex three-dimensional physical and magnetic field structures. We present simulations of the emergence of spectral line polarization of different molecules and molecular transitions in the ALMA wavelength regime. We find that molecules that thermalize at high densities, such as HCN, are also the most susceptible to polarization. We find that such molecules are expected to be significantly polarized in protoplanetary disks, while molecules that thermalize at low densities, such as CO, are only significantly polarized in the outer disk regions. We present the simulated polarization maps at a range of inclinations and magnetic field morphologies, and we comment on the observational feasibility of ALMA linear polarization observations of protoplanetary disks.