Spectral line polarization in protoplanetary disks.

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 not always faithfully traces the magnetic field structure of protoplanetary disks, which leaves spectral line polarization as a promising method of 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. Methods. We use PORTAL (POlarized Radiative Transfer Adapted to Lines) in conjunction with LIME (Line Emission Modeling Engine). Together, they allow 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 and we comment on the observational feasibility of ALMA linear polarization observations of protoplanetary disks. We find that molecules that thermalize at high densities, such as HCN, are also 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.