Global Hydromagnetic Simulations of Protoplanetary Disks with Stellar Irradiation and Simplified Thermochemistry

Abstract

Outflows driven by large-scale magnetic fields likely play an important role in the evolution and dispersal of protoplanetary disks and in setting the conditions for planet formation. We extend our 2D-axisymmetric nonideal MHD model of these outflows by incorporating radiative transfer and simplified thermochemistry, with the dual aims of exploring how heating influences wind launching and illustrating how such models can be tested through observations of diagnostic spectral lines. Our model disks launch magnetocentrifugal outflows primarily through magnetic tension forces, so the mass-loss rate increases only moderately when thermochemical effects are switched on. For typical field strengths, thermochemical and irradiation heating are more important than magnetic dissipation. We furthermore find that the entrained vertical magnetic flux diffuses out of the disk on secular timescales as a result of nonideal MHD. Through postprocessing line radiative transfer, we demonstrate that spectral line intensities and moment-1 maps of atomic oxygen, the HCN molecule, and other species show potentially observable differences between a model with a magnetically driven outflow and one with a weaker, photoevaporative outflow. In particular, the line shapes and velocity asymmetries in the moment-1 maps could enable the identification of outflows emanating from the disk surface.

Wind–MRI interactions in local models of protoplanetary discs – I. Ohmic resistivity

“…Local simulations (Bai & Stone 2013, 2014Bai 2015) found that discs may settle into the slanted symmetry, whether in the ideal MHD regime or not. Although such a configuration would not be physical at large z, as it would imply that one part of the field is bending towards the star, recent global simulations (Bai & Stone 2017;Bai 2017;Béthune et al 2017;Gressel et al 2020) have suggested that in certain radial locations such a symmetry is indeed adopted throughout the vertical extent of the disc region before field lines bend back in the normal manner away from the star further up in the atmosphere. Hence it would be useful to relax the symmetry assumptions of the solution to explore what factors contribute to the disc adopting a particular configuration.…”

“…By nature of the slanted symmetry, a disc with such configuration cannot (at least in the local model) support a net radial steady state transport of matter or B z flux, as contributions from both sides cancel out. Global simulations (Bai 2017;Béthune et al 2017;Gressel et al 2020;Riols et al 2020) have also shown that such symmetry may lead to a reduction in both overall accretion and flux transport rate, and may even cause the disc wind and accretion stream to be restricted to one hemisphere only. Since the slanted symmetry steady state is more easily reached when the local B z flux is strong, it may contribute to an automatic shut-down mechanism for the flux transport when the local build-up of B z flux becomes too strong and the disc transitions to the slanted state.…”

“…Previous authors (Bai 2017;Béthune et al 2017) have invoked non-ideal MHD effects such as the Hall-shear instability (Kunz 2008), which arises from the presence of Hall drift, to explain the development of the strong radial and toroidal net flux characteristic of the slanted solution. Others (Gressel et al 2020) have suggested that it could be a manifestation of corrugation of the mid-plane by the vertical shear instability (VSI) (Urpin 2003;Urpin & Brandenburg 1998). However, settling to the slanted symmetry state was also observed in global simulations where Ohmic and ambipolar diffusion were the only non-ideal effects present (Gressel et al 2020), and even in purely Ohmic (Rodenkirch et al 2020) or ambipolar (Riols et al 2020) discs.…”

“…Others (Gressel et al 2020) have suggested that it could be a manifestation of corrugation of the mid-plane by the vertical shear instability (VSI) (Urpin 2003;Urpin & Brandenburg 1998). However, settling to the slanted symmetry state was also observed in global simulations where Ohmic and ambipolar diffusion were the only non-ideal effects present (Gressel et al 2020), and even in purely Ohmic (Rodenkirch et al 2020) or ambipolar (Riols et al 2020) discs. The latter simulation is also locally isothermal, shutting down the VSI and ruling it out as necessary for the development of the slanted symmetry.…”

Migrating low-mass planets in inviscid dusty protoplanetary discs

“…There are several routes to enhance the solid-to-gas mass ratio in PPDs either globally or locally. These include magnetized disc winds (Bai 2017;Wang et al 2019;Gressel et al 2020); photoevaporation (Alexander & Armitage 2007;Alexander et al 2014;Wang & Goodman 2017); dusttrapping by pressure bumps (Pinilla et al 2012;Pinilla & Youdin 2017;Dullemond et al 2018); and mutual relative radial drift between dust and gas (Gonzalez et al 2017;Kanagawa et al 2017). It is therefore important to understand how planets interact with dust-rich discs (e.g.…”