From thermal to magnetic driving: spectral diagnostics of simulation-based magnetothermal disc wind models

TL;DR

This study uses spectral diagnostics and simulations to differentiate between thermal and magnetic disc winds in protoplanetary discs, finding that most observed winds align with weakly magnetized or photoevaporative models.

Contribution

It introduces a spectral diagnostic approach to distinguish between thermally and magnetically driven disc winds using synthetic emission profiles.

Findings

Broader, blueshifted profiles indicate strongly magnetized winds.

Narrower profiles with smaller blueshifts match weakly magnetized or photoevaporative winds.

Most observed low-velocity components are consistent with weak magnetic or thermal winds.

Abstract

Disc winds driven by thermal and magnetic processes are thought to play a critical role in protoplanetary disc evolution. However, the relative contribution of each mechanism remains uncertain, particularly in light of their observational signatures. We investigate whether spatially resolved emission and synthetic spectral line profiles can distinguish between thermally and magnetically driven winds in protoplanetary discs. We modelled three disc wind scenarios with different levels of magnetisation: a relatively strongly magnetised wind ($\beta$4), a rather weakly magnetised wind ($\beta$6), and a purely photoevaporative wind (PE). Using radiative transfer post-processing, we generated synthetic emission maps and line profiles for [OI] 6300 \r{A}, [NeII] 12.81 $\mathrm{\mu}$m, and o-H2 2.12 $\mathrm{\mu}$m, and compared them with observations. The $\beta$4 model generally produces broader and more blueshifted low-velocity components across all tracers, consistent with compact emission regions and steep velocity gradients. The $\beta$6 and PE models yield narrower profiles with smaller blueshifts, in better agreement with most observed narrow low-velocity components (NLVCs). We also find that some line profile diagnostics, such as the inclination at maximum centroid velocity, are not robust discriminants. However, the overall blueshift and full-width at half-maximum (FWHM) of the low-velocity components provide reliable constraints. The $\beta$4 model reproduces the most extreme blueshifted NLVCs in observations, while most observed winds are more consistent with the $\beta$6 and PE models. Our findings reinforce previous conclusions that most observed NLVCs are compatible with weakly magnetised or purely photoevaporative flows. The combination of line kinematics and emission morphology offers meaningful constraints on wind-driving physics.