The interpretation of protoplanetary disc wind diagnostic lines from X-ray photoevaporation and analytical MHD models

TL;DR

This study compares X-ray photoevaporation and MHD models to interpret wind diagnostic lines in protoplanetary discs, revealing that line broadening is often dominated by outflow velocity gradients rather than Keplerian rotation, challenging previous interpretations.

Contribution

It provides detailed synthetic line profiles from both photoevaporative and MHD models, highlighting their capabilities and limitations in explaining observed wind components.

Findings

Photoevaporation models reproduce NLVCs but not BLVCs or HVCs.

MHD models can reproduce all components but often show double peaks not commonly observed.

Line broadening is mainly due to velocity gradients, not Keplerian rotation.

Abstract

High resolution spectra of typical wind diagnostics ([OI] 6300 \r{A} and other forbidden emission lines) can often be decomposed into multiple components: high-velocity components with blueshifts up to several 100 km/s are usually attributed to fast jets, while narrow (NLVC) and broad (BLVC) low-velocity components are believed to trace slower disc winds. Under the assumption that the line-broadening is dominated by Keplerian rotation, several studies have found that the BLVCs should trace gas launched between 0.05 and 0.5 au and correlations between the properties of BLVCs and NLVCs have been interpreted as evidence for the emission tracing an extended MHD wind and not a photoevaporative wind. We calculated synthetic line profiles obtained from detailed photoionisation calculations of an X-ray photoevaporation model and a simple MHD wind model and analyzed the emission regions of different diagnostic lines and the resulting spectral profiles. The photoevaporation model reproduces most of the observed NLVCs but not the BLVCs or HVCs. The MHD model is able to reproduce all components but produces Keplerian double peaks at average inclinations that are rarely observed. The combination of MHD and photoevaporative winds could solve this problem. Our results suggest that the Gaussian decomposition does not allow for a clear distinction of flux from different wind regions and that the line broadening is often dominated by the velocity gradient in the outflow rather than by Keplerian rotation. We show that observed correlations between BLVC and NLVC do not necessarily imply a common origin in an extended MHD wind.