Evidence for an MHD disk wind via optical forbidden line spectro-astrometry

TL;DR

This study uses spectro-astrometry to provide evidence that the low velocity component of optical forbidden lines in T Tauri stars traces an MHD disk wind, offering new spatial measurements and confirming the wind's nature.

Contribution

It presents the first direct confirmation that the LVC narrow component traces an MHD disk wind and measures its spatial extent in T Tauri stars.

Findings

LVC narrow component traces an MHD disk wind.

Spatial extent of forbidden emission line LVC is approximately 40 au.

Decreasing offset with increasing velocity indicates wind height increases with disk radius.

Abstract

Spectro-astrometry is used to investigate the low velocity component (LVC) of the optical forbidden emission from the T Tauri stars RU Lupi and AS 205 N. Both stars also have high velocity forbidden emission (HVC) which is tracing a jet. For AS 205 N, analysis reveals a complicated outflow system. For RU Lupi, the [O I] 6300 and [S II] 6716, 6731 LV narrow component (NC) is offset along the same position angle (PA) as the HVC but with a different velocity gradient than the jet, in that displacement from the stellar position along the rotation axis is decreasing with increasing velocity. From the LVC NC PA and velocity gradient, it is inferred that the NC is tracing a wide angled MHD disk wind. A photoevaporative wind is ruled out. This is supported by a comparison with a previous spectro-astrometric study of the CO fundamental line. The decrease in offset with increasing velocity is interpreted as tracing an increase in the height of the wind with increasing disk radius. This is one of the first measurements of the spatial extent of the forbidden emission line LVC NC (~ 40 au, 8 au for RU~Lupi in the [S II] 6731 and [O I] 6300 lines) and the first direct confirmation that the LVC narrow component can trace an MHD disk wind.