Optical Spectro-astrometry as a Test of Wind Driven Accretion

Disks around young stars provide the material needed to build planets. Understanding how material in the disk disperses and evolves over time is vital in understanding planet formation. In recent years, theory has shifted from the classic picture of disk evolution via magneto-rational instability inducted turbulence in favor of a radially extended magnetohydrodynamic (MHD) disk winds which regulate accretion in disks and therefore are drivers of disk evolution. Recent surveys of young stars suggest that the low velocity component narrow component (LVC - NC) of the optical forbidden emission lines (FELs) traces a region close in (< 4 au from the star), the base of the MHD wind but doesn’t rule out the possibility of a thermal photoevaporative wind. In a pilot study of RU Lupi (Whelan et al. 2021) the LVC of the [OI] 6300Å and [SII] 6731Å forbidden lines was spatially resolved using spectro-astrometry (SA) and the thermal wind possibility for the origin of the LVC was ruled out. We now expand this pilot study to a sample of young low mass stars, with accretion rates spanning over the range over which disks evolve, to test whether the MHD origin of the LVC-NC is typical of other young low mass stars. This study will also test the main prediction of wind driven accretion models namely, that the mass accretion rate is approximately equal to the wind mass loss rate.