Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission

TL;DR

This study analyzes forbidden emission lines in T Tauri stars to characterize slow disk winds, revealing their kinematic properties and potential driving mechanisms, which are crucial for understanding protoplanetary disk evolution.

Contribution

First detailed analysis of low velocity forbidden line components in T Tauri stars, distinguishing between MHD and photoevaporative wind signatures.

Findings

LVC emission present in most stars with [O I] detection.

Broad and narrow components show different kinematic behaviors.

FWHM correlates with disk inclination, indicating Keplerian origin.

Abstract

Using Keck/HIRES spectra {\Delta}v ~ 7 km/s, we analyze forbidden lines of [O I] 6300 {\AA}, [O I] 5577 {\AA} and [S II] 6731 {\AA} from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high velocity component (HVC) associated with microjets, we study the properties of the low velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (MHD) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O I] but only in 2 out of eight with detected [S II] , so our analysis is largely based on the properties of the [O I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The FWHM of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 AU and 0.5 to 5 AU, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.