A New Look at T Tauri Star Forbidden Lines: MHD-driven Winds from the Inner Disk

Min Fang,Andrea K Dupree,Ilaria Pascucci,Patrick Hartigan,Suzan Edwards,Andrea Banzatti,Gregory J Herczeg,Mario Flock,Uma Gorti

doi:10.3847/1538-4357/aae780

Abstract

Abstract Magnetohydrodynamic (MHD) and photoevaporative winds are thought to play an important role in the evolution and dispersal of planet-forming disks. We report the first high-resolution (Δv ∼ 6 km s−1) analysis of [S ii] λ4068, [O i] λ5577, and [O i] λ6300 lines from a sample of 48 T Tauri stars. Following Simon et al. we decompose them into three kinematic components: a high-velocity component (HVC) associated with jets, and low-velocity narrow (LVC-NC) and broad (LVC-BC) components. We confirm previous findings that many LVCs are blueshifted by more than 1.5 km s−1 and thus most likely trace a slow disk wind. We further show that the profiles of individual components are similar in the three lines. We find that most LVC-NC and LVC-BC line ratios are explained by thermally excited gas with temperatures between 5000 and 10,000 K and electron densities of ∼107–108 cm−3. The HVC ratios are better reproduced by shock models with a pre-shock H number density of ∼106–107 cm−3. Using these physical properties, we estimate for the LVC and for the HVC. In agreement with previous work, the mass carried out in jets is modest compared to the accretion rate. With the likely assumption that the LVC-NC wind height is larger than the LVC-BC, the LVC-BC is found to be higher than the LVC-NC. These results suggest that most of the mass loss occurs close to the central star, within a few au, through an MHD-driven wind. Depending on the wind height, MHD winds might play a major role in the evolution of the disk mass.

Highlights

Circumstellar disks form as a result of angular momentum conservation during the protostellar core collapse (Shu 1977) and play an important role in both star and planet formation
The focus of our paper is on forbidden-line luminosities and line ratios, here we show that the kinematics of the low-velocity component (LVC)-broad component (BC) and LVC-narrow component (NC) of our sample are consistent with previous findings
By including detections and upper limits, we found that the high-velocity component (HVC) SII40/OI63 ratios are statistically higher than the LVC and that there is a low probability that the BC OI55/63 ratios are drawn from the same parent population as the NC and HVC (Table 4)

Summary

Introduction

Circumstellar disks form as a result of angular momentum conservation during the protostellar core collapse (Shu 1977) and play an important role in both star and planet formation. A significant fraction of the stellar mass is accreted through the disk, and what is not accreted or dispersed via other mechanisms provides the raw material to build planets. There are three main stages of disk evolution and dispersal (e.g., Gorti et al 2016; Ercolano & Pascucci 2017 for recent reviews). For most of the disk lifetime (Stage 1), evolution is primarily set by accretion. When the disk accretion rate through this radius drops below the wind mass-loss rate, photoevaporation limits the supply of gas to the inner disk, a gap is formed (Stage 2), and the inner disk drains onto the star on the local viscous timescale—of order 100,000 yr. In the last stage (Stage 3), there is no accretion onto the star, and the disk is rapidly cleared from the inside out by stellar high-energy photons directly irradiating the outer disk

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