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

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 (∆v ∼ 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. (2016), we decompose them into three kinematic components: a high-velocity component (HVC) associated with jets, and a 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 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-BC and NC line ratios are explained by thermally excited gas with temperatures between 5,000−10,000 K and electron densities ∼ 10 7 − 10 8 cm −3 . The HVC ratios are better reproduced by shock models with a pre-shock H number density of ∼ 10 6 − 10 7 cm −3 . Using these physical properties, we estimateṀ wind /Ṁ acc for the LVC andṀ jet /Ṁ acc 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 NC wind height is larger than the BC, the LVC-BCṀ wind /Ṁ acc 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.

Dust Transport and Processing in Centrifugally Driven Protoplanetary Disk Winds

“…The parameter values of the adopted T g profiles are summarized in Table 1. At the comparatively low altitudes (z/r<0.3) attained by the intermediate-size grains that are of interest to us, T g likely does not exceed ∼10 3 K; however, there is evidence from the analysis of forbidden line emission in protostellar systems that the gas temperature in disk winds can reach much higher values (∼10 4 K) further up (e.g., Fang et al 2018). Note, however, that the Blandford & Payne (1982) wind model that we employ is "cold," in the sense that the thermal pressure force has a negligible effect on the flow dynamics: this remains true for all the T g profiles that we use.…”

Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds

“…Because magnetic field strengths for observed stellar systems are still uncertain (Fang et al, 2018;Vlemmings et al, 2019), it is still difficult to evaluate the importance of magnetic versus photoevaporative winds. Our results show that the transition between these two wind-launching mechanisms strongly depends on the magnetization of the disk.…”

Spectral Evolution and Radial Dust Transport in the Prototype Young Eruptive System EX Lup*

“…Very low-velocity disk winds have been identified in EX Lup via forbidden line emission by Fang et al (2018) and Banzatti et al (2019). These works revealed significant differences in the observed wind velocities in quiescence and in outburst.…”

“…In quiescence, a low velocity component was detected at −1.5 km/s (Banzatti et al 2019). In outburst, however, velocities were measured at −12 km/s (Banzatti et al 2019) and in the range of −15 to −18 km/s (Fang et al 2018). Very slow winds and slow outflows have also been detected in FUors (Ruíz-Rodríguez et al 2017).…”