Variability of the DG Tau Forbidden Emission Line Low Velocity Component

Abstract

Optical Forbidden Emission Lines (FELs) come from transitions with long radiative decay times needing low density gas where collisions between atoms are rare. They are produced in the outflows driven by young stellar objects. These lines trace distinct velocity components, including a Low Velocity Component (LVC), which may be tracing a magneto hydrodynamic (MHD) or photoevaporative (PE) wind. We study the jet and LVC of the star DG Tau, whose jet velocity has decreased since 2006. We aim to investigate a link between the high velocity jet and the LVC and clarify the LVC origin as an MHD or PE wind by studying spectral \& spatial changes over time. Using kinematic fitting \& spectro-astrometry, we analyse three epochs of spectra spanning ~18 years. A ~100 km/s decrease in velocity from 2003 to 2021 aligns with known slowing of the jet. Fitting of the [O I] λ6300, [O I] λ5577, and [S II] λ6731 lines reveal complex FEL profiles, with up to six blue-shifted components and a redshifted wing, in agreement with Chou et al. (2025). We see three LVC sub-components (LVC-H, LVC-M, and LVC-L) that are consistent with entrained jet material, disk wind, and dense upper disk atmosphere respectively. While jet components vary in time, the LVC remains quite stable, with changes in the relative brightness of each sub-component. The results cannot distinguish between a MHD or PE wind origin for the LVC. A limit of less than 2 au is put on the de projected height of the LVC-M in [O I] λ5577, where there is no jet contribution. This supports a disk wind and may favor an MHD wind origin. The near constant peak velocity of LVC-M needs further study in context of a shared origin for jets and MHD winds. Future work needs observations with higher spectral resolution and time cadence to resolve blended components and examine a possible time lag between changes in the jet and LVC.

9 pages. 4 figures. 3 tables. Accepted to Astronomy and Astrophysics on 22/09/2025

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