Variability of the DG Tau forbidden emission line low velocity component^⋆

¹ Department of Physics, Maynooth University, Maynooth, Co.Kildare, Ireland
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
³ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan, R.O.C
⁴ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
⁵ Department of Physics, Texas State University, 749 N Comanche Street, San Marcos, TX 78666, USA

^⋆⋆ Corresponding author: noah.otten@mu.ie

Received: 28 April 2025
Accepted: 22 September 2025

Abstract

Context. Optical forbidden emission lines (FELs) come from transitions with long radiative decay times (≈100 s) that require low density environments where collisions between atoms are rare. They are produced in the low-density gas found in the outflows (jets and winds) driven by low-mass young stellar objects (YSOs). Moreover, they frequently reveal distinct velocity components within the outflow, including a so-called low velocity component (LVC). A question pertinent to the removal of excess angular momentum in YSOs is whether the LVC traces a magnetohydrodynamic (MHD) wind or a photoevaporative wind. Here, we study the jet and LVC of the classical T Tauri Star DG Tau. The velocity of the DG Tau jet has been decreasing since 2006, making it a particularly interesting source for this work.

Aims. The aim is to investigate the connection between the high-velocity jet and the LVC in DG Tau and to better understand the origin of the LVC by examining spectral and spatial changes over time.

Methods. Kinematic fitting and spectro-astrometry were applied to three epochs of high spectral resolution data spanning ≈18 years to conduct a detailed study of the changes in the LVC over time.

Results. A decrease in velocity of ≈100 kms⁻¹ from 2003 to 2021 is in agreement with the known slowing of the DG Tau jet. The kinematic fitting of the [O I]λ6300, [O I] λ5577, and [S II] λ6731 lines over the three epochs of data reveal the complex nature of the optical FELs. In agreement with a recent study of the DG Tau optical FELs, up to six blueshifted components in the FEL line profiles alongside a redshifted wing are identified. The three observed LVC sub-components (LVC-high, LVC-medium, and LVC-low) are consistent with entrained jet material, a disk wind, and a dense upper disk atmosphere, respectively. Despite the strong variability of the jet components over the three epochs, the LVC is found to be far more stable, and only the relative brightness of the three LVC sub-components is seen to change. A constraint of ≥2 au is placed on the minimum de-projected height of the LVC-M in [O I] λ5577 where there is no contribution from the jet.

Conclusions. The results support a disk wind origin for the LVC-M sub-component but cannot distinguish between a photoevaporative or MHD wind origin. The minimum [O I] λ5577 LVC-M height of ≥2 au indicates that this wind is launched inside the gravitational potential well of DG Tau and favours an MHD wind origin for the LVC-M. The fact that the peak velocity of the LVC-M does not change significantly requires further investigation in the context of a common origin for jets and MHD disk winds. Future studies will benefit from higher spectral resolution data to reduce blending between the outflow components and higher cadence sampling in time to explore a time lag between changes in the jet and the LVC.