Multi-epoch Sub-arcsecond [Fe II] Spectroimaging of the DG Tau Outflows with NIFS. I. First data epoch

摘要

Investigating the outflows emanating from young stellar objects (YSOs) onsub-arcsecond scales provides important clues to the nature of the underlyingaccretion-ejection process occurring near the central protostar. We haveinvestigated the structures and kinematics of the outflows driven by the YSO DGTauri, using the Near-infrared Integral Field Spectrograph (NIFS) on GeminiNorth. The blueshifted outflow shows two distinct components in [Fe II] 1.644micron emission, which are separated using multi-component line fitting. Astationary recollimation shock is observed, in agreement with previous X-rayand FUV observations. The presence of this shock indicates that the innermoststreamlines of the high-velocity component are launched at a very small radius,0.01-0.15 AU, from the central star. The jet accelerates and expands downstreamof the recollimation shock; the 'acceleration' is likely a sign of velocityvariations in the jet. No evidence of rotation is found, and we compare thisnon-detection to previous counter-claims. Moving jet knots, likely the resultof the jet velocity variations, are observed. One of these knots moves moreslowly than previously observed knots, and the knot ejection interval appearsto be non-periodic. An intermediate-velocity component surrounds this centraljet, and is interpreted as the result of a turbulent mixing layer along the jetboundaries. Such lateral entrainment requires the presence of a magnetic fieldof strength a few mG or less at hundreds of AU above the disc surface, which isargued to be a reasonable proposition. In H2 1-0 S(1) 2.1218 micron emission, awide-angle, intermediate-velocity blueshifted outflow is observed. Bothoutflows are consistent with being launched by a magnetocentrifugal disc wind,although an X-wind origin for the high-velocity jet cannot be ruled out.