A Micro-Molecular Bipolar Outflow from HL Tauri

Abstract

We present detailed geometry and kinematics of the inner outflow toward HL Tau observed using Near Infrared Integral Field Spectrograph (NIFS) at the Gemini-North 8 m Observatory. We analyzed H 2 2.122 mm emission and [Fe ii] 1.644 mm line emission as well as the adjacent continuum observed at a !0.2Љ resolution. The H 2 emission shows (1) a bubblelike geometry to the northeast of the star, as briefly reported in the previous paper, and (2) faint emission in the southwest counterflow, which has been revealed through careful analysis. The emission on both sides of the star shows an arc 1.0Љ away from the star, exhibiting a bipolar symmetry. Different brightnesses and morphologies in the northeast and southwest flows are attributed to absorption and obscuration of the latter by a flattened envelope and a circumstellar disk. The H 2 emission shows a remarkably different morphology from the collimated jet seen in [Fe ii] emission. The positions of some features coincide with scattering continuum, indicating that these are associated with cavities in the dusty envelope. Such properties are similar to millimeter CO outflows, although the spatial scale of the H 2 outflow in our image (∼150 AU) is strikingly smaller than the millimeter outflows, which often extend over 1000-10000 AU scales. The positionvelocity diagrams of the H 2 and [Fe ii] emission do not show any evidence for kinematic interaction between these flows. All results described above support the scenario that the jet is surrounded by an unseen wide-angled wind, which interacts with the ambient gas and produces the bipolar cavity and shocked H 2 emission.

Origin of the wide-angle hot H2in DG Tauri

“…Therefore, the cavity would have expanded to its observed half-width of 0. 2 = 30 AU in only 15 years, meaning that it would trace a very recent wind "outburst", even younger than in the case of HL Tau (70 years; Takami et al 2007). Since it would be improbable to catch a rare event at such an early age, this phenomenon would have to be recurrent on timescales of a few decades, similar to the successive "bubbles" seen in XZ Tau (Krist et al 2008).…”

“…This scenario is traditionally invoked to explain CO molecular outflow cavities seen on larger scales around young protostars (Arce et al 2007). It was proposed by Takami et al (2007) to explain the small scale V-shaped biconical cavity of H 2 1-0 S(1) emission around HL Tau, which bears strong resemblance to that in DG Tau; it is thus interesting to investigate whether the same scenario could apply to DG Tau itself. In the following discussion, we will neglect turbulent mixing along the cavity walls between the shocked ambient gas and the shocked wind.…”

“…The velocity range of [Fe II]1.64 μm emission in the blue lobe of DG Tau Takami et al (2007), who then interpreted the V-shaped H 2 as a shocked outflow cavity swept-up by an unseen wide-angle wind.…”

“…Schneider et al (2013a) also favor a molecular MHD disc wind, but heated by shocks rather than by ambipolar diffusion. In HL Tau, on the other hand, Takami et al (2007) interpret the V-shaped H 2 emission encompassing the atomic jet as a shocked cavity driven into the envelope by an unseen wide-angle wind. In any case, extended H 2 emission in TTS clearly holds key information on their wide-angle wind structure, the radial extension of any MHD disc wind present, and its impact on protoplanetary disc physics.…”

VLT integral field spectroscopy of embedded protostars: using near-infrared emission lines as tracers of accretion and outflow

“…Molecular bipolar outflow morphologies often support the existence of wide-angled winds (Lee et al 2000;Arce & Sargeant 2006). A relatively broad, low velocity molecular wind has also recently been detected surrounding the fast, HH jet from the young T Tauri star HL Tauri (Takami et al 2007) and from the DG Tau jet (Beck et al 2008). The mere presence of excited H 2 emission at the base of each HH jet tends to support a disk-wind model (or at the very least a flow component that is ejected from large disk radii), since the conditions in the jet must permit the survival of H 2 and, at the same time, excite forbidden emission lines from ionised species such as S + and Fe + .…”