A Micro Molecular Bipolar Outflow From HL Tau

TL;DR

This paper presents high-resolution observations of the inner outflow from HL Tau, revealing a bipolar H2 emission structure with distinct morphology from the jet, supporting a model of a jet surrounded by a wide-angled wind.

Contribution

It provides detailed geometry and kinematics of HL Tau's inner outflow, highlighting the differences between H2 and [Fe II] emissions and supporting the wide-angled wind scenario.

Findings

H2 emission shows a bubble-like bipolar structure.

H2 and [Fe II] emissions have different morphologies.

The H2 outflow scale (~150 AU) is smaller than typical millimeter CO outflows.

Abstract

We present detailed geometry and kinematics of the inner outflow toward HL Tau observed using Near Infrared Integral Field Spectograph (NIFS) at the Gemini-North 8-m Observatory. We analyzed H2 2.122 um emission and [Fe II] 1.644 um line emission as well as the adjacent continuum observed at a <0".2 resolution. The H2 emission shows (1) a bubble-like 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 show an arc 1".0 away from the star, exhibiting a bipolar symmetry. Different brightness 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 H2 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 H2 outflow in our image (~150 AU) is strikingly smaller than the mm outflows, which often extend over 1000-10000 AU scales. The position-velocity diagram of the H2 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 produce the bipolar cavity and shocked H2 emission.