Interaction of the central jet with the surrounding gas in the protostellar outflow from IRAS 04166+2706

Abstract

$Context.$ The outflow from the Class 0 protostar IRAS 04166+2706 (hereafter IRAS 04166) contains a remarkably symmetric jet-like component of extremely high-velocity (EHV) gas. $Aims.$ We studied the IRAS 04166 outflow and investigated the relation between its EHV component and the slower outflow gas. $Methods.$ We mosaicked the CO(2--1) emission from the IRAS 04166 outflow using the 12m and the Compact Arrays of ALMA. We also developed a ballistic toy model of the gas ejected laterally from a jet to interpret the data. $Results.$ In agreement with previous observations, the ALMA data show that the slow outflow component is distributed in two opposed conical lobes and has a shear-flow pattern with velocity increasing toward the axis. The EHV gas consists of a series of arc-like condensations that span the full width of the conical lobes and merge with their walls, suggesting that the fast and slow outflow components are physically connected. In addition, position--velocity diagrams along the outflow axis show finger-like extensions that connect the EHV emission with the origin of the diagram, as if part of the EHV gas had been decelerated by its interaction with the low-velocity outflow. A ballistic model can reproduce these finger-like extensions assuming that the EHV gas consists of jet material that has been ejected laterally over a short period of time and has transferred part of its momentum to the surrounding shear flow. $Conclusions.$ The EHV gas in the IRAS 04166 outflow seems to play a role in the acceleration of the slower gas component. The presence of similar finger-like extensions in the position-velocity diagrams of other outflows suggests that this process may be occurring in other systems, even if the EHV component is not seen because it has an atomic composition.

Figures (14)

• Figure 1: Maps of the CO(2--1) intensity integrated over the EHV and SHV velocity regimes. The velocity ranges of integration are given inside square brackets and are measured with respect to a cloud LSR velocity of 6.7 km s$^{-1}$. The map coordinates are offsets measured with respect to the position of IRAS 04166, and the colorintensity scales at the top are in units of K km s$^{-1}$. We note the different geometry of the SHV and EHV components.
• Figure 2: Maps of the combined blueshifted and redshifted EHV emission of CO(2--1), SiO(5--4), and SO(6,5--5,4) toward the inner $25"\times 25"$ of the IRAS 04166 outflow. For CO(2--1) and SiO(5--4), the first contour and contour interval are 9 K km s$^{-1}$, while for the weaker SO(6,5--5,4) they are 3 K km s$^{-1}$. The map offsets are referred to the position of IRAS 04166.
• Figure 3: Maps of CO(2--1) intensity integrated over 8 km s$^{-1}$ intervals to illustrate the different velocity regimes of the IRAS 04166 outflow. For easier comparison, the maps of blueshifted emission (top) have been rotated clockwise by $30\fdg4$, while the maps of redshifted emission (bottom) have been rotated by the same amount and reflected in the vertical direction. The interval of integration is given at the top and is measured with respect to the ambient LSR velocity of 6.7 km s$^{-1}$. All maps are in linear scale with the extreme values adjusted for maximum contrast. The position of IRAS 04166 coincides with the origin of coordinates, and the blanked areas indicate regions not covered by the ALMA mosaic.
• Figure 4: Maps of the absolute value of the velocity centroid of CO(2--1) for the SHV (left) and EHV (right) regimes. IRAS 04166 is located at the origin of coordinates, and the color scales at the top are in units of km s$^{-1}$. We note the shear pattern in the SHV regime and the velocity oscillations along the outflow axis in the EHV regime.
• Figure 5: Position--velocity diagrams of the CO(2--1) emission along the axis of the blue (top) and red (bottom) lobes of the IRAS 04166 outflow. The black arrows indicate some of the finger-like features that connect the EHV emission with the origin of the diagram. The velocity scale is measured in absolute value with respect to the cloud LSR velocity (6.7 km s$^{-1}$), and the wedge scales on the right are in units of K.