The ALMA Survey of 70$\mu$m Dark High-mass Clumps in Early Stages (ASHES) III. A Young Molecular Outflow Driven by a Decelerating Jet

TL;DR

This study analyzes ALMA CO observations of a young, massive molecular outflow with a unique S-shaped velocity structure, proposing a decelerating jet model that reveals inhomogeneous ambient conditions and estimates the outflow's age.

Contribution

It introduces a novel spatio-kinematical model explaining the peculiar outflow morphology as a decelerating jet entraining ambient material.

Findings

The outflow exhibits an S-shaped velocity pattern not seen before.

A decelerating jet model fits the observed kinematics.

Estimated outflow age is approximately 1300 years.

Abstract

We present a spatio-kinematical analysis of the CO~($J$=2$\rightarrow$1) line emission, observed with the Atacama Large Millimter/submillimter Array (ALMA), of the outflow associated with the most massive core, ALMA1, in the 70 $\mu$m dark clump G010.991$-$00.082. The position-velocity (P-V) diagram of the molecular outflow exhibits a peculiar $\mathsf{S}$-shaped morphology that has not been seen in any other star forming region. We propose a spatio-kinematical model for the bipolar molecular outflow that consists of a decelerating high-velocity component surrounded by a slower component whose velocity increases with distance from the central source. The physical interpretation of the model is in terms of a jet that decelerates as it entrains material from the ambient medium, which has been predicted by calculations and numerical simulations of molecular outflows in the past. One side of the outflow is shorter and shows a stronger deceleration, suggesting that the medium through which the jet moves is significantly inhomogeneous. The age of the outflow is estimated to be $\tau$$\approx$1300 years, after correction for a mean inclination of the system of $\approx$57$^{\circ}$.