An Isothermal Outflow in High-mass Star-forming Region G240.31+0.07

TL;DR

This study uses multi-line CO observations to analyze a high-mass star-forming region, revealing an isothermal, high-velocity bipolar outflow driven by a wide-angle wind, supporting disk-mediated accretion models.

Contribution

It provides the first detailed multi-line analysis of the physical conditions of the outflow in G240.31+0.07, confirming an isothermal, wide-angle wind driven outflow in a high-mass star-forming region.

Findings

Outflow is approximately isothermal at ~50 K.

CO column density decreases with outflow velocity.

Outflow likely driven by a wide-angle wind.

Abstract

We present Atacama Pathfinder EXperiment (APEX) observations toward the massive star-forming region G240.31+0.07 in the CO J = 3--2, 6--5, and 7--6 lines. We detect a parsec-sized, bipolar, and high velocity outflow in all the lines, which allow us, in combination with the existing CO J = 2--1 data, to perform a multi-line analysis of physical conditions of the outflowing gas. The CO 7--6/6--5, 6--5/3--2, and 6--5/2--1 ratios are found to be nearly constant over a velocity range of $\sim$5--25 km s$^{-1}$ for both blueshifted and redshifted lobes. We carry out rotation diagram and large velocity gradient (LVG) calculations of the four lines, and find that the outflow is approximately isothermal with a gas temperature of $\sim$50 K, and that the the CO column density clearly decreases with the outflow velocity. If the CO abundance and the velocity gradient do not vary much, the decreasing CO column density indicates a decline in the outflow gas density with velocity. By comparing with theoretical models of outflow driving mechanisms, our observations and calculations suggest that the massive outflow in G240.31+0.07 is being driven by a wide-angle wind and further support a disk mediated accretion at play for the formation of the central high-mass star.