2018ApJ...860..106L

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 ∼5-25 km s^–1 for both blueshifted and redshifted lobes. We carry out rotation diagram and large velocity gradient calculations of the four lines and find that the outflow is approximately isothermal with a gas temperature of ∼50 K and that 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.