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

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