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