Expanding CO Shells in the Orion A Molecular Cloud
  We present the discovery of expanding spherical shells around low to
intermediate-mass young stars in the Orion A giant molecular cloud using
observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio
Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and
expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is
comparable to protostellar outflows in the region. Together, shells and
outflows inject enough energy and momentum to maintain the cloud turbulence.
The mass-loss rates required to power the observed shells are two to three
orders of magnitude higher than predicted for line-driven stellar winds from
intermediate-mass stars. This discrepancy may be resolved by invoking
accretion-driven wind variability. We describe in detail several shells in this
paper and present the full sample in the online journal.
