Can the removal of molecular cloud envelopes by external feedback affect
  the efficiency of star formation?
  We investigate how star formation efficiency can be significantly decreased
by the removal of a molecular cloud's envelope by feedback from an external
source. Feedback from star formation has difficulties halting the process in
dense gas but can easily remove the less dense and warmer envelopes where star
formation does not occur. However, the envelopes can play an important role
keeping their host clouds bound by deepening the gravitational potential and
providing a constraining pressure boundary. We use numerical simulations to
show that removal of the cloud envelopes results in all cases in a fall in the
star formation efficiency (SFE). At 1.38 free-fall times our 4 pc cloud
simulation experienced a drop in the SFE from 16 to six percent, while our 5 pc
cloud fell from 27 to 16 per cent. At the same time, our 3 pc cloud (the least
bound) fell from an SFE of 5.67 per cent to zero when the envelope was lost.
The star formation efficiency per free-fall time varied from zero to $\approx$
0.25 according to $\alpha$, defined to be the ratio of the kinetic plus thermal
to gravitational energy, and irrespective of the absolute star forming mass
available. Furthermore the fall in SFE associated with the loss of the envelope
is found to even occur at later times. We conclude that the SFE will always
fall should a star forming cloud lose its envelope due to stellar feedback,
with less bound clouds suffering the greatest decrease.
