  We present the results of a numerical simulation in which star formation
proceeds from an initially unbound molecular cloud core. The turbulent motions,
which dominate the dynamics, dissipate in shocks leaving a quiescent region
which becomes gravitationally bound and collapses to form a small multiple
system. Meanwhile, the bulk of the cloud escapes due to its initial supersonic
velocities. In this simulation, the process naturally results in a star
formation efficiency of 50%. The mass involved in star formation depends on the
gas fraction that dissipates sufficient kinetic energy in shocks. Thus, clouds
with larger turbulent motions will result in lower star formation efficiencies.
This implies that globally unbound, and therefore transient giant molecular
clouds (GMCs), can account for the low efficiency of star formation observed in
our Galaxy without recourse to magnetic fields or feedback processes.
Observations of the dynamic stability in molecular regions suggest that GMCs
may not be self-gravitating, supporting the ideas presented in this letter.
