Molecular Cloud Evolution IV: Magnetic Fields, Ambipolar Diffusion, and
  the Star Formation Efficiency
  We investigate the formation and evolution of giant molecular clouds (GMCs)
by the collision of convergent warm neutral medium (WNM) streams in the
interstellar medium, in the presence of magnetic fields and ambipolar diffusion
(AD), focusing on the evolution of the star formation rate (SFR) and efficiency
(SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming
clouds. We find that: 1) Clouds formed by supercritical inflow streams proceed
directly to collapse, while clouds formed by subcritical streams first contract
and then re-expand, oscillating on the scale of tens of Myr. 2) Our suite of
simulations with initial magnetic field strength of 2, 3, and 4 $\mu\G$ show
that only supercritical or marginal critical streams lead to reasonable star
forming rates. 3) The GMC's M2FR is a generally increasing function of time,
whose growth rate depends on the details of how mass is added to the GMC from
the WNM. 4) The M2FR is a highly fluctuating function of position in the
clouds. 5) In our simulations, the SFE approaches stationarity, because mass is
added to the GMC at a similar rate at which it converts mass to stars. In such
an approximately stationary regime, the SFE provides a proxy of the
supercritical mass fraction in the cloud. 6) We observe the occurrence of
buoyancy of the low-M2FR regions within the gravitationally-contracting GMCs,
so that the latter naturally segregate into a high-density, high-M2FR "core"
and a low-density, low-M2FR "envelope", without the intervention of AD.
(Abridged)
