Dependence of the Star Formation Efficiency on the Parameters of
  Molecular Cloud Formation Simulations
  We investigate the response of the star formation efficiency (SFE) to the
main parameters of simulations of molecular cloud formation by the collision of
warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and
magnetic fields. The parameters we vary are the Mach number of the inflow
velocity of the streams, Msinf, the rms Mach number of the initial background
turbulence in the WNM, and the total mass contained in the colliding gas
streams, Minf. Because the SFE is a function of time, we define two estimators
for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's
evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of
star formation in each simulation (sferel). The latter is close to the "star
formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both
estimators decrease with increasing Minf, although by no more than a factor of
2 as Msinf increases from 1.25 to 3.5. Increasing levels of background
turbulence similarly reduce the SFE, because the turbulence disrupts the
coherence of the colliding streams, fragmenting the cloud, and producing
small-scale clumps scattered through the numerical box, which have low SFEs.
Finally, the SFE is very sensitive to the mass of the inflows, with sferel
decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding
streams increases from ~0.15 to ~1.5. This trend is in partial agreement with
the prediction by Krumholz & McKee (2005), since the latter lies within the
same range as the observed efficiencies, but with a significantly shallower
slope. We conclude that the observed variability of the SFE is a highly
sensitive function of the parameters of the cloud formation process, and may be
the cause of significant scatter in observational determinations.
