Quiescent cores and the efficiency of turbulence-accelerated, magnetically regulated star formation

The efficiency of star formation, defined as the ratio of the stellar to total (gas and stellar) mass, is observed to vary from a few percent in regions of dispersed star formation to about a third in cluster-forming cores. This difference may reflect the relative importance of magnetic fields and turbulence in controlling star formation. We investigate the interplay between (decaying) turbulence and magnetic fields using numerical simulations, in a sheetlike geometry. The geometry allows for an accurate and expedient treatment of ambipolar diffusion, a key ingredient for star formation. We demonstrate that star formation with an efficiency of a few percent can occur over several gravitational collapse times in moderately magnetically subcritical clouds that are supersonically turbulent. In turbulent clouds that are marginally magnetically supercritical, the star formation efficiency is higher but can still be consistent with the values inferred for nearby embedded clusters. A phenomenological prescription for protostellar outflow is included in our model to stop mass accretion after a star has obtained a given mass and to disperse away the remaining core material. Within a reasonable range of strength, the outflow does not affect the efficiency of star formation much and contributes little to turbulence replenishment in subcritical and marginally supercritical clouds. If not regulated by magnetic fields at all, star formation in a multi-Jeans mass cloud endowed with a strong initial turbulence proceeds rapidly, with the majority of cloud mass converted into stars in a gravitational collapse time in the absence of constant turbulence driving. The efficiency is formally higher than the values inferred for nearby cluster- forming cores, indicating that magnetic fields are dynamically important even for cluster formation. In turbulent, magnetically subcritical clouds, the turbulence accelerates star formation by reducing the time for dense core formation. The dense cores produced in such clouds are predominantly quiescent. For example, 8 out of the 10 cores produced at the middle point of our standard simulation have subsonic internal motions. The cores tend to be moderately supercritical and thus remain magnetically supported to a large extent. They contain a small fraction of the cloud mass and have lifetimes ranging from similar to 1.5 to 10 times their local gravitational collapse time. Some of the cores collapse to form stars, while others disperse away without star formation, in agreement with previous work. All these factors, as well as core-outflow interaction, contribute to the low efficiency of the star formation in these clouds of dispersed star formation.