Present-Day Star Formation: Protostellar Outflows and Clustered Star Formation

Stars form predominantly in clusters inside dense clumps of turbulent, magnetized molecular clouds. The typical size and mass of the cluster-forming clumps are \sim 1 pc and \sim 10^2 - 10^3 M_\odot, respectively. Here, we discuss some recent progress on theoretical and observational studies of clustered star formation in such parsec-scale clumps with emphasis on the role of protostellar outflow feedback. Recent simulations indicate that protostellar outflow feedback can maintain supersonic turbulence in a cluster-forming clump, and the clump can keep a virial equilibrium long after the initial turbulence has decayed away. In the clumps, star formation proceeds relatively slowly; it continues for at least several global free-fall times of the parent dense clump (t_{ff}\sim a few x 10^5 yr). The most massive star in the clump is formed at the bottom of the clump gravitational potential well at later times through the filamentary mass accretion streams that are broken up by the outflows from low-mass cluster members. Observations of molecular outflows in nearby cluster-forming clumps appear to support the outflow-regulated cluster formation model.Comment: 8 pages, 5 figures proceedings of the First Stars IV 2012 Conference held in Kyoto, Japa

Theory of Cluster Formation: Effects of Magnetic Fields

Stars form predominantly in clusters inside dense clumps of molecular clouds that are both turbulent and magnetized. The typical size and mass of the cluster-forming clumps are $\sim 1$ pc and $\sim 10^2 -$ 10$^3$ M$_\odot$, respectively. Here, we discuss some recent progress on numerical simulations of clustered star formation in such parsec-scale dense clumps with emphasis on the role of magnetic fields. The simulations have shown that magnetic fields tend to slow down global gravitational collapse and thus star formation, especially in the presence of protostellar outflow feedback. Even a relatively weak can retard star formation significantly, because the field is amplified by supersonic turbulence to an equipartition strength. However, in such a case, the distorted field component dominates the uniform one. In contrast, if the field is moderately strong, the uniform component remains dominant. Such a difference in the magnetic structure is observed in simulated polarization maps of dust thermal emission. Recent polarization measurements show that the field lines in nearby cluster-forming clumps are spatially well-ordered, indicative of a rather strong field. In such strongly-magnetized clumps, star formation should proceed relatively slowly; it continues for at least several global free-fall times of the parent dense clump ($t_{\rm ff}\sim$ a few $\times 10^5$ yr).Comment: 8 pages, proceedings of Computational Star Formation (IAU 270

Magnetically Regulated Star Formation in Turbulent Clouds

We investigate numerically the combined effects of supersonic turbulence, strong magnetic fields and ambipolar diffusion on cloud evolution leading to star formation. We find that, in clouds that are initially magnetically subcritical, supersonic turbulence can speed up star formation, through enhanced ambipolar diffusion in shocks. The speedup overcomes a major objection to the standard scenario of low-mass star formation involving ambipolar diffusion, since the diffusion time scale at the average density of a molecular cloud is typically longer than the cloud life time. At the same time, the strong magnetic field can prevent the large-scale supersonic turbulence from converting most of the cloud mass into stars in one (short) turbulence crossing time, and thus alleviate the high efficiency problem associated with the turbulence-controlled picture for low-mass star formation. We propose that relatively rapid but inefficient star formation results from supersonic collisions of somewhat subcritical gas in strongly magnetized, turbulent clouds. The salient features of this shock-accelerated, ambipolar diffusion-regulated scenario are demonstrated with numerical experiments.Comment: 10 pages, 3 figures, accepted for publication in ApJ