ON THE STAR FORMATION EFFICIENCY OF TURBULENT MAGNETIZED CLOUDS

Abstract

We study the star formation efficiency (SFE) in simulations and observations\nof turbulent, magnetized, molecular clouds. We find that the probability\ndensity functions (PDFs) of the density and the column density in our\nsimulations with solenoidal, mixed, and compressive forcing of the turbulence,\nsonic Mach numbers of 3-50, and magnetic fields in the super- to the\ntrans-Alfvenic regime, all develop power-law tails of flattening slope with\nincreasing SFE. The high-density tails of the PDFs are consistent with\nequivalent radial density profiles, rho ~ r^(-kappa) with kappa ~ 1.5-2.5, in\nagreement with observations. Studying velocity-size scalings, we find that all\nthe simulations are consistent with the observed v ~ l^(1/2) scaling of\nsupersonic turbulence, and seem to approach Kolmogorov turbulence with v ~\nl^(1/3) below the sonic scale. The velocity-size scaling is, however, largely\nindependent of the SFE. In contrast, the density-size and column density-size\nscalings are highly sensitive to star formation. We find that the power-law\nslope alpha of the density power spectrum, P(rho,k) ~ k^alpha, or equivalently\nthe Delta-variance spectrum of column density, DV(Sigma,l) ~ l^(-alpha),\nswitches sign from alpha < 0 for SFE ~ 0 to alpha > 0 when star formation\nproceeds (SFE > 0). We provide a relation to compute the SFE from a measurement\nof alpha. Studying the literature, we find values ranging from alpha = -1.6 to\n+1.6 in observations covering scales from the large-scale atomic medium, over\ncold molecular clouds, down to dense star-forming cores. From those alpha\nvalues, we infer SFEs and find good agreement with independent measurements\nbased on young stellar object (YSO) counts, where available. Our SFE-alpha\nrelation provides an independent estimate of the SFE based on the column\ndensity map of a cloud alone, without requiring a priori knowledge of\nstar-formation activity or YSO counts.\n