THE DYNAMICS OF MASSIVE STARLESS CORES WITH ALMA

Abstract

How do stars that are more massive than the Sun form, and thus how is the\nstellar initial mass function (IMF) established? Such intermediate- and\nhigh-mass stars may be born from relatively massive pre-stellar gas cores,\nwhich are more massive than the thermal Jeans mass. The Turbulent Core\nAccretion model invokes such cores as being in approximate virial equilibrium\nand in approximate pressure equilibrium with their surrounding clump medium.\nTheir internal pressure is provided by a combination of turbulence and magnetic\nfields. Alternatively, the Competitive Accretion model requires strongly\nsub-virial initial conditions that then lead to extensive fragmentation to the\nthermal Jeans scale, with intermediate- and high-mass stars later forming by\ncompetitive Bondi-Hoyle accretion. To test these models, we have identified\nfour prime examples of massive (~100Msun) clumps from mid-infrared extinction\nmapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration\nfractions of N2H+ in these objects, which are consistent with them being\nstarless. Here we present ALMA observations of these four clumps that probe the\nN2D+(3-2) line at 2.3" resolution. We find six N2D+ cores and determine their\ndynamical state. Their observed velocity dispersions and sizes are broadly\nconsistent with the predictions of the Turbulent Core model of\nself-gravitating, magnetized (with Alfven Mach number m_A~1) and virialized\ncores that are bounded by the high pressures of their surrounding clumps.\nHowever, in the most massive cores, with masses up to ~60Msun, our results\nsuggest that moderately enhanced magnetic fields (so that m_A~0.3) may be\nneeded for the structures to be in virial and pressure equilibrium.\nMagnetically regulated core formation may thus be important in controlling the\nformation of massive cores, inhibiting their fragmentation, and thus helping to\nestablish the stellar IMF.\n