Jonathan C. Tan, Shuo Kong, Michael J. Butler, Paola Caselli, Francesco Fontani
How do stars that are more massive than the Sun form, and thus how is the stellar initial mass function (IMF) established? Such intermediate and high-mass stars may be born from relatively massive pre-stellar gas cores, which are more massive than the thermal Jeans mass. The Turbulent Core Accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Their internal pressure is provided by a combination of turbulence and magnetic fields. On the other hand, the Competitive Accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate and high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (~100 Msun) clumps in Infrared Dark Clouds (IRDCs). Fontani et al. found high deuteration fractions of N2H+ in these objects, which are consistent with them being starless. Here we present ALMA Cycle 0 observations of these 4 clumps that probe the N2D+(3-2) line at 2.3" resolution. We find 6 N2D+ cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of virialized, magnetized (with Alfven Mach number m_A ~ 1) and self-gravitating cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ~60 Msun, our results suggest that moderately enhanced magnetic fields (so that m_A ~ 0.3) may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF.
View original:
http://arxiv.org/abs/1303.4343
No comments:
Post a Comment