The Dynamics and Chemistry of Massive Starless Cores


Paper:	The Dynamics and Chemistry of Massive Starless Cores
Volume:	476, New Trends in Radio Astronomy in the ALMA Era
Page:	123
Authors:	Tan, J. C.; Kong, S.; Butler, M. J.; Caselli, P.; Fontani, F.
Abstract:	How do massive stars form? They may be born from massive pre-stellar gas cores that are much more massive than the Jeans mass. The Turbulent Core Accretion model invokes such cores as being in approximate virial and 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 high-mass stars later forming by competitive Bondi-Hoyle accretion. To test these models, we have identified four prime examples of massive (∼ 100 M_☉) clumps from mid-infrared (MIR) extinction mapping of Infrared Dark Clouds (IRDCs). At ∼16″ resolution, we found high deuteration fractions of N₂H⁺ in these objects, consistent with them being starless. We then observed these 4 clumps with ALMA in Cycle 0 to probe the N₂D⁺(3-2) line at ∼2″ resolution, finding 6 N₂D⁺ cores. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of virialized, magnetized (with Alfvén Mach number m_A ∼ 1), self-gravitating cores that are bounded by the high pressures of their surrounding clumps. However, the most massive core with ∼ 60 M_☉, appears to require moderately enhanced magnetic fields to be in virial equilibrium, implying m_A ≃ 0.3. If confirmed, this suggests magnetic fields play a greater role than turbulence in setting the initial conditions of massive star formation. In this case the timescale for the core to be assembled may be significantly longer than a local dynamical or free-fall time. This is consistent with astrochemical modeling of the deuteration ages of the cores, which indicates a core age similar to the ambipolar diffusion time.