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 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. Alternatively, 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 M ) clumps from mid-infrared extinction mapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration fractions of N 2 H + in these objects, which are consistent with them being starless. Here we present ALMA observations of these four clumps that probe the N 2 D + (3-2) line at 2.3 resolution. We find six N 2 D + cores and determine their dynamical state. Their observed velocity dispersions and sizes are broadly consistent with the predictions of the Turbulent Core model of self-gravitating, magnetized (with Alfvén Mach number m A ∼ 1) and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, with masses up to ∼ 60 M , 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.

The earliest phases of high-mass star formation, as seen in NGC 6334 by Herschel-HOBYS

“…infrared dark clouds are much less concentrated and tend to follow a Mass(<r) ∝ r 2 relation (Butler & Tan 2012;Tan et al 2013). Note that the only known starless MDC N40 in Cygnus-X follows a similar relation (see Bontemps et al 2010, Fig.…”

“…This relation suggests these MDCs have a centrally concentrated density distribution of ρ(r) ∝ r −2 or steeper, similar to those of MDCs or hubs on ∼0.1 -1 pc scales (e.g., Beuther et al 2002;Didelon et al 2015) or low-mass protostellar envelopes on 0.01−0.1 pc scales (e.g., Motte & André 2001). By contrast, the starless structures found by submillimeter surveys or within A77, page 13 of 83 Butler & Tan 2012;Tan et al 2013;Duarte-Cabral et al 2014;Wang et al 2014, in blue open circles and crosses), respectively. NGC 6334 protostellar MDCs, displayed in filled red circles, are themselves compared to five protostellar MDCs and their embedded high-mass protostars Bontemps et al 2010;Tan et al 2016, in red open circles and crosses), respectively.…”

“…In the end, MDC #5 is our best candidate starless MDC with its unresolved size, ≤0.08 pc, and high mass, ∼210 M . Interestingly, these characteristics make it at least five times denser and thus more favorable to host a high-mass prestellar core than the starless MDC candidate of Tan et al (2013).…”

The ATLASGAL survey: The sample of young massive cluster progenitors

“…8. We mark two massive cores with M MDC = 60 M ⊙ (C1-S; Tan et al 2013) and 55 M ⊙ (CygX-N63; Bontemps et al 2010). For comparison IRDC clumps (Kainulainen & Tan 2013) and cores are shown (Butler & Tan 2012).…”

Follow-Up Observations Toward Planck Cold Clumps With Ground-Based Radio Telescopes

“…Candidates for massive PSCs have been found in recent observations, e.g. G028 C1-S (Tan et al, 2013). When compared to low-mass prestellar cores, which are thermally supported, these candidates are characterized by a supersonic non-thermal velocity dispersion and their internal pressure is provided by a combination of turbulence and magnetic field (Tan et al, 2013).…”