[Paper Review] ATOMS: ALMA Three-millimeter Observations of Massive Star-forming regions -- V. Hierarchical fragmentation and gas dynamics in IRDC G034.43+00.24

Hongli Liu, Anandmayee Tej+37 more|arXiv (Cornell University)|Nov 3, 2021

Astrophysics and Star Formation Studies75 references31 citations

TL;DR

This study presents ALMA 3 mm observations of the massive protostellar clump MM1 in IRDC G034.43+00.24, revealing hierarchical fragmentation across cloud, clump, and core scales driven by turbulence and dynamical mass inflow. The key finding is a cascade of scale-dependent accretion—evidenced by outflows, virialized cores, and velocity gradients—supporting a dynamical mass inflow/accretion scenario linked to multi-scale fragmentation in high-mass star formation.

ABSTRACT

We present new 3-mm continuum and molecular lines observations from the ATOMS survey towards the massive protostellar clump, MM1, located in the filamentary infrared dark cloud (IRDC), G034.43+00.24 (G34). The lines observed are the tracers of either dense gas (e.g. HCO+/H13CO+ J = 1-0) or outflows (e.g. CS J = 2-1). The most complete picture to date of seven cores in MM1 is revealed by dust continuum emission. These cores are found to be gravitationally bound, with virial parameter, $\alpha_{vir}<2$. At least four outflows are identified in MM1 with a total outflowing mass of $\sim 45 M_\odot$, and a total energy of $\sim 1 imes 10^{47}$ erg, typical of outflows from a B0-type star. Evidence of hierarchical fragmentation, where turbulence dominates over thermal pressure, is observed at both the cloud and the clump scales. This could be linked to the scale-dependent, dynamical mass inflow/accretion on clump and core scales. We therefore suggest that the G34 cloud could be undergoing a dynamical mass inflow/accretion process linked to the multiscale fragmentation, which leads to the sequential formation of fragments of the initial cloud, clumps, and ultimately dense cores, the sites of star formation.

Motivation & Objective

• Investigate the hierarchical fragmentation and gas dynamics in the massive protostellar clump MM1 within IRDC G034.43+00.24.
• Address the open question of whether high-mass star formation proceeds via core-accretion or competitive-accretion by analyzing fragmentation scales and mass inflow rates.
• Determine the role of multi-scale mass inflow/accretion in driving fragmentation and protostellar growth in massive clumps.
• Characterize the kinematics and outflow properties to assess the role of turbulence and feedback in shaping the fragmentation process.
• Establish a link between observed fragmentation patterns and theoretical models of high-mass star formation, particularly global hierarchical collapse and inertial-inflow models.

Proposed method

• Conducted high-resolution ALMA 3 mm continuum and molecular line observations (HCO+/H13CO+ J=1–0, CS J=2–1) toward MM1 and MM2 in IRDC G034.43+00.24.
• Used dust continuum emission to identify and characterize 9 dense cores (7 in MM1, 2 in MM2), measuring their physical properties such as mass, size, and density.
• Applied virial parameter analysis (αvir < 2) to assess gravitational boundness of the cores.
• Identified outflows via CS J=2–1 emission and estimated total outflowing mass (~45 M⊙) and energy (~1×10⁴⁷ ergs), consistent with a B0-type star.
• Mapped large-scale velocity gradients in H13CO+ (1–0) to infer rotational or inflowing motions on scales ~0.18 pc, suggesting a rotating structure rather than a small disk.
• Quantified mass inflow rates (Ṁinf ~ 10⁻⁴ M⊙ yr⁻¹) and core-scale accretion rates (Ṁacc ~ 10⁻⁵ M⊙ yr⁻¹) using momentum conservation between infall and outflows.

Experimental results

Research questions

• RQ1 What is the degree and scale of hierarchical fragmentation in the massive clump MM1 within IRDC G034.43+00.24?
• RQ2 How do the kinematics and dynamics—particularly velocity gradients and outflows—support or challenge existing models of high-mass star formation?
• RQ3 What is the role of multi-scale mass inflow/accretion in driving fragmentation and protostellar growth in this system?
• RQ4 How do the observed outflow properties and accretion rates compare with theoretical expectations for a B0-type protostar?
• RQ5 Can the observed fragmentation and inflow patterns be explained by a dynamical, scale-dependent mass inflow/accretion scenario?

Key findings

• Nine dust cores were identified, with seven in MM1 and two in MM2; the seven cores in MM1 represent the most complete core population yet observed for this clump.
• All nine cores are gravitationally bound, with virial parameters αvir < 2, indicating they are likely to collapse and form stars.
• The total outflowing mass in MM1 is ∼45 M⊙ and the total energy is ∼1×10⁴⁷ ergs, consistent with outflows powered by a B0-type protostar.
• A large-scale, butterfly-shaped velocity gradient in H13CO+ (1–0) emission, spanning ∼0.18 pc, suggests the presence of a rotating structure rather than a small-scale disk.
• Intermediate-scale velocity gradients of 3–8 km s⁻¹ pc⁻¹ in both MM1 and MM2 indicate gas inflow onto core clusters, with a mass inflow rate of order 10⁻⁴ M⊙ yr⁻¹.
• Core-scale accretion rates (Ṁacc ~ 10⁻⁵ M⊙ yr⁻¹) are lower than clump-scale inflow rates (Ṁinf ~ 10⁻⁴ M⊙ yr⁻¹), indicating a cascade of mass inflow from larger to smaller scales, supporting a dynamical, hierarchical accretion process.