ATOMS-QUARKS survey: Inflow and infall in massive protocluster G318.049+00.086: Evidence of competitive accretion

Technical Summary: ATOMS-QUARKS survey: Inflow and infall in massive protocluster G318.049+00.086

Problem Statement
The formation mechanism of massive stars ($\ge 8 M_\odot$) remains a subject of debate, primarily due to their rarity, rapid evolution, and deeply embedded clustered environments. Two primary theoretical frameworks compete: the turbulent core model, which posits that massive stars form from the collapse of isolated, massive prestellar cores, and clump-fed models (including competitive accretion, inertial-inflow, and global hierarchical collapse), which suggest massive stars grow by accreting gas from large-scale reservoirs via converging flows. While observational evidence for large-scale inflows along filaments has emerged, simultaneous multi-scale analysis of gas dynamics—from parsec-scale filaments down to individual core infall—is required to distinguish between these scenarios. This study addresses the gas kinematics of the massive protocluster G318.049+00.086 to investigate whether its dynamics support a competitive accretion scenario.

Methodology
The authors conducted a multi-scale gas kinematic study of G318.049+00.086 (distance $\approx 2.9$ kpc) utilizing data from three ALMA surveys:

1. ATOMS (Band 3): Used for $H^{13}CO^+$ (1–0), CCH (1–0), and CS (2–1) lines to trace global dynamics and derive column densities.
2. QUARKS (Band 6): Provided high-resolution ($\sim 0.3''$) 1.3 mm dust continuum maps to identify filamentary structures and dense cores.
3. ASSEMBLE (Band 6): Provided high-density tracers ($H^{13}CN$ (4–3), $CH_3OH$) to determine systemic velocities ($V_{lsr}$) and confirm optical depth effects.

The analysis involved:

• Filament Identification: Applying the FilFinder algorithm to both $H^{13}CO^+$ moment-0 maps and 1.3 mm continuum maps to identify the hub-filament system (HFS).
• Column Density Mapping: Deriving $N(H_2)$ maps from $H^{13}CO^+$ emission under Local Thermodynamic Equilibrium (LTE) assumptions, assuming an abundance ratio of $10^{-11}$.
• Kinematic Analysis: Constructing Position-Velocity (PV) diagrams along filament skeletons to measure velocity gradients and mass inflow rates ($\dot{M}_{\parallel}$) using a cylindrical model.
• Core Dynamics: Analyzing spectral profiles of $H^{13}CO^+$ and CCH to identify asymmetric line shapes (blue vs. red profiles) indicative of infall or expansion. Infall velocities ($V_{in}$) were derived by fitting observed spectra with the Hill5 radiative transfer model.
• Mass Accretion: Calculating mass infall rates ($\dot{M}_{in}$) for individual cores and comparing them with filamentary inflow rates to assess mass transfer efficiency.

Key Contributions and Results

• Hub-Filament System Identification: The study confirms G318.049+00.086 as a hub-filament system where four primary filaments (F1–F4) converge into a dense central hub. The filaments identified in the 1.3 mm continuum align strongly with those traced by $H^{13}CO^+$.
• Filamentary Inflow: PV analysis of filaments F1, F3, and F4 reveals high velocity gradients ranging from 10.9 to 22.1 km s$^{-1}$ pc$^{-1}$. These gradients imply a global mass inflow rate along the filaments of $10^3$ to $5 \times 10^3 M_\odot$ Myr$^{-1}$, sufficient to feed the central hub.
• Core Kinematics and Evolution:
  • Infall Signatures: The majority of protostellar cores (5, 7, 8, 9) exhibit blue-asymmetric line profiles in $H^{13}CO^+$ and CCH, confirming gravitational collapse.
  • Expansion/Redistribution: Several cores, including some prestellar candidates (10, 11, 13) and protostellar cores in the massive regime, show red-asymmetric profiles, indicating gas expansion or mass loss.
  • Infall Rates: Derived mass infall rates for hub cores range from $\sim 20$ to $196 \times 10^{-5} M_\odot$ yr$^{-1}$. Notably, cores with red profiles (e.g., 10, 11) exhibit negative infall rates, suggesting they are losing mass.
• Mass-Radius Relationship: While most cores reside in the low-mass star formation regime, cores 10, 11, and 14 lie in the massive star formation regime ($m(r) > 870 M_\odot (r/\text{pc})^{1.33}$). However, the presence of red profiles in these massive candidates suggests they are currently losing material rather than accreting.
• Hierarchical Mass Accretion: The study establishes a hierarchical flow:
  • Global scale ($R \sim 0.74$ pc): $\dot{M} \sim 10^4 M_\odot$ Myr$^{-1}$.
  • Filament scale ($R \sim 0.3$ pc): $\dot{M} \sim 10^3 M_\odot$ Myr$^{-1}$.
  • Core scale: $\dot{M} \sim 10^2 M_\odot$ Myr$^{-1}$.
    Crucially, cores located along the filaments but outside the hub (e.g., 10, 11) appear to be funneling mass toward the central hub, while hub cores (e.g., 7, 8) show the highest accretion rates.

Significance and Claims
The authors argue that their results support a competitive accretion scenario for massive star formation in G318.049+00.086. The evidence suggests that gas is not solely accreting onto isolated massive cores but is being dynamically funneled from less gravitationally dominant cores (located along filaments) toward the cores situated in the gravitationally favorable central hub.

The paper claims that the high mass inflow rates along the filaments ($> 10^3 M_\odot$ Myr$^{-1}$) can sustain the central hub's growth. Assuming a star formation efficiency (SFE) of 20–30%, the accumulated mass over a timescale of $2 \times 10^4$ years would be sufficient to form massive stars. The observation of red profiles in massive candidates outside the hub, contrasted with blue profiles in the hub, indicates an active mass redistribution process where the central potential well dominates the accretion flow, consistent with the predictions of competitive accretion models.