Global and Local Infall in the ASHES Sample (GLASHES). III. Physical Parameters of Contracting Motion onto Dense Cores in 70 mu m Dark Massive Clumps

The gist: The study presents infall velocities and mass infall rates derived from blue-asymmetric line profiles toward cores embedded in massive 70 µm-dark clumps, providing the first statistical constraints on core-scale mass accumulation during the earliest stages of high-mass star formation

Introduction to Core Growth Scenarios

The fundamental question addressed is how gas is accumulated from molecular-cloud scales onto individual stars Molecular clouds exhibit a hierarchical structure consisting of clouds, clumps, filaments, and dense cores (e.g., E. A. Bergin & M. Tafalla 2007), where dense cores are generally regarded as the immediate precursors of stars It remains actively debated whether most of the final stellar mass originates from gas initially bound within the natal core (the core-fed scenario; C. F. McKee & J. C. Tan 2003) or from material accreted from the surrounding clump on scales substantially larger than the initial core size (the clump-fed scenario; I. A. Bonnell et al. 2001; E. V´azquez-Semadeni et al. 2019; P. Padoan et al. 2020)

Observational Methodology and Sample Selection

The GLASHES (Global and Local Infall in the ASHES Sample) project utilized ALMA observations to target 31 massive clumps among the ASHES targets, covering approximately 400 cores in total by single-pointing observations The main scope of this paper is to derive the infall velocity and mass infall rate for the best candidates showing blue-asymmetric profiles In the sample selection, 135 cores were analyzed, with 108 cores selected as the best candidates showing blue asymmetric profiles only with a single optically-thin velocity component The Hill5 model was applied to these profiles using an affine-invariant Markov Chain Monte Carlo (MCMC) algorithm implemented in the emcee Python package

Physical Parameters of Contracting Motion

The Hill5 model is an analytic radiative transfer model designed to reproduce the asymmetric, double-peaked line profiles characteristic of molecular emission from collapsing dense cores The five free parameters of the model are defined as τ: the central optical depth of the cloud, vin: the infall velocity, σ: the velocity dispersion, vsys,thick: the systemic velocity of the core in local standard of rest frame, and Tpeak: the peak excitation temperature reached in core interior The fitted infall velocities range from 0.11 to 1.60 km s−1 and mass infall rates range from ∼10−5 to 5 × 10−3 M⊙ yr−1 (median ∼10−4 M⊙ yr−1)

Dependence of Infall Properties on Core Evolution

The study found that both quantities increase systematically from prestellar candidates to warm cores and outflow-associated cores The inferred infall velocities are close to the cores’ free-fall velocities (mean vin/vff ∼ 0.75) and tin = R/vin is comparable to the free-fall time, indicating dynamical fast contraction The mass infall rate follows a power-law relation with core mass, M˙ = 1.3×10−4(M/M⊙)0.83±0.06 M⊙ yr−1 across 0.1 M⊙ ≲ M ≲ 30 M⊙ The power-law index increases from ∼0.65 for prestellar cores to ∼1.10 for outflow-associated cores, likely tracing an evolution in the dominant physical process

Comparison with Other Environments and Proposed Scenario

The GLASHES cores show systematically higher Hill5 infall velocities and mass infall rates than nearby low-mass dense cores, yet remain below the clumpscale inflow rates measured in more evolved high-mass star-forming regions This intermediate position supports a hierarchical, clump-fed picture in which core-scale accretion is continuously replenished by larger-scale clump infall The proposed scenario suggests that the earliest stage involves dense fragments seeding future high-mass stars, which begin to grow immediately through external infall from the surrounding clump with a velocity comparable to the free-fall velocity and at rates of ≳ 10−4 M⊙ yr−1

Current Limitations

Limitations include an evolutionary bias, as the sample is restricted to 70 µm-dark clumps representing early phases of high-mass star formation The inferred infall velocities depend on the adopted model, and current angular resolution is insufficient to directly determine internal density profiles Furthermore, uncertainties associated with geometry and projection effects remain difficult to quantify

Conclusion

The GLASHES cores demonstrate that 70 µm-dark clumps already exhibit organized infall motions whose strength scales systematically with core mass and evolutionary stage, providing new statistical constraints on how mass accretion proceeds in the earliest phases of high-mass star and cluster formation The results offer a benchmark against which future, higher resolution observations and more detailed infall models can be compared

How it works

The Hill5 model is an analytic radiative transfer model designed to reproduce the asymmetric, double-peaked line profiles characteristic of molecular emission from collapsing dense cores It describes a core with a continuous excitation temperature gradient along the line of sight, rising from a cooler foreground value T0 to a warmer peak value Tpeak toward the core interior, superimposed with systematic infall motions The brightness temperature is described by an equation involving optical depth and front/rear sides The five parameters used in the fit are τ: the central optical depth of the cloud, vin: the infall velocity, σ: the velocity dispersion, vsys,thick: the systemic velocity of the core in local standard of rest frame, and Tpeak: the peak excitation temperature reached in core interior

Mass Infall Rate Calculation

The mass infall rate is calculated from the fitted infall velocity assuming a spherically symmetric, gravity-driven infall using M˙ = 4πR2ρ vin The core radius and mean density are taken from 1.3 mm continuum emission The uncertainty in the mass accretion rate (M˙) was evaluated by propagating the errors of the core mass, vin, and R

Evolutionary Trends in Accretion

The power-law index for the mass infall rate scales with core mass as M˙ ∝ M0.83±0.06 across the full sample (0.1–30 M⊙) The power-law index is shallower for prestellar candidates (0.65±0.11), consistent with tidal-lobe accretion, and steeper for outflow-associated cores (1.10 ± 0.26), suggestive of a transition toward a different dominant accretion mode This evolutionary steepening, from ∼2/3 at the prestellar stage to ∼1.1 in the outflow phase, suggests a gradual transition in the dominant accretion mechanism as cores evolve

Contribution of Infall Motions to Non-thermal Linewidth

The consistency with the pilot study suggests that organized infall motions contribute to observed non-thermal linewidths rather than purely tracing turbulent motions This may lead to an overestimation of quantities that depend on the non-thermal velocity dispersion, such as the virial parameter and Mach number if the non-thermal term is assumed to be purely turbulent

Mass Infall Rate Scaling

The observed scaling relation can also be compared with recent attempts to reproduce the evolution of the ASHES core mass function through core growth The derived M˙ = 1.3×10−4(M/1 M⊙)0.83 M⊙ yr−1 for the full sample and M˙ = 9.9 × 10−5(M/1 M⊙)0.65 M⊙ yr−1 for the prestellar core candidates agree with normalization factors from core growth models

Mass Infall Rate vs. Core Properties

The mass infall rate is largely determined by the density or mass, as more massive or denser regions with lower virial parameters are implied to be favorable sites for achieving larger infall rates and for growing more efficiently The observed trend is not simply described by a pure M˙ ∝ α−1vir relation, as the best-fit power-law indices for the prestellar and warm cores are −0.63 and −0.

1. The AI system can accurately estimate core-scale mass accumulation by implementing a power-law relation for mass infall rate, specifically using M˙ = 1.3×10−4 (M/M⊙) 0.83±0.06 M⊙ yr−1 across the observed range, which is crucial for comparing results with models like the one proposed by K. Morii et al. (2026b).

2. The system can differentiate between accretion regimes by analyzing the power-law index of mass infall rate, noting that the power-law index increases from ∼0.65 for prestellar cores to ∼1.10 for outflow-associated cores, likely tracing an evolution in the dominant physical process.

3. The AI system can assess the dynamical significance of observed line widths by correlating Hill5 infall velocity correlates moderately with the non-thermal velocity dispersion and Mach number measured from optically thin tracers, indicating that organized infall contributes to the observed line widths.

4. The system can evaluate the physical state of a core by calculating whether its contraction is dynamically significant, as evidenced by comparing inferred infall velocities are close to the cores’ free-fall velocities (mean vin/vff ∼ 0.75).

5. The AI system can distinguish between core evolutionary stages based on the derived Hill5 parameters, identifying them as outflow cores, warm cores, and prestellar core candidates in this paper by analyzing the distribution of best-fit Tpeak and vin in Figure 2.

6. The system can identify potential contamination from outflows using an automated wing-subtraction procedure described in Appendix A, allowing for a more robust fit by subtracting the broad component that captures the high-velocity wing or tail attributed to outflow contamination.

7. The AI system can quantify the relationship between infall timescale and free-fall time by determining if cores lie within a factor of two of the tin = tff relation, which is used to support a picture where many cores, especially prestellar cores, are undergoing rapid, gravity-driven contraction.

8. The system can characterize core growth efficiency by analyzing the mass infall rate scaling relation, noting that for prestellar and warm cores, the index is 0.65±0.11 and 0.58±0.10, while for outflow cores it steepens to 1.10 ± 0.26.

9. The AI system can compare the derived infall properties against other environments, recognizing that GLASHES cores show systematically higher Hill5 infall velocities and mass infall rates than nearby low-mass dense cores, yet remain below the clumpscale inflow rates measured in more evolved highmass star-forming regions.