Filamentary Accretion Flows in the Infrared Dark Cloud G14.225-0.506 Revealed by ALMA

Abstract

Filaments are ubiquitous structures in molecular clouds and play an important role in the mass assembly of stars. We present results of dynamical stability analyses for filaments in the infrared dark cloud G14.225$-$0.506, where a delayed onset of massive star formation was reported in the two hubs at the convergence of multiple filaments of parsec length. Full-synthesis imaging is performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to map the $\mathrm{N_2H^+} \; (1-0)$ emission in two hub-filament systems with a spatial resolution of $\sim 0.034 \; \mathrm{pc}$. Kinematics are derived from sophisticated spectral fitting algorithm that accounts for line blending, large optical depth, and multiple velocity components. We identify five velocity coherent filaments and derive their velocity gradients with principal component analysis. The mass accretion rates along the filaments are up to $10^{-4} \; \mathrm{M_\odot \, \mathrm{yr^{-1}}}$ and are significant enough to affect the hub dynamics within one free-fall time ($\sim 10^5 \; \mathrm{yr}$). The $\mathrm{N_2H^+}$ filaments are in equilibrium with virial parameter $\alpha_\mathrm{vir} \sim 1.2$. We compare $\alpha_\mathrm{vir}$ measured in the $\mathrm{N_2H^+}$ filaments, $\mathrm{NH_3}$ filaments, $870 \; \mu\mathrm{m}$ dense clumps, and $3 \; \mathrm{mm}$ dense cores. The decreasing trend in $\alpha_\mathrm{vir}$ with decreasing spatial scales persists, suggesting an increasingly important role of gravity at small scales. Meanwhile, $\alpha_\mathrm{vir}$ also decreases with decreasing non-thermal motions. In combination with the absence of high-mass protostars and massive cores, our results are consistent with the global hierarchical collapse scenario.