Hierarchical Hub Formation in Mon~R2: Evidence for Cloud-to-core Gas Flows

Abstract

We present a multi-scale kinematic study of the hub-filament system in Mon R2 using JCMT $^{13}$CO(3--2) and ALMA C$^{18}$O(1--0) observations to investigate how molecular gas converges toward the central hub and connects across cloud-to-core scales. The $^{13}$CO data reveal several velocity-coherent filaments with diverse position angles converging toward the central region hosting massive stars and stellar clusters, where distinct velocity components become spatially superposed. Two dominant velocity regimes at $\sim$8.8--10.2 and $\sim$10.4--11.8 km s$^{-1}$ are identified, with their velocity separation decreasing toward the central hub. The position--position--velocity and longitude--velocity maps of the $^{13}$CO emission exhibit a butterfly/X-shaped morphology, consistent with converging motions. A phenomenological rotating-and-infalling envelope model reproduces the longitude-velocity morphology, indicating that rotation and infall may contribute to the observed kinematics, although the model does not uniquely identify the underlying mechanism. At higher resolution, the ALMA C$^{18}$O observations resolve the molecular gas into multiple velocity-coherent filaments spanning $\sim$8--13 km s$^{-1}$, with distinct kinematic domains at $\sim$9--10.5, 10.5--11.5, and 12--13 km s$^{-1}$ converging toward the hub. The ALMA and JCMT position-velocity diagrams show consistent velocity structures across their respective spatial scales, with the ALMA-resolved features embedded within the broader molecular-gas velocity field traced by JCMT, demonstrating a multi-scale kinematic connection. These results support a hierarchical picture in which converging flows and filamentary accretion contribute to hub growth and ongoing star formation in Mon R2, while other dynamical processes may also contribute to the observed kinematics.