From Interfilamentary Gas to Filaments and Hubs: Gas Flows in the Monoceros R2 Hub–Filament System

Hub–filament systems (HFSs) play an important role in the formation of massive stars and star clusters. Although the velocity structures along dense filaments have been studied, the gas kinematics in the low-density interfilament regions has not been investigated. We use 13 CO ( J = 1–0) and C 18 O ( J = 1–0) observations obtained with the Nobeyama 45 m telescope to study the gas dynamics toward the Monoceros R2 (Mon R2) HFS. From the 13 CO and C 18 O data, tracing low- and high-density gas, respectively, we identify velocity-coherent structures and divide them into Fs and IFs. We estimate the velocity gradients (Δ v ) and mass accretion rates ( M ̇ ) along (∥) and across (⊥) the filaments (F) and interfilaments (IFs). The mean ratio of M ̇ ∥ to M ̇ ⊥ in Fs is 6.8, while that in IFs is 1.5. These results show that the overall gas within both Fs and IFs flows directly into the hub and the gas flows faster along the Fs than the IFs. In addition, we find that at least 30% of the gas mass in the IFs may flow toward the Fs, replenishing the latter with new matter. Our study reveals the importance of considering the total gas-mass reservoir, both low- and high-density, infalling into the hub and promoting the formation of massive stars, which are preferentially located in the hub of Mon R2.