From inter-filamentary gas to filaments and hubs: gas flows in the Mon R2 hub-filament system

Abstract

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 inter-filament 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 towards 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 filaments (Fs) and inter-filamentary regions (IFs). We estimate velocity gradients ($Δv$) and mass accretion rates ($\dot{M}$) along ($\parallel$) and across ($\perp$) the Fs and IFs. The mean ratio of $\dot{M}_\parallel$ to $\dot{M}_\perp$ 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 found that at least 30\% of the gas mass in the IFs may flow towards 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.