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

TL;DR

This study investigates gas flows in the Mon R2 hub-filament system using molecular line observations, revealing that gas flows faster along filaments and that inter-filament gas replenishes the filaments, fueling star formation.

Contribution

It provides the first detailed analysis of low-density inter-filament gas dynamics and its role in feeding filaments and the hub in a massive star-forming region.

Findings

Gas flows are faster along filaments than across them.

At least 30% of inter-filament gas flows into filaments.

Total gas reservoir, including low-density gas, feeds the hub for star formation.

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 ($\Delta v$) 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.