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 ($\Delta v$) 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.

This paper was accepted for publication in ApJL on 16 May 2026

References in corpus (13)

• Unifying low and high mass star formation through density amplified hubs of filaments
• Filamentary Accretion Flows in the Infrared Dark Cloud G14.225-0.506 Revealed by ALMA
• High abundance ratio of ${}^{13}$CO to C${}^{18}$O toward photon-dominated regions in the Orion-A giant molecular cloud
• Filament coalescence and hub structure in MonR2: Implications to massive star and cluster formation
• Direct observational evidence of the multi-scale, dynamical mass accretion toward a high-mass star forming hub-filament system
• Far-infrared observations of a massive cluster forming in the Monoceros R2 filament hub
• The JCMT BISTRO Survey: A Spiral Magnetic Field in a Hub-filament Structure, Monoceros R2
• DR 21 South Filament: a Parsec-sized Dense Gas Accretion Flow onto the DR 21 Massive Young Cluster
• TRAO Survey of the nearby filamentary molecular clouds, the universal nursery of stars (TRAO FUNS). II. Filaments and Dense cores in IC 5146
• TRAO Survey of Nearby Filamentary Molecular clouds, the Universal Nursery of Stars (TRAO FUNS) I. Dynamics and Chemistry of L1478 in the California Molecular Cloud
• An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass Protostellar Systems in the Large Magellanic Cloud
• Kinematics and star formation of hub-filament systems in W49A
• From theory to observation: understanding filamentary flows in high-mass star-forming clusters