Mon R2: A Hub-Filament System with an Infrared Bubble at the Hub center

Abstract: A multi-wavelength, multi-scale study of the Mon R2 hub-filament system (HFS) reveals a spiral structure, with the central hub containing more mass than its filaments. ALMA C$^{18}$O(1-0) emission reveals several accreting filaments connected to a molecular ring (size $\sim$0.18 pc $\times$ 0.26 pc). The molecular ring surrounds the infrared (IR) ring (size $\sim$0.12 pc $\times$ 0.16 pc), which is not usually observed. The IR ring encircles IR dark regions and a population of embedded near-IR sources, including the massive stars IRS 1 and IRS 2. ALMA HNC(3-2) line data reveal a mirrored B-shaped feature (extent $\sim$19000 AU $\times$ 39000 AU) toward the eastern part of the molecular ring, suggesting expansion at $\sim$2.25 km s$^{-1}$. Distinct HNC sub-structures in both redshifted and blueshifted velocity components are investigated toward the B-shaped feature. The presence of these braid-like substructures in each velocity component strongly suggests instability in photon-dominated regions. A dusty shell-like feature (extent $\sim$0.04 pc $\times$ 0.07 pc; mass $\sim$7 M$_{\odot}$) hosting IRS 1 is identified in the ALMA 1.14 mm continuum map, centered toward the base of the B-shaped feature. The IR and dense molecular rings are likely shaped by feedback from massive stars, driven by high pressure values between 10$^{-8}$-10$^{-10}$ dynes cm$^{-2}$, observed within a 1 pc range of the B0 ZAMS star powering the ultracompact HII region. Overall, these outcomes support that the Mon R2 HFS transitioned from IR-quiet to IR-bright, driven by the interaction between gas accretion and feedback from massive stars.