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

A multiwavelength, multiscale study of the Mon R2 hub-filament system (HFS) reveals a spiral structure, with the central hub containing more mass than its filaments. C¹⁸O(1-0) emission detected by the Atacama Large Millimeter/submillimeter Array (ALMA) reveals several accreting filaments connected to a molecular ring (size ∼0.18 pc × 0.26 pc). The molecular ring surrounds the infrared (IR) ring (size ~0.12 pc × 0.16 pc), which is not usually observed. The IR ring encircles IR-quiet 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 ∼19,000 au × 39,000 au) toward the eastern part of the molecular ring, suggesting expansion at ~2.25 km s⁻¹. Distinct HNC substructures 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 ~0.04 pc × 0.07 pc; mass ~7 M_⊙) 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 pressures of between 10⁻⁸ and 10⁻¹⁰ dyn cm⁻², observed within a 1 pc range of the B0 zero-age main-sequence star powering the ultracompact H ii region. Overall, these outcomes support the idea that the Mon R2 HFS transitioned from IR-quiet to IR-bright, driven by the interaction between gas accretion and feedback from massive stars.