Coalescence of Molecular Filaments
Abstract Star-forming molecular filaments are found to display a spectrum of line-masses (mass per unit length)1. This spectrum is thought to influence key observational parameters of star formation2 including the core and stellar initial mass function1. The exact mechanism producing the wide-range of line-masses is unknown, even though, higher surface densities are often observed at the intersection of filaments in hub-filament systems3. Here we show that cascades of lower density filaments coalescing to form higher density filaments and eventually hubs. By performing a multi-scale decomposition of surface density maps of the MonR2 star-forming region, which displays a spiral-shaped hub-filament system4, the coalescence effect is detected in two consecutive cascading steps (the surface density jumps by an order of magnitude at each step) before merging at the central hub which is found to be a dense network of short high-density filaments (as opposed to its view as a massive clump). The radial density structure of the dense-gas component of the hub-filament system shows a power-law dependence of NH2 ∝ r−2 over the scale of ∼5 pc, a feature previously found only at scales of 0.1 pc in star-forming cores5. It appears that the hub-filament system is mimicking the radial profile of an isothermal sphere, at parsec scales, a feature not known until now. This behavior is not seen for the diffuse cloud (NH2 ∝ r−0.5) which holds nearly equal mass. The filamentary nature of the hub implies that only some (embedded in the filaments), and not all, stellar seeds within the hub can become massive stars.