Unifying low- and high-mass star formation through density-amplified hubs of filaments

Abstract

Context. Star formation takes place in giant molecular clouds, resulting in mass-segregated young stellar clusters composed of Sun-like stars, brown dwarfs, and massive O-type(50–100 M⊙) stars. Aims. We aim to identify candidate hub-filament systems (HFSs) in the Milky Way and examine their role in the formation of the highest mass stars and star clusters. Methods. The Herschel survey HiGAL has catalogued about 105 clumps. Of these, approximately 35 000 targets are detected at the 3σ level in a minimum of four bands. Using the DisPerSE algorithm we detect filamentary skeletons on 10′ × 10′ cut-outs of the SPIRE 250 μm images (18′′ beam width) of the targets. Any filament with a total length of at least 55′′ (3 × 18′′) and at least 18′′ inside the clump was considered to form a junction at the clump. A hub is defined as a junction of three or more filaments. Column density maps were masked by the filament skeletons and averaged for HFS and non-HFS samples to compute the radial profile along the filaments into the clumps. Results. Approximately 3700 (11%) are candidate HFSs, of which about 2150 (60%) are pre-stellar and 1400 (40%) are proto-stellar. The filaments constituting the HFSs have a mean length of ~10–20 pc, a mass of ~5 × 104 M⊙, and line masses (M∕L) of ~2 × 103 M⊙ pc−1. All clumps with L > 104 L⊙ and L > 105 L⊙ at distances within 2 and 5 kpc respectively are located in the hubs of HFSs. The column densities of hubs are found to be enhanced by a factor of approximately two (pre-stellar sources) up to about ten (proto-stellar sources). Conclusions. All high-mass stars preferentially form in the density-enhanced hubs of HFSs. This amplification can drive the observed longitudinal flows along filaments providing further mass accretion. Radiation pressure and feedback can escape into the inter-filamentary voids. We propose a “filaments to clusters” unified paradigm for star formation, with the following salient features: (a) low-intermediate-mass stars form slowly (106 yr) in the filaments and massive stars form quickly (105 yr) in the hub, (b) the initial mass function is the sum of stars continuously created in the HFS with all massive stars formed in the hub, (c) feedback dissipation and mass segregation arise naturally due to HFS properties, and explain the (d) age spreads within bound clusters and the formation of isolated OB associations.

Coalescence of Molecular Filaments

“…The hub itself is found to be a complex network of intertwined, very dense, short filaments and coincide with the dense cluster of young stars in the embedded cluster. This view of the hub as a close network of dense filaments stands in contrast to currently pervading ideas that hubs are similar to massive clumps 3 . Even though matter can be transported and fed to the stars inside the hub through longitudinal flows, in the new view, whether a star will benefit from it and gain mass depends on whether the star is embedded inside one of the dense filaments within the hub network.…”

Clustered star formation at early evolutionary stages

“…This is in agreement with the proposed scenario by Kumar et al (2020) that low-mass star formation occurs in filaments, while high-mass star formation occurs in hubs which take longer to assemble. Thus, high-mass protostars are observed to be surrounded by more evolved low-mass protostars.…”

“…4.2.1). This also supports the theory that star formation occurs at the intersection of filaments/gas hubs, and that the growing massive cores could be fed by fresh matter from/via these filaments (e.g., Myers 2009;Peretto et al 2013;Tigé et al 2017;Kumar et al 2020). However, it is difficult from an observational point of view to observe this possible gas accretion from large to small scales.…”

“…In some regions, there is a single dominant massive core (e.g., Beuther et al 2018;Maud et al 2019), while in other regions a large number of cores are found (e.g., Palau et al 2014Palau et al , 2015Beuther et al 2018Beuther et al , 2021. In addition, it has been suggested that HMSF could occur at hubs of filamentary structures (e.g., Myers 2009;Galván-Madrid et al 2010;Schneider et al 2012;Galván-Madrid et al 2013;Tigé et al 2017;Kumar et al 2020). It is not fully understood yet what drives the accretion of the surrounding gas and dust from cloud/clump scales onto the cores (e.g., Smith et al 2009).…”

Dust polarized emission observations of NGC 6334

“…2) The "hub-dominated" group, composed by several starforming (mostly high-density) filaments merging into a highdensity "hub" (e.g., Myers 2009;Peretto et al 2013Peretto et al , 2014Williams et al 2018). These hubs have also been identified to host young star-clusters, suggesting the important role of hubfilament configurations in star-cluster formation (e.g., Schneider et al 2012;Kumar et al 2020).…”