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We present a statistical study of a sample of 17 hub-filament-system (HFS) clouds of high-mass star formation using high-angular resolution (∼1–2″) ALMA 1.3  mm and 3  mm continuum data. The sample includes 8 infrared (IR)-dark and 9 IR-bright types, which correspond to an evolutionary sequence from the IR-dark to IR-bright stage. The central massive clumps and their associated most massive cores are observed to follow a trend of increasing mass (M) and mass surface density (Σ) with evolution from IR-dark to IR-bright stage. In addition, a mass-segregated cluster of young stellar objects (YSOs) are revealed in both IR-dark and IR-bright HFSs with massive YSOs located in the hub and the population of low-mass YSOs distributed over larger areas. Moreover, outflow feedback in all HFSs are found to escape preferentially through the inter-filamentary diffuse cavities, suggesting that outflows would render a limited effect on the disruption of the HFSs and ongoing high-mass star formation therein. From the above observations, we suggest that high-mass star formation in the HFSs can be described by a multi-scale mass accretion/transfer scenario, from hub-composing filaments through clumps down to cores, that can naturally lead to a mass-segregated cluster of stars.

We present a statistical study on the orientation of outflows with respect to large-scale filaments and the magnetic fields. Although filaments are widely observed toward Galactic star-forming regions, the exact role of filaments in star formation is unclear. Studies toward low-mass star-forming regions revealed both preferred and random orientation of outflows respective to the filament long-axes, while outflows in massive star-forming regions mostly oriented perpendicular to the host filaments, and parallel to the magnetic fields at similar physical scales. Here, we explore outflows in a sample of 11 protoclusters in H ii regions, a more evolved stage compared to IRDCs, using ALMA CO (3-2) line observations. We identify a total of 105 outflow lobes in these protoclusters. Among the 11 targets, 7 are embedded within parsec-scale filamentary structures detected in 13 CO line and 870 µm continuum emissions. The angles between outflow axes and corresponding filaments (γ Fil ) do not show any hint of preferred orientations (i.e., orthogonal or parallel as inferred in numerical models) with respect to the position angle of the filaments. Identified outflow lobes are also not correlated with the magnetic fields and Galactic plane position angles. Outflows associated with filaments aligned along the large-scale magnetic fields are also randomly orientated. Our study presents the first statistical results of outflow orientation respective to large-scale filaments and magnetic fields in evolved massive star-forming regions. The random distribution suggests a lack of alignment of outflows with filaments, which may be a result of the evolutionary stage of the clusters.

Hydroxyacetone (CH3COCH2OH) is one of the smallest molecules that contain both hydroxyl and carbonyl group on neighboring carbon atoms. This steric configuration is characteristic of saccharides and determines their biochemical activity. The attempt to search for hydroxyacetone toward the massive star formation region Sagittarius B2(N) was unsuccessful. Here we report the first detection of CH3COCH2OH in the solar-type protostar IRAS 16293–2422 B, using the Atacama Large Millimeter Array science verification data at Band 4. In a total of 11 unblended transitions of CH3COCH2OH with upper level energies ranging from 86 to 246 K are identified. From our local thermodynamic equilibrium analysis, we derived that the rotational temperature of CH3COCH2OH is 160±21 K and the column density is (1.2±1.0) ×1016 cm−2, which results in a fractional abundance of 7×10−10 with respect to molecular hydrogen. In this work, we present the identification of CH3COCH2OH in IRAS 16293–2422 B and propose a simple formation mechanism. The unambiguous identification of hydroxyacetone may provide the basis for future study of the origin and evolution of saccharides in the interstellar medium.