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  1. Context.Traditionally, supersonic turbulence is considered to be one of the most likely mechanisms slowing the gravitational collapse in dense clumps, thereby enabling the formation of massive stars. However, several recent studies have raised differing points of view based on observations carried out with sufficiently high spatial and spectral resolution. These studies call for a re-evaluation of the role turbulence plays in massive star-forming regions.

    Aims.Our aim is to study the gas properties, especially the turbulence, in a sample of massive star-forming regions with sufficient spatial and spectral resolution, which can both resolve the core fragmentation and the thermal line width.

    Methods.We observed NH3metastable lines with the Very Large Array (VLA) to assess the intrinsic turbulence.

    Results.Analysis of the turbulence distribution histogram for 32 identified NH3cores reveals the presence of three distinct components. Furthermore, our results suggest that (1) sub- and transonic turbulence is a prevalent (21 of 32) feature of massive star-forming regions and those cold regions are at early evolutionary stage. This investigation indicates that turbulence alone is insufficient to provide the necessary internal pressure required for massive star formation, necessitating further exploration of alternative candidates; and (2) studies of seven multi-core systems indicate that the cores within each system mainly share similar gas properties and masses. However, two of the systems are characterized by the presence of exceptionally cold and dense cores that are situated at the spatial center of each system. Our findings support the hub-filament model as an explanation for this observed distribution.

     
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  2. Abstract

    At the core‐mantle boundary, most observed ultralow velocity zones (ULVZs) cluster along the edges of the large low shear velocity provinces (LLSVPs) and provide key information on the composition, dynamics, and evolution of the lower mantle. However, their detailed structure near slab‐like structures beneath the mid‐Pacific remains particularly challenging because of the lack of station coverage. While most studies of ULVZs concentrate on SKS‐complexity, here we report on the multipathing of ScS, which expands the sampling for ULVZs. We find the strongest multipathing along a ULVZ patch located just south of Hawaii and the far northeastern edge of the LLSVP, in a zone ~200 km in width and extending 600 km southward. The anomalous ScS travel times and distortedSdiffwaveforms further reveal patches interrupted by observed enhanced D″ indicative of slab‐debris influence on the complexity of the northeastern boundary of the mid‐Pacific LLSVP.

     
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