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Long and skinny molecular filaments running along Galactic spiral arms are known as "bones", since they make up the skeleton of the Milky Way. However, their origin is still an open question. Here, we compare spectral images of HI taken by FAST with archival CO and Herschel dust emission to investigate the conversion from HI to H$_2$ in two typical Galactic bones, CFG028.68-0.28 and CFG047.06+0.26. Sensitive FAST HI images and an improved methodology enabled us to extract HI narrow self-absorption (HINSA) features associated with CO line emission on and off the filaments, revealing the ubiquity of HINSA towards distant clouds for the first time. The derived cold HI abundances, [HI]/[H$_2$], of the two bones range from $\sim$(0.5 to 44.7)$\times10^{-3}$, which reveal different degrees of HI-H$_2$ conversion and are similar to that of nearby, low-mass star forming clouds, Planck Galactic cold clumps and a nearby active high-mass star forming region G176.51+00.20. The HI-H$_2$ conversion has been ongoing for 2.2 to 13.2 Myr in the bones, a timescale comparable to that of massive star formation therein. Therefore, we are witnessing young giant molecular clouds with rapid massive star formation. Our study paves the way of using HINSA to study cloud formation in Galactic bones, and more generally, in distant giant molecular clouds, in the FAST era., Comment: 18 pages, 6 figures, 1 table, accepted to ApJL

The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles (ASSEMBLE) aims to investigate the process of mass assembly and its connection to high-mass star formation theories in protoclusters in a dynamic view. We observed 11 massive (Mclump>1000 Msun), luminous (Lbol>10,000 Lsun), and blue-profile (infall signature) clumps by ALMA with resolution of 2200-5500 au at 350 GHz (870 um) in continuum and line emission. 248 dense cores were identified, including 106 cores showing protostellar signatures and 142 prestellar core candidates. Compared to early-stage infrared dark clouds (IRDCs) by ASHES, the core mass and surface density within the ASSEMBLE clumps exhibited significant increment, suggesting concurrent core accretion during the evolution of the clumps. The maximum mass of prestellar cores was found to be 2 times larger than that in IRDCs, indicating evolved protoclusters have the potential to harbor massive prestellar cores. The mass relation between clumps and their most massive core (MMCs) is observed in ASSEMBLE but not in IRDCs, which is suggested to be regulated by multiscale mass accretion. The mass correlation between the core clusters and their MMCs has a steeper slope compared to that observed in stellar clusters, which can be due to fragmentation of the MMC and stellar multiplicity. We observe a decrease in core separation and an increase in central concentration as protoclusters evolve. We confirm primordial mass segregation in the ASSEMBLE protoclusters, possibly resulting from gravitational concentration and/or gas accretion., Comment: 37 pages, 13 figures, 5 tables; accepted for publication in ApJS