Unraveling the Observational Signatures of Cloud-Cloud Collision and Hub-filament Systems in W31

To understand the formation process of massive stars, we present a multiscale and multiwavelength study of the W31 complex hosting two extended H II regions (i.e., G10.30-0.15 (hereafter, W31-N) and G10.15-0.34 (hereafter, W31-S)) powered by a cluster of O-type stars. Several Class I protostars and a total of 49 ATLASGAL 870 μm dust clumps (at d = 3.55 kpc) are found toward the H II regions where some of the clumps are associated with the molecular outflow activity. These results confirm the existence of a single physical system hosting the early phases of star formation. The Herschel 250 μm continuum map shows the presence of a hub-filament system (HFS) toward both W31-N and W31-S. The central hubs harbor H II regions and they are depicted with extended structures (with T _d ~ 25-32 K) in the Herschel temperature map. In the direction of W31-S, an analysis of the NANTEN2 ¹²CO(J = 1-0) and SEDIGISM ¹³CO(J = 2-1) line data supports the presence of two cloud components around 8 and 16 km s^-1, and their connection in velocity space. A spatial complementary distribution between the two cloud components is also investigated toward W31-S, where the signposts of star formation, including massive O-type stars, are concentrated. These findings favor the applicability of cloud-cloud collision (CCC) around ~2 Myr ago in W31-S. Overall, our observational findings support the theoretical scenario of CCC in W31, which explains the formation of massive stars and the existence of HFSs.