New insights in the HII region G18.88-0.49: hub-filament system and accreting filaments

Abstract: We present an analysis of multi-wavelength observations of an area of 0.27 deg x 0.27 deg around the Galactic HII region G18.88-0.49, which is powered by an O-type star (age ~10^5 years). The Herschel column density map reveals a shell-like feature of extension ~12 pc x 7 pc and mass ~2.9 x 10^4 Msun around the HII region; its existence is further confirmed by the distribution of molecular (12CO, 13CO, C18O, and NH3) gas at [60, 70] km/s. Four subregions are studied toward this shell-like feature, and show a mass range of ~0.8-10.5 x 10^3 Msun. These subregions associated with dense gas are dominated by non-thermal pressure and supersonic non-thermal motions. The shell-like feature is associated with the HII region, Class I protostars, and a massive protostar candidate, illustrating the ongoing early phases of star formation (including massive stars). The massive protostar is found toward the position of the 6.7 GHz methanol maser, and is associated with outflow activity. Five parsec-scale filaments are identified in the column density and molecular maps, and appear to be radially directed to the dense parts of the shell-like feature. This configuration is referred to as a "hub-filament" system. Significant velocity gradients (0.8-1.8 km/s/pc) are observed along each filament, suggesting that the molecular gas flows towards the central hub along the filaments. Overall, our observational findings favor a global non-isotropic collapse scenario as discussed in Motte et al. (2018), which can explain the observed morphology and star formation in and around G18.88-0.49.