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