New Insights into the H ii 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 × 0 • .27 around the Galactic H ii 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 × 7 pc and mass ∼2.9 ×10 4 M around the H ii region; its existence is further confirmed by the distribution of molecular ( 12 CO, 13 CO, C 18 O, and NH 3 ) gas at [60, 70] km s −1 . Four subregions are studied toward this shell-like feature, and show a mass range of ∼0.8-10.5 ×10 3 M . 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 H ii 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 −1 pc −1 ) 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.

Unraveling the inner substructure of new candidate hub-filament system in the HII region G25.4NW

“…The color-composite map is also overlaid with the ATLASGAL 870 µm continuum contours, and enables us to visually examine embedded filaments in the direction of G25.4NW or J18354N, which are indicated by solid curves. Based on the observed configuration of the filaments, we find the existence of a "hubfilament" system in G25.4NW (e.g., Myers 2009;Schneider et al 2012;Baug et al 2015;Dewangan et al 2015Dewangan et al , 2018Dewangan et al , 2020.…”

“…Ultimately the global non-isotropic collapse scenario suggests that an infrared-bright massive protostar forms via the inflow from larger scales, and produces an H ii region (in a time of few 10 5 -10 6 yr) through its the stellar UV radiation. Some known sites hosting a hub-filament system are Ophiuchus (Myers 2009), Taurus (Myers 2009), Rosette (Schneider et al 2012), IRDC G14.225−0.506 (Busquet et al 2013), SDC335.579−0.292 (Peretto et al 2013), Sh 2-138 (Baug et al 2015), W42 (Dewangan et al 2015), IRAS 05480+2545 (Dewangan et al 2017b), Sh 2-53 (Baug et al 2018), and Monoceros R2 (Treviño-Morales et al 2019), and G18.88−0.49 (Dewangan et al 2020). More details of the global non-isotropic collapse scenario can be found in Tigé et al (2017); Motte et al (2018) (see also Treviño-Morales et al 2019;Dewangan et al 2020).…”

“…In order to further explore the embedded filaments, we processed the Herschel 160 µm image through an edge detection algorithm (i.e. Difference of Gaussian (DoG); see Gonzalez & Woods 2011;Assirati et al 2014;Dewangan et al 2017bDewangan et al , 2020. In Figure 2b, we display a two colorcomposite map made using the Herschel 160 µm image and the DoG processed 160 µm image.…”

Sh 2-301: a blistered H II region undergoing star formation

“…These massive stars can further create H ii region/s which can trigger formation of the second generation of stars. In literature, there are multiple examples of H ii regions found in the HFSs (Myers 2009;Deharveng et al 2015;Dewangan et al 2017aDewangan et al , 2020a.…”

“…Dale et al (2015) have discussed other star formation processes like cloud-cloud collision, filamentary interactions etc. In recent years many authors have pointed out the active role of filamentary structures and their subsequent interaction in star formation (Schneider et al 2012;Dewangan et al 2017bDewangan et al , 2020a. Filamentary structures are often seen harboring young stellar clusters and massive starforming clumps, yet their precise role in star formation is not very well understood.…”