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

Abstract

ABSTRACT We present multiscale and multiwavelength data of the Galactic H ii region G25.4−0.14 (hereafter G25.4NW, distance ∼5.7 kpc). The SHARC-II 350-μm continuum map displays a hub-filament configuration containing five parsec-scale filaments and a central compact hub. Through the 5-GHz radio continuum map, four ionized clumps (i.e. Ia–Id) are identified towards the central hub, and are powered by massive OB-stars. The Herschel temperature map depicts the warm dust emission (i.e. T$_{\rm d}\, \sim$23–39 K) towards the hub. High-resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.3-mm continuum map (resolution ∼0.82 × 0.58 arcsec2) reveals three cores (c1–c3; mass ∼80–130 M⊙) towards the ionized clumps Ia, and another one (c4; mass ∼70 M⊙) towards the ionized clump Ib. A compact near-infrared (NIR) emission feature (extent ∼0.2 pc) is investigated towards the ionized clump Ia excited by an O8V-type star, and contains at least three embedded K-band stars. In the direction of the ionized clump Ia, the ALMA map also shows an elongated feature (extent ∼0.2 pc) hosting the cores c1–c3. All these findings together illustrate the existence of a small cluster of massive stars in the central hub. Considering the detection of the hub-filament morphology and the spatial locations of the mm cores, a global non-isotropic collapse (GNIC) scenario appears to be applicable in G25.4NW, which includes the basic ingredients of the global hierarchical collapse and clump-fed accretion models. Overall, the GNIC scenario explains the birth of massive stars in G25.4NW.