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Register a new userUnraveling the inner substructure of new candidate hub-filament system in the HII region G25.4NW
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Added by
Lokesh Kumar Dewangan
Publication date
2021
fields
Physics
and research's language is
English
Authors
L. K. Dewangan
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We present multi-scale and multi-wavelength data of the Galactic HII region G25.4-0.14 (hereafter G25.4NW, distance ~5.7 kpc). The SHARC-II 350 micron 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 toward the central hub, and are powered by massive OB-stars. The Herschel temperature map depicts the warm dust emission (i.e., Td ~23-39 K) toward the hub. High resolution Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm continuum map (resolution ~0.82 X 0.58) reveals three cores (c1-c3; mass ~80-130 Msun) toward the ionized clumps Ia, and another one (c4; mass ~70 Msun) toward the ionized clump Ib. A compact near-infrared (NIR) emission feature (extent ~0.2 pc) is investigated toward 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.
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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.
We performed a large-scale mapping observation toward the W33 complex and its surroundings, covering an area of $1.3^circ times 1.0^circ$ , in $^{12}$CO (1-0), $^{13}$CO (1-0), and C$^{18}$O (1-0) lines from the Purple Mountain Observatory (PMO). We found a new hub--filament system ranging from 30 to 38.5 kms located at the W33 complex. Three supercritical filaments are directly converging into the central hub W33. Velocity gradients are detected along the filaments and the accretion rates are in order of $rm 10^{-3},M_odot, yr^{-1}$. The central hub W33 has a total mass of $rmsim 1.8times10^5,M_odot$, accounting for $sim 60%$ of the mass of the hub--filament system. This indicates that the central hub is the mass reservoir of the hub-filament system. Furthermore, 49 ATLASGAL clumps are associated with the hub--filament system. We find $57%$ of the clumps to be situated in the central hub W33 and clustered at the intersections between the filaments and the W33 complex. Moreover, the distribution of Class I young stellar objects (YSOs) forms a structure resembling the hub--filament system and peaks at where the clumps group; it seems to suggest that the mechanisms of clump formation and star formation in this region are correlated. Gas flows along the filaments are likely to feed the materials into the intersections and lead to the clustering and formation of the clumps in the hub--filament system W33. The star formation in the intersections between the filaments and the W33 complex might be triggered by the motion of gas converging into the intersections.
The formation of filaments in molecular clouds is an important process in star formation. Hub-filament systems (HFSs) are a transition stage connecting parsec-scale filaments and proto-clusters. Understanding the origin of HFSs is crucial to reveal how star formation proceeds from clouds to cores. Here, we report JCMT POL-2 850 $mu$m polarization and IRAM 30-m C$^{18}$O (2-1) line observations toward the massive HFS G33.92+0.11. The 850 $mu$m continuum map reveals four major filaments converging to the center of G33.92+0.11 with numerous short filaments connecting to the major filaments at local intensity peaks. We estimate the local orientations of filaments, magnetic field, gravity, and velocity gradients from observations, and we examine their correlations based on their local properties. In the high-density areas, our analysis shows that the filaments tend to align with the magnetic field and local gravity. In the low-density areas, we find that the local velocity gradients tend to be perpendicular to both the magnetic field and local gravity, although the filaments still tend to align with local gravity. A global virial analysis suggests that the gravitational energy overall dominates the magnetic and kinematic energy. Combining local and global aspects, we conclude that the formation of G33.92+0.11 is predominantly driven by gravity, dragging and aligning the major filaments and magnetic field on the way to the inner dense center. Traced by local velocity gradients in the outer diffuse areas, ambient gas might be accreted onto the major filaments directly or via the short filaments.
We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 {mu}m, 160 {mu}m, 250 {mu}m, 350 {mu}m, and 500 {mu}m by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C18O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments.
We present the 850 $mu$m polarization observations toward the IC5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope (JCMT), as part of the B-fields In STar forming Regions Observations (BISTRO). This work is aimed at revealing the magnetic field morphology within a core-scale ($lesssim 1.0$ pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether or not the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function (PDF) of the observed polarization fraction for low signal-to-noise ratio (SNR) data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in $A_Vsim$ 20--300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5$pm$0.2 mG toward the central hub using the Davis-Chandrasekhar-Fermi method, corresponding to a mass-to-flux criticality of $sim$ $1.3pm0.4$ and an Alfv{e}nic Mach number of $<$0.6. These results suggest that gravity and magnetic field is currently of comparable importance in the HFS, and turbulence is less important.
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