FIRESTORM I: Stellar Feedback and Gas Kinematics in the Evolved W40 Hub-Filament System

TL;DR

This paper investigates the W40 hub-filament system as a nearby laboratory for how stellar feedback and filamentary accretion govern massive-star formation. By combining TRAO dense-gas tracer maps with archival multi-telescope data, it identifies six velocity-coherent filaments converging toward the central massive-star cluster IRS 1A South, and characterizes their widths, kinematics, and stability. A bridge-like feature in C$^{18}$O PV space and complementary spatial structures support a cloud–cloud collision origin for the hub, with subsequent OB-star feedback driving increased temperature and turbulence and triggering dense clump formation at the HII region interface. The results suggest W40 represents a late-stage hub-filament system where CCC and feedback together shape star formation and gas dynamics, providing a benchmark for how feedback reshapes dense gas in massive-star-forming environments.

Abstract

The FIRESTORM project--Feedback-Induced Regions and Emission from Star-forming Tracers of ObseRvable Molecular Gas--has targeted four star-forming regions to quantify the impact of stellar feedback on star formation. In this paper, we present multiwavelength results for one of the targets, the nearby high-mass star-forming region W40. Using dense-gas tracers C$^{18}$O(1--0) and H$^{13}$CO$^+$(1--0), we identified six velocity-coherent filaments: five at \vlsr $\sim$\,7.5\kms\! and one at \vlsr $\sim$\,5\kms. Four of these converge towards an infrared-bright cluster hosting the most massive star of the region (IRS 1A South, O9.5V), forming a hub-filament system (HFS). Key physical parameters, including filament lengths, widths, masses, velocity dispersions, and line masses, are derived. Five dense clumps traced by N$_2$H$^+$(1--0) exhibit subsonic to transonic turbulence, contrasting with the supersonic motions of their parental filaments, indicating turbulence dissipation. A deficit of emission at \vlsr $\sim$\,7\kms\! in several molecular lines, along with a blueshifted absorption dip in the HCN(1--0) profile, suggests that emission from OB-heated gas is being absorbed by a cold foreground cloud. A bridge-like feature in position-velocity space connects the \vlsr $\sim$\,5 and $\sim$\,7.5\kms\! filaments, and spatially coinciding with dense condensations and radio continuum peaks. These findings suggest that a past interaction--likely a cloud-cloud collision--triggered the formation of HFS and ultimately the central massive cluster.

Figures (11)

• Figure 1: (a) H$_2$ column density, $N_\mathrm{H_2}$ map from the Herschel Gould Belt Survey (HGBS) andre_2010 overlaid with 887.5 MHz contour from Rapid ASKAP Continuum Survey (RACS) Data Release 1. Contour levels are [0.01, 0.05, 0.09, 0.13, 0.17, 0.21] Jy/beam. (b) Dust temperature, $T_\text{dust}$ map from HGBS with overlaid positions of Class I (black) and II (cyan) YSOs from Sun_2022. (c) Composite image combining Herschel 250 (red), Spitzer 8 (green), and UKIRT H$\alpha$ emission (blue). (d) JCMT SCUBA-2 850 , overlaid with Herschel dense cores konyves2015. In each panel, the field of view of the TRAO observation is indicated by a white box, and the scale bar is drawn at a distance of 502 pc. Star symbols in panel (d) indicate the positions of massive stars identified by Shuping_2012, with the most massive star, IRS 1A South, marked with a red star symbol in all the panels.
• Figure 2: Colour composite moment-0 maps of molecular lines. Each map shares the same FoV and spatial axes. Each colour represents a moment-0 map integrated over the velocity ranges indicated in the figurea. These ranges were selected to represent the four main velocity components associated with the W40 cloud (3, 5, 7, and 10$\mathrm{\,km\,s^{-1}}$). The white boxes in panels b), e), f), g), h), i), j), and k) indicate the extent of the corresponding map. The FoV of each map is the same as in Fig. \ref{['fig: w40_complex']}. White circle in the lower left corner of each panel denotes the angular resolution of the respective data cube (see Table \ref{['tab: molecular_line']} for the resolutions). The location of IRS 1A South is marked with a star symbol in each panel. White contours show the Herschel column density map with contour levels of [1, 2, 3, 4] $\times10^{22}\,\mathrm{cm}^{-2}$. The facility of which the data is obtained is indicated at the upper left corner of the respective panel.
• Figure 3: (a) Integrated intensity map of HCN over the velocity range [--0.2, +0.2]$\mathrm{\,km\,s^{-1}}$, centred on the F(0--1) hyperfine line of the 7$\mathrm{\,km\,s^{-1}}$ component. Filamentary structures in the southeastern region (labelled SE1 and SE2 in Fig. \ref{['fig: filament_vel_info']}a) are clearly visible. (b--e) HCN spectra at selected positions, illustrating the effect of foreground absorption at 7$\mathrm{\,km\,s^{-1}}$, with spectra of C$^{18}$O, HCO$^+$ and H$^{13}$CN at the same positions as references. The orange dotted spectrum in each panel represents the expected profile under Local Thermodynamic Equilibrium (LTE) and optically thin ($\tau_m=0.1$) conditions, with excitation temperature $T_\mathrm{ex}$ value indicated at the upper right of each panel. The transitions of HCN hyperfine lines are indicated in panel (b). (b) At the 7$\mathrm{\,km\,s^{-1}}$ component , the F(2--1) and F(1--1) lines are completely absorbed, while the F(0--1) line is less affected. (c) At the 3$\mathrm{\,km\,s^{-1}}$ component , the F(1--1) line (which lies at 7$\mathrm{\,km\,s^{-1}}$) shows suppressed emission due to absorption. (d) At the 5$\mathrm{\,km\,s^{-1}}$ component , the F(2--1) and F(1--1) lines exhibit steeper red wings, suggesting partial absorption. (e) At the 7$\mathrm{\,km\,s^{-1}}$ component , a blueshifted dip is observed, indicating possible expansion of the absorbing foreground layer. The HCN spectra in panels (b) and (c), as well as the H$^{13}$CN spectrum, are smoothed to 0.5$\mathrm{\,km\,s^{-1}}$, while the spectra in panel (e) are smoothed to 0.2$\mathrm{\,km\,s^{-1}}$.
• Figure 4: (a) Spatial distribution of filament skeletons analysed in this study. The skeletons are colour-coded with their $V_{\mathrm{LSR}}$ profiles, linearly interpolated from the sampled points (black dots). Each filament is labelled with its designation. White arrows illustrate the direction of increasing filament length (corresponding to the positive x-axis in Fig. \ref{['fig: filament_T_vlsr_dv_profile']}). Note that N1 is shown with a distinct colourmap for clarity. (b) Skeletons of filaments detected from HGBS konyves2015. Both panels share the same FoV and background image (Herschel column density map). The scale bar and star symbol in each panel follow the definitions in Fig. \ref{['fig: w40_complex']}.
• Figure 5: (a) Profiles of centroid velocity ($V_{\mathrm{LSR}}$) (green $\star$), velocity dispersion ($\sigma_\text{obs}$) (blue $\cdot$), and peak temperature $T_\mathrm{peak}$ (red $\times$) for each identified filament, derived from single-component Gaussian fitting. Green dashed lines in sub-panel i), ii), v), and vi) indicate the best linear fit to the $V_{\mathrm{LSR}}$ profile (velocity gradient, $\nabla V$), with its value and fitting error denoted on the upper right of the sub-panel. (b) Profiles of thermal (red $\times$), non-thermal (brown $\star$) velocity dispersions ($\sigma_\mathrm{th}$, $\sigma_\mathrm{nt}$), turbulent Mach number ($\sigma_\mathrm{nt}/\sigma_\mathrm{th}$) (blue $+$) and stability ratio ($M_\mathrm{line}/M_\mathrm{line}^\mathrm{vir}$) (green $\cdot$) of each identified filament. In all panels, the shaded region of respective colour represents the standard deviation of each data point, and inverted coloured triangles at the bottom of each panel indicate the positions of Herschel-identified dense cores konyves2015: blue for starless, green for prestellar, and red for protostellar. In panel group (b), blue dotted lines represent Mach numbers of 1 and 2, while green dotted lines represent $M_\mathrm{line}/M_\mathrm{line}^\mathrm{vir}=0.5$. Brown hexagon scatters in sub-panels i), iii), and iv) denote the $\sigma_\mathrm{nt}$ of N$_2$H$^+$ clumps (see Section \ref{['sec: clumps_analysis']}). Axis labels shown in the top-left panels are common to all six sub-panels.