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Dense Molecular Ring-like structure in gaseous CO depletion region G34.74-0.12

Shuting Lin, Siyi Feng, Fengwei Xu, Ke Wang, Patricio Sanhueza, Junzhi Wang, Zhi-Yu Zhang, Yichen Zhang, Kaho Morii, Hauyu Baobab Liu, Sheng-Yuan Liu, Lile Wang, Giovanni Sabatini, Hui Li, Willem Baan, Zhi-Kai Zhu, Shanghuo Li

TL;DR

This study reports the discovery of a dense, pc-scale ring-like structure in the infrared-dark cloud G34.74-0.12, revealed by ALMA tracers C$^{18}$O (2–1), HNC (1–0), and N$_2$H$^+$ (1–0). The ring, with a diameter of 0.68 pc and mass of ~109 M$_\odot$, is redshifted relative to V$_{sys}$ and coincides with dust cores and protostellar outflows but shows no associated free-free emission or H II region, indicating an early evolutionary stage. Kinematic analysis via PV diagrams suggests slow expansion consistent with an expanding shell, with a best-fit V$_{exp}$ ≈ 2.3 km s$^{-1}$ and radius ≈ 0.39 pc, yielding a dynamical timescale ~10$^5$ yr. The authors discuss four plausible origins—relic wind-blown bubble, cloud–cloud interaction hollow, temperature-gradient ring, or line-of-sight superposition—and advocate further multi-line observations to distinguish among scenarios and to understand the initial dynamical processes in massive star formation.

Abstract

We report the discovery of a dense molecular ring-like structure in a dense (10$^5$ cm$^{-3}$), cold (pc-scale CO depletion at a factor of 5), and young (10$^4$ year) star-forming region G34.74-0.12, revealed by C$^{18}$O (2-1), HNC (1-0), and N$_2$H$^+$ (1-0) observations with the Atacama Large Millimeter/submillimeter Array (ALMA). The ring-like structure is redshifted with respect to the clump, spanning from $V_{\rm sys,lsr} + 0.9$ to $V_{\rm sys,lsr} + 2.9$ km s$^{-1}$, with a total mass of 109 $M_{\odot}$. It is spatially coincident with 1.3 mm and 3.0 mm dust continuum emission from cores, and several protostellar outflows. However, no free-free emission or H\textsc{ii} region is detected in association with this structure. With a slow expansion speed indicated by the position-velocity diagram, this ring structure differs from rings previously identified in more evolved star-forming regions. Possible explanations for the ring-like structure include a relic wind-blown bubble produced by a deeply embedded young stellar object, a hollow cavity formed by cloud-cloud interactions, a gas ring resulting from a temperature gradient, or a line-of-sight superposition of multiple outflows or dense clouds. This discovery offers a rare observational glimpse into the earliest dynamical processes involved in massive star formation.

Dense Molecular Ring-like structure in gaseous CO depletion region G34.74-0.12

TL;DR

This study reports the discovery of a dense, pc-scale ring-like structure in the infrared-dark cloud G34.74-0.12, revealed by ALMA tracers CO (2–1), HNC (1–0), and NH (1–0). The ring, with a diameter of 0.68 pc and mass of ~109 M, is redshifted relative to V and coincides with dust cores and protostellar outflows but shows no associated free-free emission or H II region, indicating an early evolutionary stage. Kinematic analysis via PV diagrams suggests slow expansion consistent with an expanding shell, with a best-fit V ≈ 2.3 km s and radius ≈ 0.39 pc, yielding a dynamical timescale ~10 yr. The authors discuss four plausible origins—relic wind-blown bubble, cloud–cloud interaction hollow, temperature-gradient ring, or line-of-sight superposition—and advocate further multi-line observations to distinguish among scenarios and to understand the initial dynamical processes in massive star formation.

Abstract

We report the discovery of a dense molecular ring-like structure in a dense (10 cm), cold (pc-scale CO depletion at a factor of 5), and young (10 year) star-forming region G34.74-0.12, revealed by CO (2-1), HNC (1-0), and NH (1-0) observations with the Atacama Large Millimeter/submillimeter Array (ALMA). The ring-like structure is redshifted with respect to the clump, spanning from to km s, with a total mass of 109 . It is spatially coincident with 1.3 mm and 3.0 mm dust continuum emission from cores, and several protostellar outflows. However, no free-free emission or H\textsc{ii} region is detected in association with this structure. With a slow expansion speed indicated by the position-velocity diagram, this ring structure differs from rings previously identified in more evolved star-forming regions. Possible explanations for the ring-like structure include a relic wind-blown bubble produced by a deeply embedded young stellar object, a hollow cavity formed by cloud-cloud interactions, a gas ring resulting from a temperature gradient, or a line-of-sight superposition of multiple outflows or dense clouds. This discovery offers a rare observational glimpse into the earliest dynamical processes involved in massive star formation.

Paper Structure

This paper contains 16 sections, 7 equations, 8 figures.

Table of Contents

  1. Introduction
  2. Observation AND DATA REDUCTION
  3. Observational Results
  4. A Ring-like structure
  5. Analysis & Discussion
  6. Kinematic structure of the ring
  7. Physical structure of the ring
  8. Large-scale environment and Young stellar object
  9. CO Depletion
  10. Possible origins of Ring-like Structure
  11. Conclusion
  12. Channel map
  13. Continuum images
  14. Core mass estimation
  15. PV diagrams of C$^{18}$O (2-1) and N$_2$H$^+$ (1-0)
  16. ...and 1 more sections

Figures (8)

  • Figure 1: (a): Integrated intensity map of C$^{18}$O (2–1) from ALMA 12m+7m+TP data with full UV coverage, integrated over the velocity range 79.9–81.9 km s$^{-1}$, overlaid on large scale two-color maps of G34.74-0.12. The red and cyan colors represent the integrated intensities of C$^{18}$O (2–1) and DCO$^+$ (1–0), respectively, over the velocity range of 72-85 km s$^{-1}$, obtained from IRAM-30m observations Feng2020ApJ...901..145F. White contours represent the dust continuum emission from the combined Planck and APEX 870 $\mu$m map of the ATLASGAL survey, shown at contour levels of 4, 5, 6, 7, and 8 $\sigma$, where $\sigma = 0.18$Jy beam$^{-1}$. The orange contours represents the C$^{18}$O (2-1) gas ring-like structure with levels start from 10 $\sigma$ and increase in steps of 3 $\sigma$ (1 $\sigma$ =0.02 Jy beam$^{-1}$ km s$^{-1}$). The relative coordinate center (0,0) denotes the center of the ring. (b): The greyscale map and black contours represent the integrated intensity of C$^{18}$O (2-1) ring (integrated velocity range: 79.9 - 81.9 km s$^{-1}$). The contour level are same as Fig. \ref{['fig:C18O-ring']} (a). The blue and red contours represent the blue- and red-shifted components of outflows traced by CO (2-1), respectively 2025arXiv250714564L. The yellow contours represent the integrated intensity of H$_2$CO (3$_{0,3}$-2$_{0,2}$) within the same integrated velocity range of C$^{18}$O (2-1), showing a similar ring-like morphology. The contour levels of H$_2$CO start from 4 $\sigma$ and increase in steps of 4 $\sigma$ (1$\sigma$ =0.01 Jy beam$^{-1}$ km s$^{-1}$). The white contours represents the 1.3 mm dust continuum from ASHES observations. Black crosses mark the positions of core 1, 2, 4, and 10, as labeled by Morii2023ApJ...950..148M. (c): C$^{18}$O (2–1) ring overlaid on the integrated intensity map of HNC (1-0). The black contours represents the C$^{18}$O (2-1) gas ring-like structure, same as Fig. \ref{['fig:C18O-ring']} (a). The white contours represents the HNC (1-0) emission with levels start from 3 $\sigma$ and increase in steps of 2 $\sigma$ (1 $\sigma$ = 0.012 Jy beam$^{-1}$ km s$^{-1}$), integrated over the same integrated velocity range of C$^{18}$O (2-1). The grey arrows indicate the paths along four different directions, corresponding to PA of 0$^{\circ}$, 45$^{\circ}$, 90$^{\circ}$, and 135$^{\circ}$. (d): C$^{18}$O (2–1) ring overlaid on the integrated intensity map of N$_2$H$^+$ (1-0), derived by stacking its hyperfine components. The black contours represents the C$^{18}$O (2-1) gas ring-like structure, same as panel (a). The white contours represents the N$_2$H$^+$ (1-0) emission with levels start from 3 $\sigma$ and increase in steps of 2 $\sigma$ (1 $\sigma$ = 0.019 Jy beam$^{-1}$ km s$^{-1}$), integrated over the same integrated velocity range of C$^{18}$O (2-1). A, B, and C mark the positions where HNC (1-0), N$_2$H$^+$ (1-0), and C$^{18}$O (2–1) lines all show emission S/N $>$ 4. The synthesized beam for each line is shown in the bottom right corner of each panel.
  • Figure 2: The line profiles of C$^{18}$O (2–1), HNC (1-0), and N$_2$H$^+$ (1-0), extracted from synthesized beam–averaged regions centered on core10, positions A, B, and C (as shown in Fig. \ref{['fig:C18O-ring']}). The vertical red dashed line indicates the systemic velocity, and the shaded region shows the integrated velocity range for the ring component.
  • Figure 3: PV diagrams of HNC (1-0) along four cuts with PA of 0$^{\circ}$, 45$^{\circ}$, 90$^{\circ}$, and 135$^{\circ}$ (labeled in Fig. \ref{['fig:C18O-ring']}) of the ring-like structure. Contour levels are set at 3, 4 and 5 $\sigma$, where $\sigma$=0.008 Jy beam$^{-1}$. The vertical black dashed line marks the center of ring-like structure, while the horizontal black dashed line indicates the systemic velocity. The gray curve in the PV diagram represents the best-fit result from the spherical expanding shell model.
  • Figure A1: C$^{18}$O (2-1) channel map. The contour levels start from 5 $\sigma$ and increase in steps of 6 $\sigma$ (1 $\sigma$ =0.007 Jy beam$^{-1}$). The cyan crosses indicate the position of core 1, 2, 4, and 10, as labeled in Fig. \ref{['fig:C18O-ring']}. The synthesized beam is given in the bottom right.
  • Figure A2: HNC (1-0) channel map. The observational velocity resolution is 0.21 kms$^{-1}$; here, we smooth the data to 0.42 kms$^{-1}$. The contour levels start from 4 $\sigma$ and increase in steps of 10 $\sigma$ (1 $\sigma$ =0.008 Jy beam$^{-1}$). The cyan crosses indicate the position of core 1, 2, 4, and 10, as labeled in Fig. \ref{['fig:C18O-ring']}. The synthesized beam is given in the bottom right.
  • ...and 3 more figures