Triggered high-mass star formation in the HII region W28A2: A cloud-cloud collision scenario

Abstract

We report on a study of the high-mass star formation in the the HII region W28A2 by investigating the molecular clouds extended over ~5-10 pc from the exciting stars using the 12CO and 13CO (J=1-0) and 12CO (J=2-1) data taken by the NANTEN2 and Mopra observations. These molecular clouds consist of three velocity components with the CO intensity peaks at V_LSR ~ -4 km s${}^{-1}$, 9 km s${}^{-1}$ and 16 km s${}^{-1}$. The highest CO intensity is detected at V_LSR ~ 9 km s${}^{-1}$, where the high-mass stars with the spectral types of O6.5-B0.5 are embedded. We found bridging features connecting these clouds toward the directions of the exciting sources. Comparisons of the gas distributions with the radio continuum emission and 8 um infrared emission show spatial coincidence/anti-coincidence, suggesting physical associations between the gas and the exciting sources. The 12CO J=2-1 to 1-0 intensity ratio shows a high value (> 0.8) toward the exciting sources for the -4 km s${}^{-1}$ and +9 km s${}^{-1}$ clouds, possibly due to heating by the high-mass stars, whereas the intensity ratio at the CO intensity peak (V_LSR ~ 9 km s${}^{-1}$) lowers down to ~0.6, suggesting self absorption by the dense gas in the near side of the +9 km s${}^{-1}$ cloud. We found partly complementary gas distributions between the -4 km s${}^{-1}$ and +9 km s${}^{-1}$ clouds, and the -4 km s${}^{-1}$ and +16 km s${}^{-1}$ clouds. The exciting sources are located toward the overlapping region in the -4 km s${}^{-1}$ and +9 km s${}^{-1}$ clouds. Similar gas properties are found in the Galactic massive star clusters, RCW 38 and NGC 6334, where an early stage of cloud collision to trigger the star formation is suggested. Based on these results, we discuss a possibility of the formation of high-mass stars in the W28A2 region triggered by the cloud-cloud collision.

arXiv comment26 pages, 16 figures, Accepted for publication in PASJ

References in corpus (41)

• Toward Understanding Massive Star Formation
• The WISE Catalog of Galactic HII Regions
• A Parallax-based Distance Estimator for Spiral Arm Sources
• Formation of Massive Molecular Cloud Cores by Cloud-cloud Collision
• Do cloud-cloud collisions trigger high-mass star formation? I. Small cloud collisions
• Molecular clouds towards RCW 49 and Westerlund 2; Evidence for cluster formation triggered by cloud-cloud collision
• Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation
• Molecular Rings and the Thickness of Star-Forming Clouds
• Revealing the physical properties of molecular gas in Orion with a large scale survey in J=2-1 lines of 12CO, 13CO and C18O
• Cloud-cloud collision as a trigger of the high-mass star formation; a molecular line study in RCW120
• The two molecular clouds in RCW 38; evidence for formation of the youngest super star cluster in the Milky Way triggered by cloud-cloud collision
• Triggered massive-star formation at the border of the HII region Sh 104
• Temperature and Density Distribution in the Molecular Gas Toward Westerlund 2: Further Evidence for Physical Association
• A new look at the molecular gas in M42 and M43; possible evidence for cloud-cloud collision which triggered formation of the OB stars in the Orion Nebula Cluster
• Triggered O star formation in M20 via cloud-cloud collision: Comparisons between high-resolution CO observations and simulations
• The Mopra Southern Galactic Plane CO Survey
• Investigation of the large-scale neutral hydrogen near the supernova remnant W28
• Massive stars in massive clusters IV: Disruption of clouds by momentum-driven winds
• Trigonometric Parallaxes of Star Forming Regions in the Scutum Spiral Arm
• Subarcsecond Submillimeter Imaging of the Ultracompact HII Region G5.89-0.39
• Detailed Investigation of the Gamma-Ray Emission in the Vicinity of SNR W28 with FERMI-LAT
• Spectroscopically resolved far-IR observations of the massive star-forming region G5.89--0.39
• Cloud-cloud collisions in the common foot point of molecular loops 1 and 2 in the Galactic Center
• RCW 36 in the Vela Molecular Ridge: Evidence for a high-mass star cluster formation triggered by Cloud-Cloud Collision
• FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) 2: Possible evidence for formation of NGC~6618 cluster in M17 by cloud-cloud collision
• Formation of the active star forming region LHA 120-N 44 triggered by tidally-driven colliding HI flows
• The Mopra Southern Galactic Plane CO Survey - Data Release 3
• Molecular clouds in the NGC 6334 and NGC 6357 region; Evidence for a 100 pc-scale cloud-cloud collision triggering the Galactic mini-starbursts
• High-mass star formation possibly triggered by cloud-cloud collision in the HII region RCW 34
• A 7mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources
• New Distance and Revised Natures of High Mass Star Formation in G5.89-0.39
• The Mopra Southern Galactic Plane CO Survey - Data Release 1
• The Possibly Remnant Massive Outflow in G5.89-0.39: I - Observations and Initial MHD Simulations
• Star Formation Activity in the molecular cloud G35.20$-$0.74: onset of cloud-cloud collision
• ALMA CO Observations of a Giant Molecular Cloud in M33: Evidence for High-Mass Star Formation Triggered by Cloud-Cloud Collisions
• Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud collisions as a trigger of massive star formation
• Triggering the formation of the supergiant H II region NGC 604 in M33
• The Extremely High-Velocity Outflow from the Luminous Young Stellar Object G5.89-0.39
• Detailed CO ($J$ = 1--0, 2--1 and 3--2) observations toward an H{\sc ii} region RCW~32 in the Vela Molecular Ridge
• CO observations toward the isolated mid-infrared bubble S44: External triggering of O-star formation by a cloud-cloud collision
• Chandra observations of the HII complex G5.89-0.39 and TeV gamma-ray source HESSJ1800-240B

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• G133.50+9.01: A likely cloud-cloud collision complex triggering the formation of filaments, cores and a stellar cluster
• Ionising feedback from an O star formed in a shock-compressed layer