The Orion Nebula Cluster toward the Hii region M42 is the most outstanding young cluster at the smallest distance 410 pc among the rich high-mass stellar clusters. By newly analyzing the archival molecular data of the 12 CO(J = 1-0) emission at 21 ′′ resolution, we identified at least three pairs of complementary distributions between two velocity components at 8 km s −1 and 13 km s −1 . We present a hypothesis that the two clouds collided with each other and triggered formation of the high-mass stars, mainly toward two regions including the nearly ten O stars, θ 1 Ori and θ 2 Ori, in M42 and the B star, NU Ori, in M43. The timescale of the collision is estimated to be ∼ 0.1 Myr by a ratio of the cloud size and velocity corrected for projection, which is consistent with the age of the youngest cluster members less than 0.1 Myr. The majority of the low-mass cluster members were formed prior to the collision in the last one Myr. We discuss implications of the present hypothesis and the scenario of high-mass star formation by comparing with the other eight cases of triggered O star formation via cloud-cloud collision.Evidence for triggered formation of O / early B star(s) by cloud-cloud collision is found in the four super star clusters, Westerlund 2, NGC 3603, RCW 38 and [DBS2003] 179 (Furukawa et al. 2009;Ohama et al. 2010;Fukui et al. 2014, Kuwahara et al. 2017, in the two Hii regions with single O stars, RCW 120 and M20 (Torii et al.

The FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) project is one of the legacy projects using the new multi-beam FOREST receiver installed on the Nobeyama 45-m telescope. This project aims to investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gas in the Galaxy at once by observing 12 CO, 13 CO, and C 18 O J = 1 − 0 lines simultaneously. The mapping regions are a part of the 1st quadrant (10• ) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (∼20 ′′ ) for the Galactic plane survey in the CO J = 1 − 0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us with an invaluable dataset for investigating the physics of the galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic scale structures to the internal structures of giant molecular clouds, such as small filament/clump/core. We present an overview of the FUGIN project, observation plan, and initial results, which reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas such as spiral arms.

We observed the Monoceros R2 molecular cloud with the ACIS-I array onboard the Chandra X-ray Observatory. From the central 3. ′ 2 × 3. ′ 2 region, we detect 154 sources above the detection limit of ∼ 5 × 10 −16 ergs s −1 cm −2 with a 100 ks-exposure. About 85% of the X-ray sources are identified with an infrared counterpart, including four high mass stars in zero age main sequence (ZAMS) and/or pre main sequence (PMS) phase. The X-ray spectra of the high mass ZAMS and PMS stars are represented with a thin thermal plasma model of a temperature above ∼ 2 keV. The X-rays are time-variable and exhibit rapid flares. These high temperature plasma and flaring activity are similar to those seen in low mass PMS stars and contrary to the behavior observed in high mass main sequence stars. The X-ray luminosity increases as the intrinsic K-band flux increases. However, the X-ray luminosity saturates at a level of ∼ 10 31 ergs s −1 . We conclude that high mass ZAMS and PMS emit X-rays, possibly due to the magnetic activity like those of low mass stars.

High-mass star formation is an important step which controls galactic evolution. GM 24 is a heavily obscured star cluster including a single O9 star with more than ∼100 lower mass stars within a 0.3 pc radius toward (l, b) ∼ (350.• 5, 0.• 96), close to the Galactic min-starburst NGC 6334. We found two velocity components associated with the cluster by new observations of 12 COJ= 2-1 emission, whereas the cloud was previously considered to be single. We found the distribution of the two components of 5 km s −1 separation shows complementary distribution; the two fit well with each other if a relative displacement of 3 pc is applied along the Galactic plane. A position-velocity diagram of the GM 24 cloud is explained by a model based on the numerical simulations of two colliding clouds, where an intermediate velocity component created by collision is taken into account. We estimate the collision time scale to be ∼Myr in projection of a relative motion titled to the line of sight by 45 degrees. The results lend further support for cloud-cloud collision as an important mechanism of high-mass star formation in the Carina-Sagittarius Arm.

We performed new large-scale 12CO, 13CO, and C18O J = 1–0 observations of the W 43 giant molecular cloud complex in the tangential direction of the Scutum arm (l ∼30°) as a part of the FUGIN project. The low-density gas traced by 12CO is distributed over 150 pc × 100 pc (l × b), and has a large velocity dispersion (20–30 km s−1). However, the dense gas traced by C18O is localized in the W 43 Main, G30.5, and W 43 South (G29.96−0.02) high-mass star-forming regions in the W 43 giant molecular cloud (GMC) complex, which have clumpy structures. We found at least two clouds with a velocity difference of ∼10–20 km s−1, both of which are likely to be physically associated with these high-mass star-forming regions based on the results of high 13CO J = 3–2 to J = 1–0 intensity ratio and morphological correspondence with the infrared dust emission. The velocity separation of these clouds in W 43 Main, G30.5, and W 43 South is too large for each cloud to be gravitationally bound. We also revealed that the dense gas in the W 43 GMC has a high local column density, while “the current SFE” (star formation efficiency) of the entire GMC is low ($\sim\!\! 4\%$) compared with the W 51 and M 17 GMC. We argue that the supersonic cloud–cloud collision hypothesis can explain the origin of the local mini-starbursts and dense gas formation in the W 43 GMC complex.

We carried out 12 CO(J = 1-0) observations of the Galactic gamma-ray supernova remnant (SNR) Kesteven 79 using the Nobeyama Radio Observatory 45 m radio telescope, which has an angular resolution of ∼ 20 arcsec. We identified molecular and atomic gas interacting with Kesteven 79 whose radial velocity is ∼ 80 km s −1 . The interacting molecular and atomic gases show good spatial correspondence with the X-ray and radio shells, which have an expanding motion with an expanding velocity of ∼ 4 km s −1 . The molecular gas associated with the radio and X-ray peaks also exhibits a high-intensity ratio of CO 3-2/1-0 > 0.8, suggesting a kinematic temperature of ∼ 24 K, owing to heating by the supernova shock. We determined the kinematic distance to the SNR to be ∼ 5.5 kpc and the radius of the SNR to be ∼ 8 pc. The average interstellar proton density inside of the SNR is ∼ 360 cm −3 , of which atomic protons comprise only ∼ 10 %. Assuming a hadronic origin for the gamma-ray emission, the total cosmic-ray proton energy above 1 GeV is estimated to be ∼ 5 × 10 48 erg.

Context. The interstellar medium is observed to be organized in filamentary structures, and in neutral (H I) and ionized (H II) bubbles. The expanding nature of these bubbles shapes the surrounding medium and possibly plays a role in the formation and evolution of the interstellar filaments. The impact of the expansion of these bubbles on the interstellar medium is not well understood. Aims. Our aim is to describe the kinematics of a filamentary molecular cloud forming high-mass stars and hosting multiple H II regions in order to study the possible environmental impact on the properties of molecular filaments. Methods. We present APEX 13CO and C18O(2–1) mapping observations of the 10 × 50 pc NGC 6334 molecular cloud complex. We investigated the gas velocity structure along and across the 50 pc long cloud and toward velocity-coherent filaments (VCFs). Results. The NGC 6334 complex is observed to have a coherent velocity structure smoothly varying by ~5 km s−1 over its 50 pc elongation parallel to the Galactic plane. We identify a sample of 75 VCFs in the C18O(2–1) position-position-velocity cube and present the properties of 47 VCFs with a length ≳1 pc (five beams). We measure a large number of velocity gradients along the VCFs. The amplitudes of these velocity gradients and the velocity dispersion measured along the crests increase with the column density of the VCFs. We derive the column density and velocity power spectra of the VCFs. These power spectra are well represented with power laws showing similar slopes for the two quantities (with a mean of about −2), although some differ by up to a factor of 2. The position velocity diagrams perpendicular to three VCFs (selected from different physical environments) show the V-shaped velocity pattern corresponding to a bent structure in velocity space with the filament at the tip of the V surrounded by an extended structure connected to it with a velocity gradient. This velocity structure is qualitatively similar to that resulting from numerical simulations of filament formation from large-scale compression from propagating shock fronts. In addition, the radial profiles perpendicular to these VCFs hint to small-scale internal impacts from neighboring H II bubbles on two of them, while the third is mostly unaffected. Conclusions. The observed opposite curvature in velocity space (V- and A-shaped) toward the VCFs points to various origins of large-scale external compressions from propagating H I bubbles. This suggests the plausible importance of multiple H I compressions, separated in space and time, in the formation and evolution of molecular clouds and their star formation history. These atomic compressions due to past and distant star formation events are complemented by the impact of H II bubbles from present time and local star formation activity.

We carried out new CO (J =1-0, 2-1 and 3-2) observations with NANTEN2 and ASTE in the region of the twin Galactic mini-starbursts NGC 6334 and NGC 6357. We detected two velocity molecular components of 12 km s −1 velocity separation, which is continuous over 3 degrees along the plane. In NGC 6334 the two components show similar two-peaked intensity distributions toward the young H II regions and are linked by a bridge feature. In NGC 6357 we found spatially complementary distribution between the two velocity components as well as a bridge feature in velocity. Based on these results we hypothesize that the two clouds in the two regions collided with each other in the past few Myr and triggered formation of the starbursts over ∼100 pc. We suggest that the formation of the starbursts happened toward the collisional region of ∼ 10-pc extents with initial high molecular column densities. For NGC 6334 we present a scenario which includes spatial variation of the colliding epoch due to non-uniform cloud separation. The scenario possibly explains the apparent age difference among the young O stars in NGC 6334 raging from 10 4 yrs to 10 6 yrs; the latest collision happened within 10 5 yrs toward the youngest stars in NGC 6334 I(N) and I which exhibit molecular outflows without H II regions. For NGC 6357 the O stars were formed a few Myrs ago, and the cloud dispersal by the O stars is significant. We conclude that cloud-cloud collision offers a possible explanation of the min-starburst over a 100-pc scale.