Search

Sh2-48 is a Galactic H iiregion, 3.8 kpc distant, with an O9.5-type star identified at its center. As a part of the FOREST Unbiased Galactic plane Imaging survey using the Nobeyama 45 m telescope (FUGIN) project, we obtained a CO J= 1–0 data set for a large area of Sh2-48 at a spatial resolution of 21″ (∼0.4 pc), and used it to find a molecular cloud with a total molecular mass of ∼3.8 × 104M⊙associated with Sh2-48. The molecular cloud has a systematic velocity shift in a velocity range of ∼42–47 km s−1. On the lower-velocity side the CO emission spatially corresponds to the bright 8 μm filament at the western rim of Sh2-48; however, the CO emission with higher velocities separates into the eastern and western sides of the 8 μm filament. This velocity variation forms a V-shaped feature in the east–west direction on the position–velocity diagram. We found that these lower- and higher-velocity components are, unlike the infrared and radio-continuum data, physically associated with Sh2-48. To interpret the observed V-shaped velocity distribution, we assess a cloud–cloud collision scenario, and found, from a comparison between observations and simulations, that the velocity distribution is an expected outcome of a collision between a cylindrical cloud corresponding to the lower-velocity component and a spherical cloud, and that the two separate higher-velocity components are interpretable as collision-broken remnants of the spherical cloud. Based on the consistency between an estimated formation timescale of the H iiregion, ∼1.3 Myr, and a timescale of the collision, we conclude that the high-mass star formation in Sh2-48 was triggered by the collision.

We report on a study of the high-mass star formation in the H iiregion W 28 A2 by investigating the molecular clouds that extend over ∼5–10 pc from the exciting stars using the 12CO and 13CO (J= 1–0) and 12CO (J= 2–1) data taken by NANTEN2 and Mopra observations. These molecular clouds consist of three velocity components with CO intensity peaks at VLSR∼ −4 km s−1, 9 km s−1, and 16 km s−1. The highest CO intensity is detected at VLSR∼ 9 km s−1, where the high-mass stars with spectral types 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 μm 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−1and +9 km s−1clouds, possibly due to heating by the high-mass stars, whereas the intensity ratio at the CO intensity peak (VLSR∼ 9 km s−1) decreases to ∼0.6, suggesting self absorption by the dense gas in the near side of the +9 km s−1cloud. We found partly complementary gas distributions between the −4 km s−1and +9 km s−1clouds, and the −4 km s−1and +16 km s−1clouds. The exciting sources are located toward the overlapping region in the −4 km s−1and +9 km s−1clouds. 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 the possibility of the formation of high-mass stars in the W 28 A2 region being triggered by cloud–cloud collision.

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.

Star formation is thought to be driven by two groups of mechanisms; spontaneous collapse and triggered collapse. Triggered star formation mechanisms further diverge into cloud–cloud collision (CCC), “collect and collapse” (C&C) and shock-induced collapse of pre-existing, gravitationally stable cores, or “radiation driven implosion” (RDI). To evaluate the contributions of these mechanisms and establish whether these processes can occur together within the same star-forming region, we performed mapping observations of radio-frequency ammonia and water maser emission lines in the S235 massive star-forming region. Via spectral analyses of main, hyperfine, and multi-transitional ammonia lines we explored the distribution of temperature and column density in the dense gas in the S235 and S235AB star-forming region. The most remarkable result of the mapping observations is the discovery of high-density gas in inter-core bridges which physically link dense molecular cores that house young proto-stellar clusters. The presence of dense gas implies the potential for future star formation within the system of cores and gas bridges. Cluster formation implies collapse, and the continuous physical links, also seen in re-imaged archival CS and 13CO maps, suggest a common origin to the molecular cores housing these clusters, i.e a structure condensed from a single, larger parent cloud, brought about by the influence of a local expanding H$\,$iiregion. An ammonia absorption feature co-locating with the center of the extended H$\,$iiregion may be attributed to an older gas component left over from the period prior to formation of the H$\,$iiregion. Our observations also detail known and new sites of water maser emission, highlighting regions of active ongoing star formation.

We report an observational study of the giant molecular cloud (GMC) associated with the Galactic infrared ring-like structure N35 and two nearby H iiregions G024.392+00.072 (H iiregion A) and G024.510−00.060 (H iiregion B), using the new CO J= 1–0 data obtained as a part of the FOREST Unbiased Galactic Plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project at a spatial resolution of 21″. Our CO data reveals that the GMC, with a total molecular mass of 2.1 × 106M⊙, has two velocity components of over ∼10–15 km s−1. The majority of molecular gas in the GMC is included in the lower-velocity component (LVC) at ∼110–114 km s−1, while the higher-velocity components (HVCs) at ∼118–126 km s−1consist of three smaller molecular clouds which are located near the three H iiregions. The LVC and HVCs show spatially complementary distributions along the line-of-sight, despite large velocity separations of ∼5–15 km s−1, and are connected in velocity by the CO emission with intermediate intensities. By comparing the observations with simulations, we discuss a scenario where collisions of the three HVCs with the LVC at velocities of ∼10–15 km s−1can provide an interpretation of these two observational signatures. The intermediate-velocity features between the LVC and HVCs can be understood as broad bridge features, which indicate the turbulent motion of the gas at the collision interfaces, while the spatially complementary distributions represent the cavities created in the LVC by the HVCs through the collisions. Our model indicates that the three H iiregions were formed after the onset of the collisions, and it is therefore suggested that the high-mass star formation in the GMC was triggered by the collisions.

We carried out a molecular-line study toward the three Spitzer bubbles S116, S117, and S118, which show active formation of high-mass stars. We found molecular gas consisting of two components with a velocity difference of ∼5 km s−1. One of them, the small cloud, has a typical velocity of −63 km s−1and the other, the large cloud, has one of −58 km s−1. The large cloud has a nearly circular intensity depression, the size of which is similar to that of the small cloud. We present an interpretation that its cavity was created by a collision between the two clouds and that this collision compressed the gas into a dense layer elongating along the western rim of the small cloud. In this scenario, the O stars including those in the three Spitzer bubbles were formed in the interface layer compressed by the collision. Assuming that the relative motion of the clouds has a tilt of 45° to the line of sight, we estimate that the collision continued for the last 1 Myr at a relative velocity of ∼10 km s−1. In the S116-S117-S118 system the H iiregions are located outside of the cavity. This morphology is ascribed to the density-bound distribution of the large cloud which caused the H iiregions to expand more easily toward the outer part of the large cloud than towards the inside of the cavity. The present case proves that a cloud–cloud collision creates a cavity without the action of O-star feedback, and suggests that the collision-compressed layer is highly filamentary.

We report on the possibility that the high-mass star located in the H iiregion RCW 34 was formed by a triggering induced by a collision of molecular clouds. Molecular gas distributions of the 12CO and 13CO J= 2–1 and 12CO J= 3–2 lines in the direction of RCW 34 were measured using the NANTEN2 and ASTE telescopes. We found two clouds with velocity ranges of 0–10 km s−1and 10–14 km s−1. Whereas the former cloud is as massive as ∼1.4 × 104M⊙and has a morphology similar to the ring-like structure observed in the infrared wavelengths, the latter cloud, with a mass of ∼600 M⊙, which has not been recognized by previous observations, is distributed to just cover the bubble enclosed by the other cloud. The high-mass star with a spectral type of O8.5V is located near the boundary of the two clouds. The line intensity ratio of 12CO J= 3–2/J= 2–1 yields high values (≳1.0), suggesting that these clouds are associated with the massive star. We also confirm that the obtained position–velocity diagram shows a similar distribution to that derived by a numerical simulation of the supersonic collision of two clouds. Using the relative velocity between the two clouds (∼5 km s−1), the collisional time scale is estimated to be ∼0.2 Myr with the assumption of a distance of 2.5 kpc. These results suggest that the high-mass star in RCW 34 was formed rapidly within a time scale of ∼0.2 Myr via a triggering of a cloud–cloud collision.