arXiv subjects

Shinji Fujita

Publications and source records attributed to Shinji Fujita.

At least 37 records · Page 2Linked to original sources

arXiv2020

Observations of the [CI] ($^3P_1$-$^3P_0$) emission toward the massive star-forming region RCW38: further evidence for highly-clumped density distribution of the molecular gas

We present observations of the $^3P_1$-$^3P_0$ fine-structure line of atomic carbon using the ASTE 10 m sub-mm telescope towards RCW38, the youngest super star cluster in the Milky Way. The detected [CI] emission is compared with the CO $J$ = 1-0 image cube presented in Fukui et al. (2016) which has an angular resolution of 40$^{\prime \prime}$ ($\sim$ 0.33 pc). The overall distribution of the [CI] emission in this cluster is similar to that of the $^{13}$CO emission. The optical depth of the [CI] emission was found to be $τ$ = 0.1-0.6, suggesting mostly optically thin emission. An empirical conversion factor from the [CI] integrated intensity to the H$_2$ column density was estimated as $X_{\rm [CI]}$ = 6.3 $\times$ 10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s (for visual extinction: $A_V$ $\le$ 10 mag) and 1.4 $\times$ 10$^{21}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s (for $A_V$ of 10-100 mag). The column density ratio of the [CI] to CO ($N_{\rm [CI]}/N_{\rm CO}$) was derived as $\sim$ 0.1 for $A_V$ of 10-100 mag, which is consistent with that of the Orion cloud presented in Ikeda et al. (2002). However, our results cover an $A_V$ regime of up to 100 mag, which is wider than the coverage found in Orion, which reach up to $\sim$ 60 mag. Such a high [CI]/CO ratio in a high $A_V$ region is difficult to be explained by the plane-parallel photodissociation region (PDR) model, which predicts that this ratio is close to 0 due to the heavy shielding of the ultraviolet (UV) radiation. Our results suggest that the molecular gas in this cluster is highly clumpy, allowing deep penetration of UV radiation even at averaged $A_V$ values of 100 mag. Recent theoretical works have presented models consistent with such clumped gas distribution with a sub-pc clump size (e.g., Tachihara et al. 2018).

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arXiv2020

Massive star formation in the Carina nebula complex and Gum 31 -- I. The Carina nebula complex

Herein, we present results from observations of the 12CO (J=1-0), 13CO (J=1-0), and 12CO (J=2-1) emission lines toward the Carina nebula complex (CNC) obtained with the Mopra and NANTEN2 telescopes. We focused on massive-star-forming regions associated with the CNC including the three star clusters Tr14, Tr15, and Tr16, and the isolated WR-star HD92740. We found that the molecular clouds in the CNC are separated into mainly four clouds at velocities -27, -20, -14, and -8 km/s. Their masses are 0.7x10^4Msun, 5.0x10^4 Msun, 1.6x10^4 Msun, and 0.7x10^4 Msun, respectively. Most are likely associated with the star clusters, because of their high 12CO (J=2-1)/12CO (J=1-0) intensity ratios and their correspondence to the Spitzer 8 micron distributions. In addition, these clouds show the observational signatures of cloud--cloud collisions. In particular, there is a V-shaped structure in the position--velocity diagram and a complementary spatial distribution between the -20 km/s cloud and the -14 km/s cloud. Based on these observational signatures, we propose a scenario wherein the formation of massive stars in the clusters was triggered by a collision between the two clouds. By using the path length of the collision and the assumed velocity separation, we estimate the timescale of the collision to be ~1 Myr. This is comparable to the ages of the clusters estimated in previous studies.

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arXiv2020

ALMA Observations of Giant Molecular Clouds in M33. II. Triggered High-mass Star Formation by Multiple Gas Colliding Events at the NGC 604 Complex

We present the results of ALMA observations in $^{12}$CO($J=2-1$), $^{13}$CO($J=2-1$), and C$^{18}$O($J=2-1$) lines and 1.3 mm continuum emission toward a massive ($\sim 10^6 M_{\odot}$) giant molecular cloud associated with the giant H II region NGC 604 in one of the nearest spiral galaxy M33 at an angular resolution of 0''.44 $\times$ 0''.27 (1.8 pc $\times$ 1.1 pc). The $^{12}$CO and $^{13}$CO images show highly complicated molecular structures composed of a lot of filaments and shells whose lengths are 5 -- 20 pc. We found three 1.3 mm continuum sources as dense clumps at edges of two shells and also at an intersection of several filaments. We examined the velocity structures of $^{12}$CO($J=2-1$) emission in the shells and filaments containing dense clumps, and concluded that expansion of the H II regions cannot explain the formation of such dense cores. Alternatively, we suggest that cloud--cloud collisions induced by an external H I gas flow and the galactic rotation compressed the molecular material into dense filaments/shells as ongoing high-mass star formation sites. We propose that multiple gas converging/colliding events with a velocity of a few tens km s$^{-1}$ are necessary to build up NGC 604, the most significant cluster-forming complex in the Local Group of galaxies.

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arXiv2020

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN): Possible evidence of cloud-cloud collisions triggering high-mass star formation in the giant molecular cloud M16 (Eagle Nebula)

M16, the Eagle Nebula, is an outstanding \HII \ region which exhibits extensive high-mass star formation and hosts remarkable "pillars". We herein obtained new $^{12}$CO $J=$1-0 data for the region observed with NANTEN2, which were combined with the $^{12}$CO $J=$1-0 data obtained using FUGIN survey. These observations revealed that a giant molecular cloud (GMC) of $\sim 1.3 \times 10^5$ \Msun \ is associated with M16, which is elongated by over 30 pc and is perpendicular to the galactic plane, at a distance of 1.8 kpc. This GMC can be divided into the northern (N) cloud, the eastern (E) filament, the southeast (SE) cloud, the southeast (SE) filament, and the southern (S) cloud. We also found two velocity components (blue and red shifted component) in the N cloud. The blue-shifted component shows a ring-like structure, as well as the red-shifted component coincides with the intensity depression of the ring-like structure. The position-velocity diagram of the components showed a V-shaped velocity feature. The spatial and velocity structures of the cloud indicated that two different velocity components collided with each other at a relative velocity of 11.6 \kms. The timescale of the collision was estimated to be $\sim 4 \times 10^5$ yr. The collision event reasonably explains the formation of the O9V star ALS15348, as well as the shape of the Spitzer bubble N19. A similar velocity structure was found in the SE cloud, which is associated with the O7.5V star HD168504. In addition, the complementary distributions of the two velocity components found in the entire GMC suggested that the collision event occurred globally. On the basis of the above results, we herein propose a hypothesis that the collision between the two components occurred sequentially over the last several $10^{6}$ yr and triggered the formation of O-type stars in the NGC6611 cluster.

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arXiv2020

ALMA CO Observations of a Giant Molecular Cloud in M33: Evidence for High-Mass Star Formation Triggered by Cloud-Cloud Collisions

We report the first evidence for high-mass star formation triggered by collisions of molecular clouds in M33. Using the Atacama Large Millimeter/submillimeter Array, we spatially resolved filamentary structures of giant molecular cloud 37 in M33 using $^{12}$CO($J$ = 2-1), $^{13}$CO($J$ = 2-1), and C$^{18}$O($J$ = 2-1) line emission at a spatial resolution of $\sim$2 pc. There are two individual molecular clouds with a systematic velocity difference of $\sim$6 km s$^{-1}$. Three continuum sources representing up to $\sim$10 high-mass stars with the spectral types of B0V-O7.5V are embedded within the densest parts of molecular clouds bright in the C$^{18}$O($J$ = 2-1) line emission. The two molecular clouds show a complementary spatial distribution with a spatial displacement of $\sim$6.2 pc, and show a V-shaped structure in the position-velocity diagram. These observational features traced by CO and its isotopes are consistent with those in high-mass star-forming regions created by cloud-cloud collisions in the Galactic and Magellanic Cloud HII regions. Our new finding in M33 indicates that the cloud-cloud collision is a promising process to trigger high-mass star formation in the Local Group.

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arXiv2020

ALMA Observations of Giant Molecular Clouds in M33 I: Resolving Star Formation Activities in the Giant Molecular Filaments Possibly Formed by a Spiral Shock

We report molecular line and continuum observations toward one of the most massive giant molecular clouds (GMCs), GMC-16, in M33 using ALMA with an angular resolution of 0$''$44 $\times$ 0$''$27 ($\sim$2 pc $\times$ 1 pc). We have found that the GMC is composed of several filamentary structures in $^{12}$CO and $^{13}$CO ($J$ = 2-1). The typical length, width, and total mass are $\sim$50-70 pc, $\sim$5-6 pc, and $\sim$10$^{5}$ $M_{\odot}$, respectively, which are consistent with those of giant molecular filaments (GMFs) as seen in the Galactic GMCs. The elongations of the GMFs are roughly perpendicular to the direction of the galaxy's rotation, and several H$\;${\sc ii} regions are located at the downstream side relative to the filaments with an offset of $\sim$10-20 pc. These observational results indicate that the GMFs are considered to be produced by a galactic spiral shock. The 1.3 mm continuum and C$^{18}$O ($J$ = 2-1) observations detected a dense clump with the size of $\sim$2 pc at the intersection of several filamentary clouds, which is referred to as the $"$hub filament,$"$ possibly formed by a cloud-cloud collision. A strong candidate for protostellar outflow in M33 has also been identified at the center of the clump. We have successfully resolved the parsec-scale local star formation activity in which the galactic scale kinematics may induce the formation of the parental filamentary clouds.

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arXiv2020

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

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.

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arXiv2020

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). VI. Dense gas and mini-starbursts in the W43 giant molecular cloud complex

We performed new large-scale $^{12}$CO, $^{13}$CO, and C$^{18}$O $J=$1--0 observations of the W43 giant molecular cloud complex in the tangential direction of the Scutum arm ($l\sim {30^\circ}$) as a part of the FUGIN project. The low-density gas traced by $^{12}$CO is distributed over 150 pc $\times$ 100 pc ($l \times b$), and has a large velocity dispersion (20-30 km s$^{-1}$). However, the dense gas traced by C$^{18}$O is localized in the W43 Main, G30.5, and W43 South (G29.96-0.02) high-mass star-forming regions in the W43 GMC complex, which have clumpy structures. We found at least two clouds with a velocity difference of $\sim$ 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 $^{13}$CO $J=$ 3-2 to $J =$ 1-0 intensity ratio and morphological correspondence with the infrared dust emission. The velocity separation of these clouds in W43 Main, G30.5, and W43 South is too large for each cloud to be gravitationally bound. We also revealed that the dense gas in the W43 GMC has a high local column density, while "the current SFE" of entire the GMC is low ($\sim 4\%$) compared with the W51 and M17 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 W43 GMC complex.

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arXiv2020

High-mass star formation in Orion B triggered by cloud-cloud collision: Merging molecular clouds in NGC 2024

We performed new comprehensive $^{13}$CO($J$=2--1) observations toward NGC 2024, the most active star forming region in Orion B, with an angular resolution of $\sim$100'' obtained with NANTEN2. We found that the associated cloud consists of two independent velocity components. The components are physically connected to the H{\sc ii} region as evidenced by their close correlation with the dark lanes and the emission nebulosity. The two components show complementary distribution with a displacement of $\sim$0.6 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that a cloud-cloud collision between the two components triggered the formation of the late O-type stars and early B stars localized within 0.3 pc of the cloud peak. The duration time of the collision is estimated to be 0.3 million years from a ratio of the displacement and the relative velocity $\sim$3 km s$^{-1}$ corrected for probable projection. The high column density of the colliding cloud $\sim$10$^{23}$ cm$^{-2}$ is similar to those in the other high-mass star clusters in RCW 38, Westerlund 2, NGC 3603, and M42, which are likely formed under trigger by cloud-cloud collision. The present results provide an additional piece of evidence favorable to high-mass star formation by a major cloud-cloud collision in Orion.

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arXiv2020

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). VII. molecular fraction of HI clouds

In this study, we analyze molecular gas formation in neutral atomic hydrogen (HI) clouds using the latest CO data obtained from the four-beam receiver system on a 45-m telescope (FOREST) unbiased Galactic plane imaging survey with the Nobeyama 45-m telescope (FUGIN) and HI data taken from the Very Large Array (VLA) Galactic plane survey (VGPS). We applied a dendrogram algorithm to the HI data cube to identify HI clouds, and we calculated the HI mass and molecular gas mass by summing the CO line intensity within each HI cloud. On the basis of the results, we created a catalog of 5,737 identified HI clouds with local standard of rest (LSR) velocity of $V_{\rm LSR}\le -20$ km s$^{-1}$ in Galactic longitude and latitude ranges of $20^\circ \le l \le 50^\circ$ and $-1^\circ \le b \le 1^\circ$, respectively. We found that most of the HI clouds are distributed within a Galactocentric distance of 16 kpc, most of which are in the Cold Neutral Medium (CNM) phase. In addition, we determined that the high-mass end of the mass HI function is well fitted with the power-law function with an index of 2.3. Although two sequences of self-gravitating and diffuse clouds are expected to appear in the M$_{\rm tot}$-M$_{{\rm H}_2}$ diagram according to previous works based on a plane-parallel model, the observational data show only a single sequence with large scattering within these two sequences. This implies that most of the clouds are mixtures of these two types of clouds. Moreover, we suggest the following scenario of molecular gas formation: An HI-dominant cloud evolved with increasing H$_2$ mass along a path of $M_{{\rm H}_2} \propto M_{\rm tot}^2$ by collecting diffuse gas before reaching and moving along the curves of the two sequences.

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arXiv2020

Molecular and Atomic Clouds toward the Wolf-Rayet Nebula NGC 2359: Possible Evidence for Isolated High-Mass Star Formation Triggered by a Cloud-Cloud Collision

NGC 2359 is an HII region located in the outer Galaxy that contains the isolated Wolf-Rayet (WR) star HD 56925. We present millimeter/submillimeter observations of $^{12}$CO($J$ = 1-0, 3-2) line emission toward the entire nebula. We identified that there are three molecular clouds at VLSR $\sim$37, $\sim$54, and $\sim$67 km s$^{-1}$, and three HI clouds: two of them are at VLSR $\sim$54 km s$^{-1}$ and the other is at $\sim$63 km s$^{-1}$. These clouds except for the CO cloud at 67 km s$^{-1}$ are limb-brightened in the radio continuum, suggesting part of each cloud has been ionized. We newly found an expanding gas motion of CO/HI, whose center and expansion velocities are $\sim$51 and $\sim$4.5 km s$^{-1}$, respectively. This is consistent with large line widths of the CO and HI clouds at 54 km s$^{-1}$. The kinematic temperature of CO clouds at 37 and 54 km s$^{-1}$ are derived to be 17 and 61 K, respectively, whereas that of the CO cloud at 67 km s$^{-1}$ is only 6 K, indicating that the former two clouds have been heated by strong UV radiation. We concluded that the 37 and 54 km s$^{-1}$ CO clouds and three HI clouds are associated with NGC 2359, even if these clouds have different velocities. Although the velocity difference including the expanding motion are typical signatures of the stellar feedback from the exciting star, our analysis revealed that the observed large momentum for the 37 km s$^{-1}$ CO cloud cannot be explained only by the total wind momentum of the WR star and its progenitor. We therefore propose an alternative scenario that the isolated high-mass progenitor of HD 56925 was formed by a collision between the CO clouds at 37 and 54 km s$^{-1}$. If we apply the collision scenario, NGC 2359 corresponds to the final phase of the cloud-cloud collision.

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arXiv2019

High-mass star formation in Orion possibly triggered by cloud-cloud collision III, NGC2068 and NGC2071

Using the NANTEN2 Observatory, we carried out a molecular line study of high-mass star forming regions with reflection nebulae, NGC 2068 and NGC 2071, in Orion in the 13CO(J=2-1) transition. The 13CO distribution shows that there are two velocity components at 9.0 and 10.5 km/s . The blue-shifted component is in the northeast associated with NGC 2071, whereas the red-shifted component is in the southwest associated with NGC 2068. The total intensity distribution of the two clouds shows a gap of ~1 pc, suggesting that they are detached at present. A detailed spatial comparison indicates that the two show complementary distributions. The blue-shifted component lies toward an intensity depression to the northwest of the red-shifted component, where we find that a displacement of 0.8 pc makes the two clouds fit well with each other. Furthermore, a new simulation of non-frontal collisions shows that observations from 60 degrees off the collisional axis agreed well with the velocity structure in this region. On the basis of these results, we hypothesize that the two components collided with each other at a projected relative velocity 3.0 km/s estimated to be 0.3 Myr for an assumed axis of the relative motion 60 degrees off the line of sight. We assume that the two most massive early B-type stars in the cloud, illuminating stars of the two reflection nebulae, were formed by collisional triggering at the interfaces between the two clouds. Given the other young high-mass star forming regions, namely, M42, M43, and NGC 2024 (Fukui et al. 2018b; Ohama et al. 2017a), it seems possible that collisional triggering has been independently working to form O-type and early B-type stars in Orion in the last Myr over a projected distance of ~80 pc.

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arXiv2019

Cluster formation induced by a cloud--cloud collision in [DBS2003]179

[DBS2003]179 is a super star cluster in the Galaxy discovered by deep near infrared observations. We carried out CO J=1-0 and J=3-2 observations of the region of [DBS2003]179 with NANTEN2, ASTE and the Mopra 22m telescope. We identified and mapped two molecular clouds which are likely associated with the cluster. The association is evidenced by the spatial correlation with the 8 micron Spitzer image, and a high ratio of the two transitions of 12CO (J=3-2 to J=1-0). The two clouds show complementary distribution in space and bridge features connecting them in velocity. We frame a hypothesis that the two clouds collided with each other 1-2 Myr ago, and the collision compressed the interface layer, triggering the formation of the cluster. This offers an additional piece of evidence for a super star cluster formed by cloud--cloud collision alongside of the four super star clusters including Wd2, NGC3603, RCW38 and R136. The findings indicate that the known super star clusters having closely associated dust emission are formed by cloud-cloud collision, lending support for the important role of cloud--cloud collision in high-mass star formation.

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arXiv2019

Nobeyama 45-m Cygnus X CO Survey: II Physical Properties of $\mathrm{C^{18}O}$ Clumps

We report the statistical physical properties of the C$^{18}$O($J=1-0$) clumps present in a prominent cluster-forming region, Cygnus X, using the dataset obtained by the Nobeyama 45-m radio telescope. This survey covers 9 deg$^2$ of the north and south regions of Cygnus X, and totally 174 C$^{18}$O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2-1 pc, velocity dispersions of $<2.2~\mathrm{km~s^{-1}}$, gas masses of 30-3000 $M_\odot$, and H$_2$ densities of (0.2-5.5)$\times10^4~\mathrm{cm^{-3}}$. We confirm that the C$^{18}$O clumps in the north region have a higher H$_2$ density than those in the south region, supporting the existence of a difference in the evolution stages, consistent with the star formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H$_2$ density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C$^{18}$O clump mass function shows two spectral index components, $α=-1.4$ in 55-140 $M_\odot$ and $α=-2.1$ in $>140~M_\odot$, which are consistent with the low- and intermediate-mass parts of the Kroupa's initial mass function. The spectral index in the star-forming clumps in $>140~M_\odot$ is consistent with that of the starless clumps in 55-140 $M_\odot$, suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star formation efficiency of molecular clumps (10%), about ten C$^{18}$O clumps having a gas mass of $>10^3~M_\odot$ will evolve into open clusters containing one or more OB stars.

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arXiv2019

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) V: Dense gas mass fraction of molecular gas in the Galactic plane

Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds have quasi-universal properties on star formation, and observational studies of extra galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of the dense gas in molecular clouds f_DG. In particular, for the Milky Way (MW), there are no previous studies resolving the f_DG disk over a scale of several kpc. In this study, the f_DG was measured over 5kpc in the first quadrant of the MW, based on the CO J=1-0 data in l=10-50 deg obtained as part of the FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses including f_DG in the region within ~30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged f_DG of 2.9^{+2.6}_{-2.6} % was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor of inefficient star formation in galaxies. It was also found that the f_DG shows large variations depending on the structures in the MW disk. The f_DG in the Galactic arms were estimated to be ~4-5%, while those in the bar and inter-arm regions were as small as ~0.1-0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to the differences in SFRs.

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arXiv2019

Massive star formation in W51A triggered by cloud-cloud collisions

W51A is one of the most active star-forming region in our Galaxy, which contains giant molecular clouds with a total mass of 10^6 Msun. The molecular clouds have multiple velocity components over ~20 km/s, and interactions between these components have been discussed as the mechanism which triggered the massive star formation in W51A. In this paper, we report an observational study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O (J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the Nobeyama 45-m telescope at 20" resolution. Our CO data resolved the four discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of ~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column densities of higher than 10^23 cm^-3, which are each embedded within the four velocity clouds. These four clumps are distributed close to each others within a small distance of 5 pc, showing a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each others by bridge features with intermediate intensities. The high intensity ratios of 13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with the HII regions. We also found these features in other HII regions in W51A. The timescales of the collisions are estimated to be several 0.1 Myrs as a crossing time of the clouds, which are consistent with the ages of the HII regions measured from the size of the HII regions in the 21 cm continuum emissions. We discuss the cloud-cloud collision scenario and massive star formation in W51A by comparing with the recent observational and theoretical studies of cloud-cloud collision.

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arXiv2018

Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud collisions as a trigger of massive star formation

Herein, we present the 12CO (J=1-0) and 13CO (J=1-0) emission line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) toward a Spitzer bubble N4. We observed clouds of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4 Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the 25-km/s cloud likely traces the ring-like structure observed at mid-infrared wavelength. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary distribution, which are discussed as observational signatures of a cloud-cloud collision, between the 16- and 25-km/s clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds; however whereas the 19-km/s cloud is possibly not a part of the interaction with N4. The time scale of collision is estimated to be 0.2-0.3 Myr, which is comparable to the estimated dynamical age of the HII region of ~0.4 Myr. In N4W, a star-forming clump located west of N4, we observed molecular outflows from young stellar objects and the observational signature of a cloud-cloud collision. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a cloud-cloud collision in N4W.

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arXiv2018

CO observations toward the isolated mid-infrared bubble S44: External triggering of O-star formation by a cloud-cloud collision

We have performed a multi-wavelength study of the mid-infrared bubble S44 to investigate the origin of isolated high-mass star(s) and the star-formation process around the bubble formed by the HII region. In this paper, we report the results of new CO observations ($^{12}$CO, $^{13}$CO $J=$1-0, and $^{12}$CO $J=$3-2) toward the isolated bubble S44 using the NANTEN2, Mopra, and ASTE radio telescopes. We found two velocity components in the direction of the bubble, at $-84$ km s$^{-1}$ and $-79$ km s$^{-1}$. These two clouds are likely to be physically associated with the bubble,both because of the enhanced $^{12}$CO $J=$3-2/1-0 intensity ratio from a ring-like structure affected by ultraviolet radiation from embedded high-mass star(s) and from the morphological correspondence between the 8 $μ$m emission and the CO distribution. Assuming a single object, we estimate the spectral type of the embedded star inside the bubble to be O8.5-9 ($\sim 20 M_{\odot}$) from the radio-continuum free-free emission. We hypothesize that the two clouds collided with each other 3 Myr ago, triggering the formation of the isolated high-mass star in S44, as also occurred with RCW 120 and RCW 79. We argue that this scenario can explain the origin of the isolated O-star inside the bubble.

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