arXiv subjects

Kengo Tachihara

Publications and source records attributed to Kengo Tachihara.

At least 91 records · Page 5Linked to original sources

arXiv2018

RCW 36 in the Vela Molecular Ridge: Evidence for a high-mass star cluster formation triggered by Cloud-Cloud Collision

A collision between two molecular clouds is one possible candidate for high-mass star formation. The HII region RCW 36, located in the Vela molecular ridge, contains a young star cluster with two O-type stars. We present new CO observations of RCW 36 with NANTEN2, Mopra, and ASTE using $^{12}$CO($J$ = 1-0, 2-1, 3-2) and $^{13}$CO($J$ = 2-1) line emissions. We have discovered two molecular clouds lying at the velocities $V_\mathrm{LSR} \sim$5.5 and 9 km s$^{-1}$. Both clouds are likely to be physically associated with the star cluster, as verified by the good spatial correspondence among the two clouds, infrared filaments, and the star cluster. We also found a high intensity ratio of $\sim$0.6-1.2 for CO $J$ = 3-2 / 1-0 toward both clouds, indicating that the gas temperature has been increased due to heating by the O-type stars. We propose that the O-type stars in RCW 36 were formed by a collision between the two clouds, with a relative velocity separation of 5 km s$^{-1}$. The complementary spatial distributions and the velocity separation of the two clouds are in good agreement with observational signatures expected for O-type star formation triggered by a cloud-cloud collision. We also found a displacement between the complementary spatial distributions of the two clouds, which we estimate to be 0.3 pc assuming the collision angle to be 45$^{\circ}$ relative to the line-of-sight. We estimate the collision timescale to be $\sim$10$^5$ yr. It is probable that the cluster age by Ellerbroek et al. (2013b) is dominated by the low-mass members which were not formed under the triggering by cloud-cloud collision, and that the O-type stars in the center of the cluster are explained by the collisional triggering independently from the low-mass star formation.

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arXiv2018

Molecular clouds in the NGC 6334 and NGC 6357 region; Evidence for a 100 pc-scale cloud-cloud collision triggering the Galactic mini-starbursts

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 HII 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 $\sim$ 100 pc. We suggest that the formation of the starbursts happened toward the collisional region of $\sim$ 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 HII 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.

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arXiv2018

Detailed CO ($J$ = 1--0, 2--1 and 3--2) observations toward an H{\sc ii} region RCW~32 in the Vela Molecular Ridge

We made CO ($J$ = 1--0, 2--1, and 3--2) observations toward an H{\sc ii} region RCW~32 in the Vela Molecular Ridge. The CO gas distribution associated with the H{\sc ii} region was revealed for the first time at a high resolution of 22 arcsec. The results revealed three distinct velocity components which show correspondence with the optical dark lanes and/or H$α$ distribution. Two of the components show complementary spatial distribution which suggests collisional interaction between them at a relative velocity of $\sim$4 km~s$^{-1}$. Based on these results, we present a hypothesis that cloud-cloud collision determined the cloud distribution and triggered formation of the exciting star ionizing RCW~32. The collision time scale is estimated from the cloud size and the velocity separation to be $\sim$2 Myrs and the collision terminated $\sim$1 Myr ago, which is consistent with an age of the exciting star and the associated cluster. By combing the previous works on the H{\sc ii} regions in the Vela Molecular Ridge, we argue that the majority, at least four, of the H{\sc ii} regions in the Ridge were formed by triggering of cloud-cloud collision.

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arXiv2017

Star formation in the Sh 2-53 region influenced by accreting molecular filaments

We present a multi-wavelength analysis of a $\sim$30$' \times$30$'$ area around the Sh 2-53 region (hereafter S53 complex), which is associated with at least three H II regions, two mid-infrared bubbles (N21 and N22), and infrared dark clouds. The $^{13}$CO line data trace the molecular content of the S53 complex in a velocity range of 36--60 km s$^{-1}$, and show the presence of at least three molecular components within the selected area along this direction. Using the observed radio continuum flux of the H II regions, the derived spectral types of the ionizing sources agree well with the previously reported results. The S53 complex harbors clusters of young stellar objects (YSOs) that are identified using the photometric 2--24 $μ$m magnitudes. It also hosts several massive condensations (3000-30000 $M_\odot$) which are traced in the {\em Herschel} column density map. The complex is found at the junction of at least five molecular filaments, and the flow of gas toward the junction is evident in the velocity space of the $^{13}$CO data. Together, the S53 complex is embedded in a very similar "hub-filament" system to those reported in Myers, and the active star formation is evident towards the central "hub" inferred by the presence of the clustering of YSOs.

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arXiv2017

Formation of the young compact cluster GM 24 triggered by a cloud-cloud collision

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 $\sim$100 lower mass stars within a 0.3 pc radius toward $(l,b)\sim$ (350.$^{\circ}$5, 0.$^{\circ}$96), close to the Galactic min-starburst NGC 6334. We found two velocity components associated with the cluster by new observations of $^{12}$CO $J=$ 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 which fits 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 $\sim$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 a major mechanism of high-mass star formation in the Carina-Sagittarius Arm.

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arXiv2017

High-mass star formation possibly triggered by cloud-cloud collision in the HII region RCW 34

We report a possibility that the high-mass star located in the HII region RCW 34 was formed by a triggering induced by a collision of molecular clouds. Molecular gas distributions of the $^{12}$CO and $^{13}$CO $J=$2-1, and $^{12}$CO $J=$3-2 lines toward RCW 34 were measured by using the NANTEN2 and ASTE telescopes. We found two clouds with the velocity ranges of 0-10 km s$^{-1}$ and 10-14 km s$^{-1}$. Whereas the former cloud as massive as ~2.7 x 10$^{4}$ Msun has a morphology similar to the ring-like structure observed in the infrared wavelengths, the latter cloud with the mass of ~10$^{3}$ Msun, which has not been recognized by previous observations, distributes just likely to cover the bubble enclosed by the other cloud. The high-mass star with the spectral types of O8.5V is located near the boundary of the two clouds. The line intensity ratio of $^{12}$CO $J=$3-2 / $J=$2-1 yields high values (~1.5) in the neighborhood of the high-mass star, suggesting that these clouds are associated with the massive star. We also confirmed that the obtained position-velocity diagram shows a similar distribution with that derived by a numerical simulation of the supersonic collision of two clouds. Using the relative velocity between the two clouds (~5 km s$^{-1}$), the collisional time scale is estimated to be $\sim$0.2 Myr with the assumption of the 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 cloud-cloud collision.

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arXiv2017

A Detached Protostellar Disk around a $\sim$0.2$M_{\odot}$ protostar in a Possible Site of a Multiple Star Formation in a Dynamical Environment in Taurus

We report ALMA observations in 0.87 mm continuum and $^{12}$CO ($J$ = 3--2) toward a very low-luminosity ($<$0.1 $L_{\odot}$) protostar, which is deeply embedded in one of the densest core MC27/L1521F, in Taurus with an indication of multiple star formation in a highly dynamical environment. The beam size corresponds to $\sim$20 AU, and we have clearly detected blueshifted/redshifted gas in $^{12}$CO associated with the protostar. The spatial/velocity distributions of the gas show there is a rotating disk with a size scale of $\sim$10 AU, a disk mass of $\sim$10$^{-4}$ $M_{\odot}$ and a central stellar mass of $\sim$0.2 $M_{\odot}$. The observed disk seems to be detached from the surrounding dense gas, although it is still embedded at the center of the core whose density is $\sim$10$^{6}$ cm$^{-3}$. The current low-outflow activity and the very low luminosity indicate that the mass accretion rate onto the protostar is extremely low in spite of a very early stage of star formation. We may be witnessing the final stage of the formation of $\sim$0.2 $M_{\odot}$ protostar. However, we cannot explain the observed low luminosity with the standard pre-main-sequence evolutionary track unless we assume cold accretion with an extremely small initial radius of the protostar ($\sim$0.65 $R_\odot$). These facts may challenge our current understanding of the low mass star formation, in particular the mass accretion process onto the protostar and the circumstellar disk.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) : Molecular clouds toward W33 ; possible evidence for a cloud-cloud collision triggering O star formation

We observed molecular clouds in the W33 high-mass star-forming region associated with compact and extended HII regions using the NANTEN2 telescope as well as the Nobeyama 45-m telescope in the $J=$1-0 transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) legacy survey. We detected three velocity components at 35 km s$^{-1}$, 45 km s$^{-1}$, and 58 km s$^{-1}$. The 35 km s$^{-1}$ and 58 km s$^{-1}$ clouds are likely to be physically associated with W33 because of the enhanced $^{12}$CO $J=$ 3-2 to $J=$1-0 intensity ratio as $R_{\rm 3-2/1-0} > 1.0$ due to the ultraviolet irradiation by OB stars, and morphological correspondence between the distributions of molecular gas and the infrared and radio continuum emissions excited by high-mass stars. The two clouds show complementary distributions around W33. The velocity separation is too large to be gravitationally bound, and yet not explained by expanding motion by stellar feedback. Therefore, we discuss that a cloud-cloud collision scenario likely explains the high-mass star formation in W33.

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arXiv2017

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results

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 12CO, 13CO, and C18O J=1-0 lines simultaneously. The mapping regions are a part of the 1st quadrant (10d < l < 50d, |b| < 1d) and the 3rd quadrant (198d < l <236d, |b| < 1d) 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.

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arXiv2017

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

We present $^{12}$CO $J=$1--0, $^{13}$CO $J=$1--0 and C$^{18}$O $J=$1--0 images of the M17 giant molecular clouds obtained as part of FUGIN (FOREST Ultra-wide Galactic Plane Survey InNobeyama) project. The observations cover the entire area of M17 SW and M17 N clouds at the highest angular resolution ($\sim$19$"$) to date which corresponds to $\sim$ 0.15 pc at the distance of 2.0 kpc. We find that the region consists of four different velocity components: very low velocity (VLV) clump, low velocity component (LVC), main velocity component (MVC), and high velocity component (HVC). The LVC and the HVC have cavities. UV photons radiated from NGC 6618 cluster penetrate into the N cloud up to $\sim$ 5 pc through the cavities and interact with molecular gas. This interaction is correlated with the distribution of YSOs in the N cloud. The LVC and the HVC are distributed complementary after that the HVC is displaced by 0.8 pc toward the east-southeast direction, suggesting that collision of the LVC and the HVC create the cavities in both clouds. The collision velocity and timescale are estimated to be 9.9 km s$^{-1}$ and $1.1 \times 10^{5}$ yr, respectively. The high collision velocity can provide the mass accretion rate up to 10$^{-3}$ $M_{\solar}$ yr$^{-1}$, and the high column density ($4 \times 10^{23}$ cm$^{-2}$) might result in massive cluster formation. The scenario of cloud-cloud collision likely well explains the stellar population and its formation history of NGC 6618 cluster proposed by Hoffmeister et al. (2008).

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arXiv2017

A new view of the giant molecular cloud M16 (Eagle Nebula) in 12CO J=1-0 and 2-1 transitions with NANTEN2

M16, the Eagle Nebula, is an outstanding HII region where extensive high-mass star formation is taking place in the Sagittarius Arm, and hosts the remarkable "pillars" observed with HST. We made new CO observations of the region in the 12CO J=1--0 and J=2--1 transitions with NANTEN2. These observations revealed for the first time that a giant molecular cloud of $\sim 1.3 \times 10^5$ \Msun \ is associated with M16, which is elongated vertically to the Galactic plane over 35 pc at a distance of 1.8 kpc. We found a cavity of the molecular gas of $\sim 10$ pc diameter toward the heart of M16 at \lbeq (16.95\degree, 0.85\degree), where more than 10 O-type stars and $\sim 400$ stars are associated, in addition to a close-by molecular cavity toward a Spitzer bubble N19 at \lbeq (17.06\degree, 1.0\degree). We found three velocity components which show spatially complementary distribution in the entire M16 giant molecular cloud (GMC) including NGC6611 and N19, suggesting collisional interaction between them. Based on the above results we frame a hypothesis that collision between the red-shifted and blue-shifted components at a relative of $\sim 10$ \kms \ triggered formation of the O-type stars in the M16 GMC in the last 1-2 Myr. The collision is two fold in the sense that one of the collisional interactions is major toward the M16 cluster and the other toward N19 with a RCW120 type, the former triggered most of the O star formation with almost full ionization of the parent gas, and the latter an O star formation in N19.

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arXiv2017

High-mass star formation in Orion triggered by cloud-cloud collision II, Two merging molecular clouds in NGC2024

We analyzed the NANTEN2 13CO (J=2-1 and 1-0) datasets in NGC 2024. We found that the cloud consists of two velocity components, whereas the cloud shows mostly single-peaked CO profiles. The two components are physically connected to the HII 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 0.4 pc. Such complementary distribution is typical to colliding clouds discovered in regions of high-mass star formation. We hypothesize that cloud-cloud collision between the two components triggered the formation of the late O stars and early B stars localized within 0.3 pc of the cloud peak. The collision timescale is estimated to be ~ 10^5 yrs from a ratio of the displacement and the relative velocity 3-4 km s-1 corrected for probable projection. The high column density of the colliding cloud 1023 cm-2 is similar to those in the other massive 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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arXiv2017

A massive molecular outflow in the dense dust core AGAL G337.916-00.477

Massive molecular outflows erupting from high-mass young stellar objects provide important clues to understanding the mechanism of high-mass star formation. Based on new CO J=3-2 and J=1-0 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and Mopra telescope facilities, we discovered a massive bipolar outflow associated with the dense dust core AGALG337.916-00.477 (AGAL337.9-S), located 3.48 kpc from the Sun. The outflow lobes have extensions of less than 1 pc -and thus were not fully resolved in the angular resolutions of ASTE and Mopra- and masses of 35-40 M_sun. The maximum velocities of the outflow lobes are as high as 35-40 km/s. Our analysis of the infrared and sub-mm data indicates that AGAL337.9-S is in an early evolutionary stage of the high-mass star formation, having the total far-infrared luminosity of ~5x10^4 L_sun. We also found that another dust core AGALG337.922-00.456 (AGAL337.9-N) located 2' north of AGAL337.9-S is a high-mass young stellar object in an earlier evolutional stage than AGAL337.9-S, although it is less bright in the mid-infrared than AGAL337.9-S.

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arXiv2017

Formation of the Young Massive Cluster R136 triggered by Tidally-driven Colliding HI Flows

Understanding of massive cluster formation is one of the important issues of astronomy. By analyzing the HI data, we have identified that the two HI velocity components (L- and D-components) are colliding toward the HI Ridge, in the southeastern end of the LMC, which hosts the young massive cluster R136 and $\sim$400 O/WR stars (Doran et al. 2013) including the progenitor of SN1987A. The collision is possibly evidenced by bridge features connecting the two HI components and complementary distributions between them. We frame a hypothesis that the collision triggered the formation of R136 and the surrounding high-mass stars as well as the HI & Molecular Ridge. Fujimoto & Noguchi (1990) advocated that the last tidal interaction between the LMC and the SMC about 0.2 Gyr ago induced collision of the L- and D-components. This model is consistent with numerical simulations (Bekki & Chiba 2007b). We suggest that a dense HI partly CO cloud of 10$^{6}$ $M_{\odot}$, a precursor of R136, was formed at the shock-compressed interface between the colliding L- and D-components. We suggest that part of the low-metalicity gas from the SMC was mixed in the tidal interaction based on the $Planck/IRAS$ data of dust optical depth (Planck Collaboration et al. 2014).

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arXiv2017

HI, CO, and Dust in the Perseus Cloud

Comparison analyses between the gas emission data (HI 21cm line and CO 2.6 mm line) and the Planck/IRAS dust emission data (optical depth at 353 GHz tau353 and dust temperature Td) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically-thick HI gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that tau353 increases as a function of the 1.3-th power of column number density of the total hydrogen (NH), and this implies dust evolution in high density regions. This calibrated tau353-NH relationship shows that the amount of the HI gas can be underestimated to be ~60% if the optically-thin HI method is used. Based on this relationship, we calculated optical depth of the 21 cm line (tauHI), and found that ~ 0.92 around the molecular cloud. The effect of tauHI is still significant even if we take into account the dust evolution. We also estimated a spatial distribution of the CO-to-H2 conversion factor (XCO), and we found its average value is ~ 1.0x10^20 cm-2 K-1 km-1 s. Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.

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arXiv2016

Triggered O star formation in M20 via cloud-cloud collision: Comparisons between high-resolution CO observations and simulations

High-mass star formation is one of the top-priority issues in astrophysics. Recent observational studies are revealing that cloud-cloud collisions may play a role in high-mass star formation in several places in the Milky Way and the Large Magellanic Cloud. The Trifid Nebula M20 is a well known galactic HII region ionized by a single O7.5 star. In 2011, based on the CO observations with NANTEN2 we reported that the O star was formed by the collision between two molecular clouds ~0.3,Myr ago. Those observations identified two molecular clouds towards M20, traveling at a relative velocity of 7.5 km/s. This velocity separation implies that the clouds cannot be gravitationally bound to M20, but since the clouds show signs of heating by the stars there they must be spatially coincident with it. A collision is therefore highly possible. In this paper we present the new CO J=1-0 and J=3-2 observations of the colliding clouds in M20 performed with the Mopra and ASTE telescopes. The high resolution observations revealed the two molecular clouds have peculiar spatial and velocity structures, i.e., the spatially complementary distribution between the two clouds and the bridge feature which connects the two clouds in velocity space. Based on a new comparison with numerical models, we find that this complementary distribution is an expected outcome of cloud-cloud collisions, and that the bridge feature can be interpreted as the turbulent gas excited at the interface of the collision. Our results reinforce the cloud-cloud collision scenario in M20.

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arXiv2016

The rotation period distributions of 4--10 Myr T Tauri stars in Orion OB1: New constraints on pre-main-sequence angular momentum evolution

Most existing studies of the angular momentum evolution of young stellar populations have focused on the youngest (1-3 Myr) T Tauri stars. In contrast, the angular momentum distributions of older T Tauri stars (4-10 Myr) have been less studied, even though they hold key insight to understanding stellar angular momentum evolution at a time when protoplanetary disks have largely dissipated and when models therefore predict changes in the rotational evolution that can in principle be tested. We present a study of photometric variability among 1,974 confirmed T Tauri members of various sub-regions of the Orion OB1 association, and with ages spanning 4-10 Myr, using optical time-series from three different surveys. For 564 of the stars (~32% of the weak-lined T Tauri stars and ~13% of the classical T Tauri stars in our sample) we detect statistically significant periodic variations which we attribute to the stellar rotation periods, making this one of the largest samples of T Tauri star rotation periods yet published. We observe a clear change in the overall rotation period distributions over the age range 4-10 Myr, with the progressively older sub-populations exhibiting systematically faster rotation. This result is consistent with angular momentum evolution model predictions of an important qualitative change in the stellar rotation periods starting at ~5 Myr, an age range for which very few observational constraints were previously available.

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arXiv2016

Revealing a detailed mass distribution of a high-density core MC27/L1521F in Taurus with ALMA

We present the results of ALMA observations of dust continuum emission and molecular rotational lines toward a dense core MC27 (aka L1521F) in Taurus, which is considered to be at a very early stage of star formation. The detailed column density distribution on size scales from a few tens AU to ~10,000 AU scale are revealed by combining the ALMA (12 m array + 7 m array) data with the published/unpublished single-dish data. The high angular resolution observations at 0.87 mm with a synthesized beam size of ~0."74 x 0."32 reveal that a protostellar source, MMS-1, is not spatially resolved and lacks associated gas emission, while a starless high-density core, MMS-2, has substructures both in dust and molecular emission. The averaged radial column density distribution of the inner part of MC27/L1521F (r $\lesssim$ 3000 AU) is N(H2) ~r$^{-0.4}$, clearly flatter than that of the outer part, ~r$^{-1.0}$. The complex velocity/spatial structure obtained with previous ALMA observations is located inside the inner flatter region, which may reflect the dynamical status of the dense core.

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