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Yasuo Fukui

Publications and source records attributed to Yasuo Fukui.

At least 91 records · Page 5Linked to original sources

Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds

The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxies - all three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the field of complex chemistry in the low-metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to galaxies in the earlier epochs of the Universe, thus the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier Universe.

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Relations Between Molecular Cloud Structure Sizes and Line Widths in the Large Magellanic Cloud

We present a comparative study of the size-line width relation for substructures within six molecular clouds in the Large Magellanic Cloud (LMC) mapped with the Atacama Large Millimeter/submillimeter Array (ALMA). Our sample extends our previous study, which compared a Planck detected cold cloud in the outskirts of the LMC with the 30 Doradus molecular cloud and found the typical line width for 1 pc radius structures to be 5 times larger in 30 Doradus. By observing clouds with intermediate levels of star formation activity, we find evidence that line width at a given size increases with increasing local and cloud-scale 8$μ$m intensity. At the same time, line width at a given size appears to independently correlate with measures of mass surface density. Our results suggest that both virial-like motions due to gravity and local energy injection by star formation feedback play important roles in determining intracloud dynamics.

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Fermi-LAT gamma-ray study of the interstellar medium and cosmic rays in the Chamaeleon Molecular-Cloud Complex: A look at the dark gas as optically thick HI

We report a Fermi-LAT $γ$-ray analysis for the Chamaeleon molecular-cloud complex using a total column density (NH) model based on the dust optical depth at 353 GHz ($τ_{353}$) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the NH model as a function of $τ_{353}$, NH $\propto$ $τ_{353}^{1/α}$ ($α$ $\geq$ 1.0), to explicitly take into account a possible nonlinear $τ_{353}$/NH ratio. We found that a nonlinear relation, $α$$\sim$1.4, gives the best fit to the $γ$-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit NH model, we derived the CO-to-H2 conversion factor (XCO) and gas mass, taking into account uncertainties of the NH model. The value of XCO is found to be (0.63-0.76) $\times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s, which is consistent with that of a recent $γ$-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) $\times$ 10$^{4}$ Msun, of which the mass of additional gas not traced by standard HI or CO line surveys is 20-40%. The additional gas amounts to 30-60% of the gas mass estimated in the case of optically thin HI and has 5-7 times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick HI and CO-dark H2. We also derived the $γ$-ray emissivity spectrum, which is consistent with the local HI emissivity derived from LAT data within the systematic uncertainty of $\sim$20%

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Discovery of recombining plasma inside the extended gamma-ray supernova remnant HB9

We present the results from the Suzaku X-ray Imaging Spectrometer observation of the mixed-morphology supernova remnant (SNR) HB9 (G160.9+2.6). We discovered recombining plasma (RP) in the western Suzaku observation region and the spectra here are well described by a model having collisional ionization equilibrium (CIE) and RP components. On the other hand, the X-ray spectra from the eastern Suzaku observation region are best reproduced by the CIE and non-equilibrium ionization model. We discuss possible scenarios to explain the origin of the RP emission based on the observational properties and concluded that the rarefaction scenario is a possible explanation for the existence of RP. In addition, the gamma-ray emission morphology and spectrum within the energy range of 0.2$-$300 GeV are investigated using $\sim$10 years of data from the Fermi Large Area Telescope (LAT). The gamma-ray morphology of HB9 is best described by the spatial template of radio continuum emission. The spectrum is well-fit to a log-parabola function and its detection significance was found to be $\sim$25$σ$. Moreover, a new gamma-ray point source located just outside the south-east region of the SNR's shell was detected with a significance of $\sim$6$σ$. We also investigated the archival HI and CO data and detected an expanding shell structure in the velocity range of $-10.5$ and $+1.8$ km s$^{-1}$ that is coinciding with a region of gamma-ray enhancement at the southern rim of the HB9 shell.

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Extinction and dust/gas ratio in the H I ridge region of the LMC based on the IRSF/SIRIUS near-infrared survey

We present a dust extinction AV map of the Large Magellanic Cloud (LMC) in the H I ridge region using the IRSF near-infrared (IR) data, and compare the AV map with the total hydrogen column density N(H) maps derived from the CO and H I observations. In the LMC H I ridge region, the two-velocity H I components (plus an intermediate velocity component) are identified, and the young massive star cluster is possibly formed by collision between them. In addition, one of the components is suggested to be an inflow gas from the Small Magellanic Cloud (SMC) which is expected to have even lower metallicity gas (Fukui et al. 2017, PASJ, 69, L5). To evaluate dust/gas ratios in the H I ridge region in detail, we derive the AV map from the near-IR color excess of the IRSF data updated with the latest calibration, and fit the resultant AV map with a combination of the N(H) maps of the different velocity components to successfully decompose it into the 3 components. As a result, we find difference by a factor of 2 in AV /N(H) between the components. In additon, the CO-to-H2 conversion factor also indicates difference between the components, implying the difference in the metallicity. Our results are likely to support the scenario that the gas in the LMC H I ridge region is contaminated with an inflow gas from the SMC with a geometry consistent with the on-going collision between the two velocity components.

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A centrally concentrated sub-solar mass starless core in the Taurus L1495 filamentary complex

The formation scenario of brown dwarfs is still unclear because observational studies to investigate its initial condition are quite limited. Our systematic survey of nearby low-mass star-forming regions using the Atacama Compact Array (aka Morita array) and the IRAM 30 m telescope in 1.2 mm continuum has identified a centrally concentrated starless condensation with a central H$_2$ volume density of $\sim$10$^6$ cm$^{-3}$, MC5-N, connected to a narrow (width $\sim$0.03 pc) filamentary cloud in the Taurus L1495 region. The mass of the core is $\sim$0.2-0.4 $M_{\odot}$, which is an order of magnitude smaller than typical low-mass prestellar cores. Taking into account a typical core to star formation efficiency for prestellar cores ($\sim$20%-40%) in nearby molecular clouds, brown dwarf(s) or very low-mass star(s) may be going to be formed in this core. We have found possible substructures at the high-density portion of the core, although much higher angular resolution observation is needed to clearly confirm them. The subsequent N$_2$H$^+$ and N$_2$D$^+$ observations using the Nobeyama 45 m telescope have confirmed the high-deuterium fractionation ($\sim$30%). These dynamically and chemically evolved features indicate that this core is on the verge of proto-brown dwarf or very low-mass star formation and is an ideal source to investigate the initial conditions of such low-mass objects via gravitational collapse and/or fragmentation of the filamentary cloud complex.

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Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI

Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from atomic hydrogen (HI) and 12CO molecule, dust optical depth at 353 GHz ($τ_{353}$), and $J$-band infrared extinction ($A_{J}$). We have found a scatter correlation between the HI integrated intensity ($W_{\rm HI}$) and $τ_{353}$ in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in HI emission ($τ_{\rm HI}$) using a total column density ($N_{\rm H}$) model based on $τ_{353}$. A nonlinear relation of $τ_{353}$ with the $\sim$1.2 power of $A_{J}$ has been found in opaque regions ($A_{J}$ $\gtrsim$ 0.3 mag), which may indicate dust evolution effect. If we apply this nonlinear relation to the $N_{\rm H}$ model (i.e., $N_{\rm H} \propto τ_{353}^{1/1.2}$) allowing arbitrary $τ_{\rm HI}$, the model curve reproduces well the $W_{\rm HI}$-$τ_{353}$ scatter correlation, suggesting optically thick HI ($τ_{\rm HI} \sim$1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the $N_{\rm H}$ model, we have then derived the CO-to-H$_{2}$ conversion factor ($X_{\rm CO}$) on $\sim$1.5$^{\circ}$ scales (corresponding to $\sim$4 persec) and found spatial variations of $X_{\rm CO}$ $\sim$(0.5-3)$\times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s across the cloud complex, possibly depending on the radiation field inside or surrounding the molecular clouds. These gas properties found in the Chamaeleon region are discussed through a comparison with other local molecular cloud complexes.

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The Polycyclic Aromatic Hydrocarbon Mass Fraction on a 10 pc scale in the Magellanic Clouds

We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the \citet{DL07} dust model. We derive the abundance of the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction (q_pah, the fraction of the dust mass in the form of PAHs) is smaller in the SMC (1.0$^{+0.3}_{-0.3}$%) than in the LMC (3.3$^{+1.4}_{-1.3}$%). We measure the PAH fraction in different gas phases (H II regions, ionized gas outside of H II regions, molecular gas, and diffuse neutral gas). H II regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way ($\sim4.6$%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: a more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.

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Relative Alignment Between the Magnetic Field and Molecular Gas Structure in the Vela C Giant Molecular Cloud using Low and High Density Tracers

We compare the magnetic field orientation for the young giant molecular cloud Vela C inferred from 500-$μ$m polarization maps made with the BLASTPol balloon-borne polarimeter to the orientation of structures in the integrated line emission maps from Mopra observations. Averaging over the entire cloud we find that elongated structures in integrated line-intensity, or zeroth-moment maps, for low density tracers such as $^{12}$CO and $^{13}$CO $J$ $\rightarrow$ 1 - 0 are statistically more likely to align parallel to the magnetic field, while intermediate or high density tracers show (on average) a tendency for alignment perpendicular to the magnetic field. This observation agrees with previous studies of the change in relative orientation with column density in Vela C, and supports a model where the magnetic field is strong enough to have influenced the formation of dense gas structures within Vela C. The transition from parallel to no preferred/perpendicular orientation appears to happen between the densities traced by $^{13}$CO and by C$^{18}$O $J$ $\rightarrow$ 1 - 0. Using RADEX radiative transfer models to estimate the characteristic number density traced by each molecular line we find that the transition occurs at a molecular hydrogen number density of approximately $10^3$ cm$^{-3}$. We also see that the Centre-Ridge (the highest column density and most active star-forming region within Vela C) appears to have a transition at a lower number density, suggesting that this may depend on the evolutionary state of the cloud.

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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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Formation of High-Mass stars in an isolated environment in the Large Magellanic Cloud

The aim of this study is to characterize the distribution and basic properties of the natal gas associated with high-mass young stellar objects (YSOs) in isolated environments in the Large Magellanic Cloud (LMC). High-mass stars usually form in Giant Molecular Clouds (GMCs) as part of a young stellar cluster, but some OB stars are observed far from GMCs. By examining the spatial coincidence between the high-mass YSOs and 12CO (J = 1-0) emission detected by NANTEN and Mopra observations, we selected ten high-mass YSOs that are located away from any of the NANTEN clouds but are detected by the Mopra pointed observations. The ALMA observations revealed that a compact molecular cloud whose mass is a few thousand solar masses or smaller is associated with the high-mass YSOs, which indicates that these compact clouds are the sites of high-mass star formation. The high-density and high-temperature throughout the clouds are explained by the severe photodissociation of CO due to the lower metallicity than in the Galaxy. The star formation efficiency ranges from several to as high as ~ 40%, indicating efficient star formation in these environments. The enhanced turbulence may be a cause of the efficient star formation therein, as judged from the gas velocity information and the association with the lower density gas.

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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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Synthetic observations of highly clumped formation of CO and CI clouds in the turbulent interstellar medium; Implications on the seed CO clouds and the CO-dark H$_{2}$ gas

We carried out synthetic observations of a theoretical model of interstellar carbon in CO, C I and C II including H2 formation. The theoretical model based on numerical simulations of H I gas converging at a velocity of 20 km/s by Inoue & Inutsuka (2012) is employed in the present study. The gas model, which is fairy realistic and consistent with the observations of the local gas located at high b, is turbulent and inhomogeneous with magnetic field. We focus on the evolution of the rare species, CO, C I and C II from 0.3 Myr to 9 Myr since the converging flows collided. In the early phase within 1 Myr, the Cold Neutral Medium (CNM) clumps of sub-pc scales are the formation sites of H2, CO and C I, and in the late phase from 3 Myr to 9 Myr the CNM clumps merge together to form pc-scale molecular clouds. The results show that C I and CO are well mixed at pc scales, and mark a significant disagreement with the classic picture which predicts layered C I and CO distributions according to AV . We suggest that the faint CO clouds in the Oph North (Tachihara et al. 2012) and the Pegasus loop (Yamamoto et al. 2006) are promising candidates for such small CO clouds. The total mass budget of the local ISM indicates the fraction of H I is about 70% and the CO-dark H2 is small, less than a few percent, contrary to the usual assumption.

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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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An Intermediate Velocity HI Cloud Falling to the Galactic Disk; Possible Evidence for Low Metallicity HI Gas Originated Outside the Galactic Disk

We found that an intermediate velocity cloud (IVC) IVC 86-36 in HI 21 cm emission shows a head-tail distribution toward the Galactic plane with marked parallel filamentary streamers, which is extended over 40 degrees in the sky. The distance of IVC 86-36 is constrained to be less than ~3 kpc from absorption of a background star as determined from opticalspectroscopy. There is a bridge feature in velocity between the IVC and the local ISM with velocity separation of ~50 km s-1, which may indicate dynamical interaction of the IVC with the disk. If the interaction is correct, the distance estimate d of the IVC ranges from 200 pc to 3 kpc, and the mass of the IVC head is estimated to be 7X103(d/1kpc)2Msol. The IVC shares similar properties to the Smith cloud located at 12 kpc, including the head-tail distribution, streamers, and bridge feature, while the mass of the IVC is less than ~0.1 of the Smith cloud. A comparison between the Hi and the Planck/IRAS dust emission indicates that the dust emission of IVC 86-36 is not detectable in spite of its HI column density of 2X10^20 cm-2, indicating low metalicity of IVC 86-36 by a factor of ~< 0.2 as compared with the solar neighbor. We conclude that IVC 86-38 is an infalling cloud which likely originated in the low-metallicity environment of the Galactic halo or the Magellanic system.

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The Dust-Selected Molecular Clouds in the Northeast Region of the Small Magellanic Cloud

We present a high-sensitivity ($1σ<1.6~\mathrm{mJy~beam^{-1}}$) continuum observation in a 343 arcmin$^2$ area of the northeast region in the Small Magellanic Cloud at a wavelength of 1.1 mm, conducted using the AzTEC instrument on the ASTE telescope. In the observed region, we identified 20 objects by contouring $10σ$ emission. Through spectral energy distribution (SED) analysis using 1.1 mm, $Herschel$, and $Spitzer$ data, we estimated the gas masses of $5\times 10^3-7\times 10^4~\mathrm{M_\odot}$, assuming a gas-to-dust ratio of 1000. Dust temperature and the index of emissivity were also estimated as $18-33$ K and $0.9-1.9$, respectively, which are consistent with previous low resolution studies. The relation between dust temperature and the index of emissivity shows a weak negative linear correlation. We also investigated five CO-detected dust-selected clouds in detail. The total gas masses were comparable to those estimated from the Mopra CO data, indicating that the assumed gas-to-dust ratio of 1000 and the $X_\mathrm{CO}$ factor of $1\times10^{21}~\mathrm{cm^{-2}~(K~km~s^{-1})^{-1}}$, with uncertainties of a factor of 2, are reliable for the estimation of the gas masses of molecular or dust-selected clouds. Dust column density showed good spatial correlation with CO emission, except for an object that associates with bright young stellar objects. The $8~\mathrm{μm}$ filamentary and clumpy structures also showed similar spatial distribution with the CO emission and dust column density, supporting the fact that polycyclic aromatic hydrocarbon emissions arise from the surfaces of dense gas and dust clouds.

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Submillimeter Polarization Spectrum of the Carina Nebula

Linear polarization maps of the Carina Nebula were obtained at 250, 350, and 500 $μ$m during the 2012 flight of the BLASTPol balloon-borne telescope. These measurements are combined with Planck 850 $μ$m data in order to produce a submillimeter spectrum of the polarization fraction of the dust emission, averaged over the cloud. This spectrum is flat to within $\pm$15% (relative to the 350 $μ$m polarization fraction). In particular, there is no evidence for a pronounced minimum of the spectrum near 350 $μ$m, as suggested by previous ground-based measurements of other molecular clouds. This result of a flat polarization spectrum in Carina is consistent with recently-published BLASTPol measurements of the Vela C molecular cloud, and also agrees with a published model for an externally-illuminated, dense molecular cloud by Bethell and collaborators. The shape of the spectrum in Carina does not show any dependence on the radiative environment of the dust, as quantified by the Planck-derived dust temperature or dust optical depth at 353 GHz.

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CO observations of the molecular gas in the galactic HII region Sh2-48; Evidence for cloud-cloud collision as a trigger of high-mass star formation

Sh2-48 is a Galactic HII region located at 3.8 kpc 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 the CO J=1-0 dataset for a large area of Sh2-48 at a spatial resolution of 21"(~0.4 pc), which we used to find a molecular cloud with a total molecular mass of ~3.8x10^4 Mo associated with Sh2-48. The molecular cloud has a systematic velocity shift within a velocity range ~42-47 km/s . On the lower velocity side the CO emission spatially corresponds with the bright 8 μm filament at the western rim of Sh2-48, while the CO emission at higher velocities is separated at the eastern and western sides of the 8μm filament. This velocity change forms V-shaped, east-west-oriented feature 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 assessed a cloud-cloud collision scenario and found from a comparison between the observations and simulations that the velocity distribution is an expected outcome of a collision between a cylindrical cloud and a spherical cloud, with the cylindrical cloud corresponding to the lower-velocity component, and the two separated components in the higher-velocity part interpretable as the collision-broken remnants of the spherical cloud. Based on the consistency of the ~1.3Myr estimated formation timescale of the HII region with that of the collision, we concluded that the high-mass star formation in Sh2-48 was triggered by the collision.

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