arXiv subjects

Fumitaka Nakamura

Publications and source records attributed to Fumitaka Nakamura.

At least 109 records · Page 6Linked to original sources

arXiv2018

Expanding CO Shells in the Orion A Molecular Cloud

We present the discovery of expanding spherical shells around low to intermediate-mass young stars in the Orion A giant molecular cloud using observations of $^{12}$CO (1-0) and $^{13}$CO (1-0) from the Nobeyama Radio Observatory 45-meter telescope. The shells have radii from 0.05 to 0.85 pc and expand outward at 0.8 to 5 km/s. The total energy in the expanding shells is comparable to protostellar outflows in the region. Together, shells and outflows inject enough energy and momentum to maintain the cloud turbulence. The mass-loss rates required to power the observed shells are two to three orders of magnitude higher than predicted for line-driven stellar winds from intermediate-mass stars. This discrepancy may be resolved by invoking accretion-driven wind variability. We describe in detail several shells in this paper and present the full sample in the online journal.

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arXiv2018

Infall Signatures in a Prestellar Core embedded in the High-Mass 70 $μ$m Dark IRDC G331.372-00.116

Using Galactic Plane surveys, we have selected a massive (1200 M$_\odot$), cold (14 K) 3.6-70 $μ$m dark IRDC G331.372-00.116. This IRDC has the potential to form high-mass stars and, given the absence of current star formation signatures, it seems to represent the earliest stages of high-mass star formation. We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.1 and 1.3 mm in dust continuum and line emission. The dust continuum reveals 22 cores distributed across the IRDC. In this work, we analyze the physical properties of the most massive core, ALMA1, which has no molecular outflows detected in the CO (2-1), SiO (5-4), and H$_2$CO (3-2) lines. This core is relatively massive ($M$ = 17.6 M$_\odot$), subvirialized (virial parameter $α_{vir}=M_{vir}/M=0.14$), and is barely affected by turbulence (transonic Mach number of 1.2). Using the HCO$^+$ (3-2) line, we find the first detection of infall signatures in a relatively massive, prestellar core (ALMA1) with the potential to form a high-mass star. We estimate an infall speed of 1.54 km s$^{-1}$ and a high accretion rate of 1.96 $\times$ 10$^{-3}$ M$_\odot$ yr$^{-1}$. ALMA1 is rapidly collapsing, out of virial equilibrium, more consistent with competitive accretion scenarios rather than the turbulent core accretion model. On the other hand, ALMA1 has a mass $\sim$6 times larger than the clumps Jeans mass, being in an intermediate mass regime ($M_{J}=2.7<M\lesssim$ 30 M$_\odot$), contrary to what both the competitive accretion and turbulent core accretion theories predict.

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arXiv2018

A First Look at BISTRO Observations of The $ρ$ Oph-A core

We present 850 $μ$m imaging polarimetry data of the $ρ$ Oph-A core taken with the Submillimeter Common-User Bolometer Array-2 (SCUBA-2) and its polarimeter (POL-2), as part of our ongoing survey project, BISTRO (B-fields In STar forming RegiOns). The polarization vectors are used to identify the orientation of the magnetic field projected on the plane of the sky at a resolution of 0.01 pc. We identify 10 subregions with distinct polarization fractions and angles in the 0.2 pc $ρ$ Oph A core; some of them can be part of a coherent magnetic field structure in the $ρ$ Oph region. The results are consistent with previous observations of the brightest regions of $ρ$ Oph-A, where the degrees of polarization are at a level of a few percents, but our data reveal for the first time the magnetic field structures in the fainter regions surrounding the core where the degree of polarization is much higher ($> 5 \%$). A comparison with previous near-infrared polarimetric data shows that there are several magnetic field components which are consistent at near-infrared and submillimeter wavelengths. Using the Davis-Chandrasekhar-Fermi method, we also derive magnetic field strengths in several sub-core regions, which range from approximately 0.2 to 5 mG. We also find a correlation between the magnetic field orientations projected on the sky with the core centroid velocity components.

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arXiv2018

The CARMA-NRO Orion Survey

We present the first results from a new, high resolution, $^{12}$CO(1-0), $^{13}$CO(1-0), and C$^{18}$O(1-0) molecular line survey of the Orion A cloud, hereafter referred to as the CARMA-NRO Orion Survey. CARMA observations have been combined with single-dish data from the Nobeyama 45m telescope to provide extended images at about 0.01 pc resolution, with a dynamic range of approximately 1200 in spatial scale. Here we describe the practical details of the data combination in uv space, including flux scale matching, the conversion of single dish data to visibilities, and joint deconvolution of single dish and interferometric data. A $Δ$-variance analysis indicates that no artifacts are caused by combining data from the two instruments. Initial analysis of the data cubes, including moment maps, average spectra, channel maps, position-velocity diagrams, excitation temperature, column density, and line ratio maps provides evidence of complex and interesting structures such as filaments, bipolar outflows, shells, bubbles, and photo-eroded pillars. The implications for star formation processes are profound and follow-up scientific studies by the CARMA-NRO Orion team are now underway. We plan to make all the data products described here generally accessible; some are already available at https://dataverse.harvard.edu/dataverse/CARMA-NRO-Orion

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arXiv2018

A Statistical Study of Massive Cluster-Forming Clumps

We report results of the observations of 15 regions in several molecular lines for a statistical study of massive cluster-forming clumps. We identified 24 clumps based on the C18O (J=1-0) data obtained by the NRO 45 m telescope, and found that 16 of them are associated with young clusters. The clumps associated with clusters have a typical mass, radius, and molecular density of ~1 X 10^3 Mo, ~0.5 pc, ~1 X 10^5 cm^-3, respectively. We categorized the clumps and clusters into four types according to the spatial coincidence of gas and star density, and discussed their evolutions: Clumps without clusters (Type 1), clumps showing good correlations with clusters (Type 2), clumps showing poor correlations with clusters (Type 3), and clusters with no associated clumps (Type 4). Analyses of the velocity structures and the chemical compositions imply that the clump + cluster systems should evolve from Type 1 to Type 4. We found that some of the Type 2 clumps are infalling on the clump-scale to form clusters at the clump center, which should commonly occur in the beginning of cluster formation. Interestingly, all of the identified Type 1 clumps are likely to be older than the Type 2 clumps in terms of chemical compositions, suggesting that they have been gravitationally stable for a long time possibly being supported by the strong magnetic field of > 1 mG.Type 1 clumps younger than the observed Type 2 clumps should be very rare to find because of their short lifetime.

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arXiv2017

GMC Collisions as Triggers of Star Formation. VI. Collision-Induced Turbulence

We investigate collisions between giant molecular clouds (GMCs) as potential generators of their internal turbulence. Using magnetohydrodynamic (MHD) simulations of self-gravitating, magnetized, turbulent, GMCs, we compare kinematic and dynamic properties of dense gas structures formed when such clouds collide compared to those that form in non-colliding clouds as self-gravity overwhelms decaying turbulence. We explore the nature of turbulence in these structures via distribution functions of density, velocity dispersions, virial parameters, and momentum injection. We find that the dense clumps formed from GMC collisions have higher effective Mach number, greater overall velocity dispersions, sustain near-virial equilibrium states for longer times, and are the conduit for injection of turbulent momentum into high density gas at high rates.

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arXiv2017

GMC Collisions as Triggers of Star Formation. V. Observational Signatures

We present calculations of molecular, atomic and ionic line emission from simulations of giant molecular cloud (GMC) collisions. We post-process snapshots of the magneto-hydrodynamical simulations presented in an earlier paper in this series by Wu et al. (2017) of colliding and non-colliding GMCs. Using photodissociation region (PDR) chemistry and radiative transfer we calculate the level populations and emission properties of $^{12}$CO $J=1-0$, [CI] $^3{\rm P}_1\rightarrow{^3{\rm P}}_0$ at $609\,μ$m, [CII] $158\,μ$m and [OI] $^3{\rm P}_1\rightarrow{^3{\rm P}}_0$ transition at $63\,μ$m. From integrated intensity emission maps and position-velocity diagrams, we find that fine-structure lines, particularly the [CII] $158\,μ$m, can be used as a diagnostic tracer for cloud-cloud collision activity. These results hold even in more evolved systems in which the collision signature in molecular lines has been diminished.

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arXiv2017

First Observation of the Submillimeter Polarization Spectrum in a Translucent Molecular Cloud

Polarized emission from aligned dust is a crucial tool for studies of magnetism in the ISM and a troublesome contaminant for studies of CMB polarization. In each case, an understanding of the significance of the polarization signal requires well-calibrated physical models of dust grains. Despite decades of progress in theory and observation, polarized dust models remain largely underconstrained. During its 2012 flight, the balloon-borne telescope BLASTPol obtained simultaneous broad-band polarimetric maps of a translucent molecular cloud at 250, 350, and 500 microns. Combining these data with polarimetry from the Planck 850 micron band, we have produced a submillimeter polarization spectrum for a cloud of this type for the first time. We find the polarization degree to be largely constant across the four bands. This result introduces a new observable with the potential to place strong empirical constraints on ISM dust polarization models in a previously inaccessible density regime. Comparing with models by Draine and Fraisse (2009), our result disfavors two of their models for which all polarization arises due only to aligned silicate grains. By creating simple models for polarized emission in a translucent cloud, we verify that extinction within the cloud should have only a small effect on the polarization spectrum shape compared to the diffuse ISM. Thus we expect the measured polarization spectrum to be a valid check on diffuse ISM dust models. The general flatness of the observed polarization spectrum suggests a challenge to models where temperature and alignment degree are strongly correlated across major dust components.

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arXiv2017

Observations of Cyanopolyynes toward Four High-Mass Star-Forming Regions Containing Hot Cores

We carried out line survey observations at the 26-30 GHz band toward the four high-mass star-forming regions containing hot cores, G10.30-0.15, G12.89+0.49, G16.86-2.16, and G28.28-0.36, with the Robert C. Byrd Green Bank Telescope. We have detected HC5N from all of the sources, and HC7N from the three sources, except for G10.30-0.15. We further conducted observations of HC5N at the 42-46 GHz and 82-103 GHz bands toward the three sources, G12.89+0.49, G16.86-2.16, and G28.28-0.36, with the Nobeyama 45 m radio telescope. The rotational lines of HC5N with the high excitation energies (Eu/k=63-100 K), which are hardly excited in the cold dark clouds, have been detected from the three sources. The rotational temperatures of HC5N are found to be 13-20 K in the three sources. The detection of the lines with the high excitation energies and the derived rotational temperatures indicate that HC5N exists in the warm gas within 0.07-0.1 pc radii around massive young stellar objects. The column densities of HC5N in the three sources are derived to be (2.0+-2.8)*10^(13) cm-2 . We compare the ratios between N(HC5N) the column density of HC5N and W(CH3OH) the integrated intensity of the thermal CH3OH emission line among the three high-mass star-forming regions. We found a possibility of the chemical differentiation in the three high-mass star-forming regions; G28.28-0.36 shows the largest N (HC5N)/W (CH3OH) ratio of > 8.0*10^(14) in units of (K km s-1 )-1 cm-2 , while G12.89+0.49 and G16.86-2.16 show the smaller values (~2*10^(13) ).

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arXiv2017

First results from BISTRO -- a SCUBA-2 polarimeter survey of the Gould Belt

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. The most important of these is the role of magnetic fields in the star formation process on the scale of individual filaments and cores in dense regions. We describe the data acquisition and reduction processes for POL-2, demonstrating both repeatability and consistency with previous data. We present a first-look analysis of the first results from the BISTRO survey in the OMC 1 region. We see that the magnetic field lies approximately perpendicular to the famous 'integral filament' in the densest regions of that filament. Furthermore, we see an 'hour-glass' magnetic field morphology extending beyond the densest region of the integral filament into the less-dense surrounding material, and discuss possible causes for this. We also discuss the more complex morphology seen along the Orion Bar region. We examine the morphology of the field along the lower-density north-eastern filament. We find consistency with previous theoretical models that predict magnetic fields lying parallel to low-density, non-self-gravitating filaments, and perpendicular to higher-density, self-gravitating filaments.

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arXiv2017

GMC Collisions as Triggers of Star Formation. III. Density and Magnetically Regulated Star Formation

We study giant molecular cloud (GMC) collisions and their ability to trigger star cluster formation. We further develop our three dimensional magnetized, turbulent, colliding GMC simulations by implementing star formation sub-grid models. Two such models are explored: (1) "Density-Regulated," i.e., fixed efficiency per free-fall time above a set density threshold; (2) "Magnetically-Regulated," i.e., fixed efficiency per free-fall time in regions that are magnetically supercritical. Variations of parameters associated with these models are also explored. In the non-colliding simulations, the overall level of star formation is sensitive to model parameter choices that relate to effective density thresholds. In the GMC collision simulations, the final star formation rates and efficiencies are relatively independent of these parameters. Between non-colliding and colliding cases, we compare the morphologies of the resulting star clusters, properties of star-forming gas, time evolution of the star formation rate (SFR), spatial clustering of the stars, and resulting kinematics of the stars in comparison to the natal gas. We find that typical collisions, by creating larger amounts of dense gas, trigger earlier and enhanced star formation, resulting in 10 times higher SFRs and efficiencies. The star clusters formed from GMC collisions show greater spatial sub-structure and more disturbed kinematics.

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arXiv2017

Comparing submillimeter polarized emission with near-infrared polarization of background stars for the Vela C molecular cloud

We present a large-scale combination of near-infrared (near-IR) interstellar polarization data from background starlight with polarized emission data at submillimeter (sub-mm) wavelengths for the Vela C molecular cloud. The near-IR data consist of more than 6700 detections probing a range of visual extinctions between $2$ and $20\,$mag in and around the cloud. The sub-mm data was collected in Antartica by the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (BLASTPol). This is the first direct combination of near-IR and sub-mm polarization data for a molecular cloud aimed at measuring the "polarization efficiency ratio" ($R_{\mathrm{eff}}$), a quantity that is expected to depend only on grain intrinsic physical properties. It is defined as $p_{500}/(p_{I}/τ_{V})$, where $p_{500}$ and $p_{I}$ are polarization fractions at $500\,μ$m and $I$-band, respectively, and $τ_{V}$ is the optical depth. To ensure that the same column density of material is producing both polarization from emission and from extinction, we conducted a careful selection of near-background stars using 2MASS, $Herschel$ and $Planck$ data. This selection excludes objects contaminated by the Galactic diffuse background material as well as objects located in the foreground. Accounting for statistical and systematic uncertainties, we estimate an average $R_{\mathrm{eff}}$ value of $2.4\pm0.8$, which can be used to test the predictions of dust grain models designed for molecular clouds when such predictions become available. $R_{\mathrm{eff}}$ appears to be relatively flat as a function of the cloud depth for the range of visual extinctions probed.

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arXiv2017

Wide-Field $^{12}$CO ($J=2-1$) and $^{13}$CO ($J=2-1$) Observations toward the Aquila Rift and Serpens Molecular Cloud Complexes. I. Molecular Clouds and Their Physical Properties

We present results of wide-field $^{12}$CO ($J = 2 - 1$) and $^{13}$CO ($J = 2 - 1$) observations toward the Aquila Rift and Serpens molecular cloud complexes (25$^\circ < l < 33^\circ$ and $1^\circ < b < 6^\circ$) at an angular resolution of 3$'$.4 ($\approx$ 0.25 pc) and at a velocity resolution of 0.079 km s$^{-1}$ with the velocity coverage of $-5$ km s$^{-1} < V_{\rm LSR} <$ 35 km s$^{-1}$. We found that the $^{13}$CO emission better traces the structures seen in the extinction map and derived the $X_{\rm ^{13}CO}$-factor of this region. Applying \texttt{SCIMES} to the $^{13}$CO data cube, we identified 61 clouds and derived their masses, radii, and line widths. The line-width-radius relation of the identified clouds basically follows those of nearby molecular clouds. Majority of the identified clouds are close to virial equilibrium although the dispersion is large. By inspecting the $^{12}$CO channel maps by eye, we found several arcs which are spatially extended to 0.2 $-$ 3 degree in length. In the longitude-velocity diagrams of $^{12}$CO, we also found the two spatially-extended components which appear to converge toward Serpens South and W40 region. The existence of two components with different velocities and arcs suggests that large-scale expanding bubbles and/or flows play a role in the formation and evolution of the Serpens South and W40 cloud.

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arXiv2016

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

We investigate giant molecular cloud (GMCs) collisions and their ability to induce gravitational instability and thus star formation. This mechanism may be a major driver of star formation activity in galactic disks. We carry out a series of three dimensional, magnetohydrodynamics (MHD), adaptive mesh refinement (AMR) simulations to study how cloud collisions trigger formation of dense filaments and clumps. Heating and cooling functions are implemented based on photo-dissociation region (PDR) models that span the atomic to molecular transition and can return detailed diagnostic information. The clouds are initialized with supersonic turbulence and a range of magnetic field strengths and orientations. Collisions at various velocities and impact parameters are investigated. Comparing and contrasting colliding and non-colliding cases, we characterize morphologies of dense gas, magnetic field structure, cloud kinematic signatures, and cloud dynamics. We present key observational diagnostics of cloud collisions, especially: relative orientations between magnetic fields and density structures, like filaments; $^{13}$CO($J$=2-1), $^{13}$CO($J$=3-2), and $^{12}$CO($J$=8-7) integrated intensity maps and spectra; and cloud virial parameters. We compare these results to observed Galactic clouds.

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arXiv2016

Dense core properties in the Infrared Dark cloud G14.225-0.506 revealed by ALMA

We have performed a dense core survey toward the Infrared Dark Cloud G14.225-0.506 at 3 mm continuum emission with the Atacama Large Millimeter/Submillimeter Array (ALMA). This survey covers the two hub-filament systems with an angular resolution of $\sim3$\arcsec ($\sim0.03$ pc). We identified 48 dense cores. Twenty out of the 48 cores are protostellar due to their association with young stellar objects (YSOs) and/or X-ray point-sources, while the other 28 cores are likely prestellar and unrelated with known IR or X-ray emission. Using APEX 870 $μ$m continuum emission, we also identified the 18 clumps hosting these cores. Through virial analysis using the ALMA N$_2$H$^+$ and VLA/Effelsberg NH$_3$ molecular line data, we found a decreasing trend in the virial parameter with decreasing scales from filaments to clumps, and then to cores. The virial parameters of $0.1-1.3$ in cores, indicate that cores are likely undergoing dynamical collapse. The cumulative Core Mass Function (CMF) for the prestellar cores candidates has a power law index of $α=1.6$, with masses ranging from 1.5 to 22 $M_\odot$. We find no massive prestellar or protostellar cores. Previous studies suggest that massive O-tpye stars have not been produced yet in this region. Therefore, high-mass stars should be formed in the prestellar cores by accreting a significant amount of gas from the surrounding medium. Another possibility is that low-mass YSOs become massive by accreting from their parent cores that are fed by filaments. These two possibilities might be consistent with the scenario of global hierarchical collapse.

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arXiv2016

Discovery of Infalling Motion with Rotation of the Cluster-Forming Clump S235AB and Its Implication to the Clump Structures

We report the discovery of infalling motion with rotation of S235AB the massive cluster-forming clump (~10^3 Mo) in the S235 region. Our C18O observations with the 45m telescope at the Nobeyama Radio Observatory have revealed the elliptical shape of the clump. Position-velocity (PV) diagram taken along its major axis exhibits two well-defined peaks symmetrically located with respect to the clump center, which is similar to that found for a dynamically infalling envelope with rotation around a single protostar modeled by N. Ohashi and his collaborators, indicating that the cluster-forming clump is also collapsing by the self-gravity toward the clump center. With analogue to Ohashi's model, we made a simple model of an infalling, rotating clump to fit the observed data. Based on the inferred model parameters as well as results of earlier observations and simulations in the literature, we discuss structures of the clump such as the relation among the global mass infall rate (~10^-3 Mo/yr), formation of a compact core (with a mass and size of ~4 Mo and < 0.1 pc) at the center, and a massive star (~11 Mo) forming in the core.

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arXiv2016

Submillimeter Polarization Spectrum in the Vela C Molecular Cloud

Polarization maps of the Vela C molecular cloud were obtained at 250, 350, and 500um during the 2012 flight of the balloon-borne telescope BLASTPol. These measurements are used in conjunction with 850um data from Planck to study the submillimeter spectrum of the polarization fraction for this cloud. The spectrum is relatively flat and does not exhibit a pronounced minimum at λ~350um as suggested by previous measurements of other molecular clouds. The shape of the spectrum does not depend strongly on the radiative environment of the dust, as quantified by the column density or the dust temperature obtained from Herschel data. The polarization ratios observed in Vela C are consistent with a model of a porous clumpy molecular cloud being uniformly heated by the interstellar radiation field.

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arXiv2016

The intrinsic abundance ratio and X-factor of CO isotopologues in L1551 shielded from FUV photodissociation

L1551 is chosen because it is relatively isolated in the Taurus molecular cloud shielded from FUV photons, providing an ideal environment for studying the target properties. Our observations cover ~40'x40' with resolution ~30", which are the maps with highest spatial dynamical range to date. We derive the X(13CO)/X(C18O) value on the sub-parsec scales in the range of ~3-27 with a mean value of 8.0+-2.8. Comparing to the visual extinction map derived from the Herschel observations, we found that the abundance ratio reaches its maximum at low Av (i.e., Av ~ 1-4mag), and decreases to the typical solar system value of 5.5 inside L1551 MC. The high X(13CO)/X(C18O) value at the boundary of the cloud is most likely due to the selective FUV photodissociation of C18O. This is in contrast with Orion-A where its internal OB stars keep the abundance ratio at a high level greater than ~10. In addition, we explore the variation of the X-factor, because it is an uncertain but widely used quantity in extragalactic studies. We found that X-factor is proportional to N(H2)^1.0 which is consistent with previous simulations. Excluding the high density region, the average X-factor is similar to the Milky Way average value.

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