Browsing by Subject "Interstellar medium"

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  • Tatematsu, Ken'ichi; Liu, Tie; Kim, Gwanjeong; Yi, Hee-Weon; Lee, Jeong-Eun; Hirano, Naomi; Liu, Sheng-Yuan; Ohashi, Satoshi; Sanhueza, Patricio; Di Francesco, James; Evans, Neal J.; Fuller, Gary A.; Kandori, Ryo; Choi, Minho; Kang, Miju; Feng, Siyi; Hirota, Tomoya; Sakai, Takeshi; Lu, Xing; Lu'o'ng, Quang Nguyen; Thompson, Mark A.; Wu, Yuefang; Li, Di; Kim, Kee-Tae; Wang, Ke; Ristorcelli, Isabelle; Juvela, Mika; Toth, L. Viktor (2020)
    We mapped two molecular cloud cores in the Orion A cloud with the 7 m Array of the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeterArray (ALMA) and with the Nobeyama 45 m radio telescope. These cores have bright N2D+ emission in single-pointing observations with the Nobeyama 45 m radio telescope, have a relatively high deuterium fraction, and are thought to be close to the onset of star formation. One is a star-forming core, and the other is starless. These cores are located along filaments observed in N2H+ and show narrow line widths of 0.41 km s(-1) and 0.45 km s(-1) in N2D+, respectively, with the Nobeyama 45 m telescope. Both cores were detected with the ALMA ACA 7 m Array in the continuum and molecular lines at Band 6. The starless core G211 shows a clumpy structure with several sub-cores, which in turn show chemical differences. Also, the sub-cores in G211 have internal motions that are almost purely thermal. The starless sub-core G211D, in particular, shows a hint of the inverse P Cygni profile, suggesting infall motion. The star-forming core G210 shows an interesting spatial feature of two N2D+ peaks of similar intensity and radial velocity located symmetrically with respect to the single dust continuum peak. One interpretation is that the two N2D+ peaks represent an edge-on pseudo-disk. The CO outflow lobes, however, are not directed perpendicular to the line connecting both N2D+ peaks.
  • Dutta, Somnath; Lee, Chin-Fei; Liu, Tie; Hirano, Naomi; Liu, Sheng-Yuan; Tatematsu, Ken'ichi; Kim, Kee-Tae; Shang, Hsien; Sahu, Dipen; Kim, Gwanjeong; Moraghan, Anthony; Jhan, Kai-Syun; Hsu, Shih-Ying; Evans, Neal J.; Johnstone, Doug; Ward-Thompson, Derek; Kuan, Yi-Jehng; Lee, Chang Won; Lee, Jeong-Eun; Traficante, Alessio; Juvela, Mika; Vastel, Charlotte; Zhang, Qizhou; Sanhueza, Patricio; Soam, Archana; Kwon, Woojin; Bronfman, Leonardo; Eden, David; Goldsmith, Paul F.; He, Jinhua; Wu, Yuefang; Pelkonen, Veli-Matti; Qin, Sheng-Li; Li, Shanghuo; Li, Di (2020)
    Planck Galactic Cold Clumps (PGCCs) are considered to be the ideal targets to probe the early phases of star formation. We have conducted a survey of 72 young dense cores inside PGCCs in the Orion complex with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm (band 6) using three different configurations (resolutions similar to 035, 10, and 70) to statistically investigate their evolutionary stages and substructures. We have obtained images of the 1.3 mm continuum and molecular line emission ((CO)-C-12, and SiO) at an angular resolution of similar to 035 (similar to 140 au) with the combined arrays. We find 70 substructures within 48 detected dense cores with median dust mass similar to 0.093 M and deconvolved size similar to 027. Dense substructures are clearly detected within the central 1000 au of four candidate prestellar cores. The sizes and masses of the substructures in continuum emission are found to be significantly reduced with protostellar evolution from Class 0 to Class I. We also study the evolutionary change in the outflow characteristics through the course of protostellar mass accretion. A total of 37 sources exhibit CO outflows, and 20 (>50%) show high-velocity jets in SiO. The CO velocity extents (Delta Vs) span from 4 to 110 km s(-1) with outflow cavity opening angle width at 400 au ranging from [Theta(obs)](400) similar to 06-39, which corresponds to 334-1257. For the majority of the outflow sources, the Delta Vs show a positive correlation with [Theta(obs)](400), suggesting that as protostars undergo gravitational collapse, the cavity opening of a protostellar outflow widens and the protostars possibly generate more energetic outflows.
  • Väisälä, Miikka (Helsingin yliopisto, 2017)
    Magnetic fields and turbulent flows pervade the interstellar medium on all scales. The magnetic turbulence that emerges on the large scales cascades towards the small scales where it influences molecular cloud structure, and star formation within the densest and coldest clumps of the clouds. Active star formation results in supernovae, and the supernova-driven turbulence takes part in the galactic dynamo process leading to the inverse cascade of turbulent energy. Such a process is one example of self-organisatory processes in the interstellar medium where order arises from chaos. Supernovae also induce and influence other important processes in the galactic disks, and this thesis examines some of them. Differentially rotating disk systems, such as galaxies, are prone to magnetorotational instability, where weak magnetic fields destabilise the otherwise hydrodynamically stable disk system, and lead to angular momentum transport outwards. However, magnetorotational instability can be quenched or even damped by another source of turbulence such as supernovae. As both magnetorotational instability and supernovae are capable of producing dynamo effects, the galactic large-scale magnetic fields are proposed to arise as an interplay of these two effects. In addition, supernovae are observed to be able to generate and sustain large-scale flows in galaxies through the anisotropic kinetic alpha effect. Thermodynamical effects have a significant influence on the properties of turbulence. Due to baroclinicity, the supernova-driven turbulence is highly vortical in nature. These types of flows produce a narrow, non-Gaussian velocity distribution with extended wings and an exponential magneti field distribution. Such effects should be taken into account when interstellar turbulence is parametrized in the form of initial conditions and forcing functions for the purpose of making smaller scale models of molecular cloud formation. The Gaussianity of the magnetic field fluctuations is a common assumption, for example, when fitting magnetic field models to explain large-scale polarization maps of the interstellar dust, and our results suggest that such assumptions should require more examination. To study these phenomena, a combination of numerical approaches and observational methods are needed. Exploring physics of turbulence requires the tools of high-performance computing and precise, high-order numerical schemes. Because of the rapidly increasing demands of computation, novel approaches have to be investigated. To improve computational efficiency this thesis shows how the sixth-order finite difference method can be accelerated with the help of graphics processing units. The properties of the interstellar medium can be examined best by the emission of atomic/molecular gas and by the emission, absorption and scattering of interstellar dust. Looking at small-scale phenomena, molecular line emission from cold prestellar cores is explored. More large-scale effects are examined with the help of polarized dust emission, by combining radiative transfer calculations with the results of a supernova-driven model of the interstellar medium, including a realistic multiphase structure and dynamo-generated small- and large-scale magnetic fields. This thesis contains seven papers. Of these, three papers examine the processes driving turbulence in differentially rotating disks via numerical modelling, while one paper looks into how graphics processing units can accelerate such calculations. Observationally, two papers study cold cores and early stages of star formation with the help of radio telescopes, and one paper examines how supernova-driven turbulence is reflected in the large-scale emission of diffuse interstellar dust. Keywords: Magnetohydrodynamics, interstellar medium, galactic dynamo, polarization, star formation, GPGPU
  • Padoan, Paolo; Pan, Liubin; Juvela, Mika; Haugbolle, Troels; Nordlund, Åke (2020)
    We address the problem of the origin of massive stars, namely the origin, path, and timescale of the mass flows that create them. Based on extensive numerical simulations, we propose a scenario where massive stars are assembled by large-scale, converging, inertial flows that naturally occur in supersonic turbulence. We refer to this scenario of massive-star formation as the inertial-inflow model. This model stems directly from the idea that the mass distribution of stars is primarily the result of turbulent fragmentation. Under this hypothesis, the statistical properties of turbulence determine the formation timescale and mass of prestellar cores, posing definite constraints on the formation mechanism of massive stars. We quantify such constraints by analyzing a simulation of supernova-driven turbulence in a 250 pc region of the interstellar medium, describing the formation of hundreds of massive stars over a time of approximately 30 Myr. Due to the large size of our statistical sample, we can say with full confidence that massive stars in general do not form from the collapse of massive cores nor from competitive accretion, as both models are incompatible with the numerical results. We also compute synthetic continuum observables in the Herschel and ALMA bands. We find that, depending on the distance of the observed regions, estimates of core mass based on commonly used methods may exceed the actual core masses by up to two orders of magnitude and that there is essentially no correlation between estimated and real core masses.