Developing Diagnostics of Molecular Clouds Using Numerical MHD Simulations

Modelling dust polarization observations of molecular clouds through MHD simulations

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx3096 ◽

2017 ◽

Vol 474 (4) ◽

pp. 5122-5142 ◽

Cited By ~ 17

Author(s):

Patrick K King ◽

Laura M Fissel ◽

Che-Yu Chen ◽

Zhi-Yun Li

Keyword(s):

Molecular Clouds ◽

Mhd Simulations

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Three-dimensional MHD simulations of molecular cloud fragmentation regulated by gravity, ambipolar diffusion, and turbulence

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030245 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 115-116

Author(s):

Takahiro Kudoh ◽

Shantanu Basu

Keyword(s):

Star Formation ◽

Magnetic Fields ◽

Time Scale ◽

Molecular Cloud ◽

Molecular Clouds ◽

Three Dimensional ◽

Core Formation ◽

Mhd Simulations ◽

Dimensional Simulation

AbstractWe find that the star formation is accelerated by the supersonic turbulence in the magnetically dominated (subcritical) clouds. We employ a fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The time-scale of collapsing core formation in subcritical clouds is a few ×107 years when starting with small subsonic perturbations. However, it is shortened to approximately several ×106 years by the supersonic flows in the clouds. We confirm that higher-spacial resolution simulations also show the same result.

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Magnetohydrodynamic Simulations of the Interaction of Magnetic Tower Jets with Interstellar Clouds including Cooling

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313000562 ◽

2012 ◽

Vol 8 (S292) ◽

pp. 96-96

Author(s):

Y. Asahina ◽

T. Ogawa ◽

R. Matsumoto

Keyword(s):

Accretion Disks ◽

Molecular Clouds ◽

Rotation Speed ◽

Propagation Speed ◽

Bow Shock ◽

Interstellar Gas ◽

Interstellar Clouds ◽

Mhd Simulations ◽

Density Enhancement ◽

Magnetohydrodynamic Simulations

AbstractWe carried out magnetohydrodynamic (MHD) simulations to reveal the formation mechanism of molecular towers observed in the central region of our galaxy. These molecular clouds can be formed by the interaction of magnetic tower jet with the interstellar gas. When the jet collides with dense HI clouds, the HI gas is compressed by the bow shock ahead of the jet. Since the density enhancement triggers the cooling instability because it increases the cooling rate, the shocked gas cools down and forms cold, dense gas. We carried out MHD simulations including the cooling. The magnetized jet which triggers the formation of the molecular column appears in global magnetohydrodynamic simulations of accretion disks, in which the magnetic loops emerging from the disk are twisted by the differential rotation between the footpoints of magnetic loops anchored to the disk. Numerical results indicate that the magnetic loops expand, and form a magnetic tower. When the ambient density is small, the propagation speed of the tower can be as large as the rotation speed of the disk. When the ambient density is high, the collision of the jet and the HI cloud forms dense molecular tower.

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Protostellar turbulence in cluster forming regions of molecular clouds

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307001640 ◽

2006 ◽

Vol 2 (S237) ◽

pp. 306-310

Author(s):

Fumitaka Nakamura ◽

Zhi-Yun Li

Keyword(s):

Magnetic Field ◽

Star Formation ◽

Molecular Clouds ◽

Cluster Formation ◽

Mhd Waves ◽

Mhd Turbulence ◽

Interstellar Turbulence ◽

Slowing Down ◽

Mhd Simulations ◽

Protostellar Outflows

AbstractWe perform 3D MHD simulations of cluster formation in turbulent magnetized dense molecular clumps, taking into account the effect of protostellar outflows. Our simulation shows that initial interstellar turbulence decays quickly as several authors already pointed out. When stars form, protostellar outflows generate and maintain supersonic turbulence that have a power-law energy spectrum of Ek ~ k−2, which is somewhat steeper than those of driven MHD turbulence simulations. Protostellar outflows suppress global star formation, although they can sometimes trigger local star formation by dynamical compression of pre-existing cores. Magnetic field retards star formation by slowing down overall contraction. Interplay of protostellar outflows and magnetic field generates large-amplitude Alfven and MHD waves that transform outflow motions into turbulent motions efficiently. Cluster forming clumps tend to be in dynamical equilibrium mainly due to dynamical support by protostellar outflow-driven turbulence (hereafter, protostellar turbulence).

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Origin of CH+ in diffuse molecular clouds

Astronomy and Astrophysics ◽

10.1051/0004-6361/201629905 ◽

2017 ◽

Vol 600 ◽

pp. A114 ◽

Cited By ~ 15

Author(s):

Valeska Valdivia ◽

Benjamin Godard ◽

Patrick Hennebelle ◽

Maryvonne Gerin ◽

Pierre Lesaffre ◽

...

Keyword(s):

Velocity Distribution ◽

Chemical Evolution ◽

Molecular Clouds ◽

Ambipolar Diffusion ◽

The Other ◽

Small Scale ◽

Neutral Drift ◽

Physical Conditions ◽

Mhd Simulations ◽

Ideal Mhd

Context. Molecular clouds are known to be magnetised and to display a turbulent and complex structure where warm and cold phases are interwoven. The turbulent motions within molecular clouds transport molecules, and the presence of magnetic fields induces a relative velocity between neutrals and ions known as the ion-neutral drift (vd). These effects all together can influence the chemical evolution of the clouds. Aims. This paper assesses the roles of two physical phenomena which have previously been invoked to boost the production of CH+ under realistic physical conditions: the presence of warm H2 and the increased formation rate due to the ion-neutral drift. Methods. We performed ideal magnetohydrodynamical (MHD) simulations that include the heating and cooling of the multiphase interstellar medium (ISM), and where we treat dynamically the formation of the H2 molecule. In a post-processing step we compute the abundances of species at chemical equilibrium using a solver that we developed. The solver uses the physical conditions of the gas as input parameters, and can also prescribe the H2 fraction if needed. We validate our approach by showing that the H2 molecule generally has a much longer chemical evolution timescale compared to the other species. Results. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low (0.3−30%) but nevertheless higher than its equilibrium value, and where the gas temperature is high (≳ 300 K). We show that warm and out of equilibrium H2 increases the integrated column densities of CH+ by one order of magnitude up to values still ~ 3−10 times lower than those observed in the diffuse ISM. We balance the Lorentz force with the ion-neutral drag to estimate the ion-drift velocities from our ideal MHD simulations. We find that the ion-neutral drift velocity distribution peaks around ~ 0.04 km s-1, and that high drift velocities are too rare to have a significant statistical impact on the abundances of CH+. Compared to previous works, our multiphase simulations reduce the spread in vd, and our self-consistent treatment of the ionisation leads to much reduced vd. Nevertheless, our resolution study shows that this velocity distribution is not converged: the ion-neutral drift has a higher impact on CH+ at higher resolution. On the other hand, our ideal MHD simulations do not include ambipolar diffusion, which would yield lower drift velocities. Conclusions. Within these limitations, we conclude that warm H2 is a key ingredient in the efficient formation of CH+ and that the ambipolar diffusion has very little influence on the abundance of CH+, mainly due to the small drift velocities obtained. However, we point out that small-scale processes and other non-thermal processes not included in our MHD simulation may be of crucial importance, and higher resolution studies with better controlled dissipation processes are needed.

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