Signatures of turbulence in the dense interstellar medium

We present an ensemble of recent observational results on molecular clouds which, taken separately, could all be understood by invoking various unrelated physical processes, but taken all together form a coherent ensemble stressing the imprints of turbulence in the physics of the cold interstellar medium. These results are first, the existence of wings in the molecular line profiles, which can be interpreted on statistical grounds as the signature of the intermittency of the velocity field in turbulent flows, second the fractal geometry of the cloud edges, with properties reminiscent of those of various surfaces studied in turbulent laboratory flows, and third, the fact that the dense gas fills only a very small fraction of the space. The last points are supported by CO multitransition observations of a few fields in nearby molecular clouds. They show that the excitation conditions are the same for the gas emitting in the linewings and in the linecores and are also remarkably uniform over a large range (factor 10) of column densities. An attractive interpretation of the molecular line data is that most of the 12CO(J=2—1) and (J=3—2) emissions arise in cold (Tk ≥ 10K) and dense (nH2 ∼ 104cm—3 or more) structures distributed on a fractal set with no characteristic scale size greater than about 1000 AU.

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An ALMA and ATCA Molecular Line Survey Toward Centaurus A

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313001269 ◽

2012 ◽

Vol 8 (S292) ◽

pp. 251-251

Author(s):

Jürgen Ott ◽

Mark McCoy ◽

David Meier

Keyword(s):

Radio Galaxy ◽

Molecular Absorption ◽

Physical Processes ◽

Molecular Line ◽

Physical Conditions ◽

Line Survey ◽

Molecular Abundances ◽

Excitation Conditions ◽

Compact Array ◽

Centaurus A

AbstractWe present Atacama Large Millimeter/submillimeter Array (ALMA) and Australia Telescope Compact Array (ATCA) data of molecular absorption lines toward the bright central core of the prominent, nearby (d ∼ 3.8 Mpc) radio galaxy Centaurus A, at 13, 7, 3, and 1mm wavelengths. The line of sight crosses the prominent dust lane and continues through the disk and eventually through gas that may be very close to the central supermassive black hole. The goal of our survey is to determine the physical conditions of the gas via analyses of molecular line tracers including molecular abundances and excitation conditions that are sensitive to changes in temperature, density, ionization, and shocks. This study allows us to derive the physical processes that are shaping each molecular environment.

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Turbulence in the interstellar medium

Nonlinear Processes in Geophysics ◽

10.5194/npg-21-587-2014 ◽

2014 ◽

Vol 21 (3) ◽

pp. 587-604 ◽

Cited By ~ 28

Author(s):

D. Falceta-Gonçalves ◽

G. Kowal ◽

E. Falgarone ◽

A. C.-L. Chian

Keyword(s):

Cosmic Rays ◽

Magnetic Fields ◽

Interstellar Medium ◽

Turbulent Flows ◽

Molecular Clouds ◽

Three Dimensional ◽

Interstellar Turbulence ◽

The Stability ◽

Observational Techniques ◽

And Diffusion

Abstract. Turbulence is ubiquitous in the insterstellar medium and plays a major role in several processes such as the formation of dense structures and stars, the stability of molecular clouds, the amplification of magnetic fields, and the re-acceleration and diffusion of cosmic rays. Despite its importance, interstellar turbulence, like turbulence in general, is far from being fully understood. In this review we present the basics of turbulence physics, focusing on the statistics of its structure and energy cascade. We explore the physics of compressible and incompressible turbulent flows, as well as magnetised cases. The most relevant observational techniques that provide quantitative insights into interstellar turbulence are also presented. We also discuss the main difficulties in developing a three-dimensional view of interstellar turbulence from these observations. Finally, we briefly present what the main sources of turbulence in the interstellar medium could be.

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Molecules in the Magellanic Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900201046 ◽

1991 ◽

Vol 148 ◽

pp. 415-420 ◽

Cited By ~ 1

Author(s):

R. S. Booth ◽

Th. De Graauw

Keyword(s):

High Resolution ◽

Molecular Clouds ◽

Short Review ◽

Large Magellanic Cloud ◽

Magellanic Cloud ◽

Magellanic Clouds ◽

Giant Molecular Clouds ◽

Interstellar Molecules ◽

First Time ◽

Excitation Conditions

In this short review we describe recent new observations of millimetre transitions of molecules in selected regions of the Magellanic Clouds. The observations were made using the Swedish-ESO Submillimetre Telescope, SEST, (Booth et al. 1989), the relatively high resolution of which facilitates, for the first time, observations of individual giant molecular clouds in the Magellanic Clouds. We have mapped the distribution of the emission from the two lowest rotational transitions of 12CO and 13CO and hence have derived excitation conditions for the molecule. In addition, we have observed several well-known interstellar molecules in the same regions, thus doubling the number of known molecules in the Large Magellanic Cloud (LMC). The fact that all the observations have been made under controlled conditions with the same telescope enables a reasonable intercomparison of the molecular column densities. In particular, we are able to observe the relative abundances among the different isotopically substituted species of CO.

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Hypersonic swizzle sticks: jets, fossil cavities and turbulence in molecular clouds

Proceedings of the International Astronomical Union ◽

10.1017/s174392130700141x ◽

2006 ◽

Vol 2 (S237) ◽

pp. 172-176

Author(s):

Andrew J Cunningham ◽

Adam Frank ◽

Eric G Blackman ◽

Alice Quillen

Keyword(s):

Turbulent Flows ◽

Molecular Clouds ◽

Turbulent Medium ◽

Young Stars ◽

Turbulence Energy ◽

Theoretical Simulation ◽

Bow Shocks ◽

Ambient Gas ◽

Energy And Momentum ◽

Ability To Drive

AbstractThe ubiquity and high density of outflows from young stars in clusters make them an intriguing candidate for the source of turbulence energy in molecular clouds. In this contribution we discuss new studies, both observational and theoretical, which address the issue of jet/outflow interactions and their ability to drive turbulent flows in molecular clouds. Our results are surprising in that they show that fossil cavities, rather than bow shocks from active outflows, constitute the mechanism of re-energizing turbulence. We first present simulations which show that collisions between active jets are ineffective at converting directed momentum and energy in outflows into turbulence. This effect comes from the ability of radiative cooling to constrain the surface area through which colliding outflows entrain ambient gas. We next discuss observational results which demonstrate that fossil cavities from “extinct” outflows are abundant in molecular material surrounding clusters such as NGC 1333. These structures, rather than the bow shocks of active outflows, comprise the missing link between outflow energy input and re-energizing turbulence. In a separate theoretical/simulation study we confirm that the evolution of cavities from decaying outflow sources leads to structures which match the observations of fossil cavities. Finally we present new results of outflow propagation in a fully turbulent medium exploring the explicit mechanisms for the transfer of energy and momentum between the driving wind and the turbulent environment.

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Magnetic Decreases (MDs) and mirror modes: two different plasma β changing mechanisms

Nonlinear Processes in Geophysics ◽

10.5194/npg-17-467-2010 ◽

2010 ◽

Vol 17 (5) ◽

pp. 467-479 ◽

Cited By ~ 10

Author(s):

B. T. Tsurutani ◽

G. S. Lakhina ◽

O. P. Verkhoglyadova ◽

E. Echer ◽

F. L. Guarnieri

Keyword(s):

Free Energy ◽

Energetic Particle ◽

Ponderomotive Force ◽

Interplanetary Space ◽

The Other ◽

Characteristic Scale ◽

Physical Processes ◽

Scale Size ◽

Mirror Mode ◽

Energetic Particle Transport

Abstract. We discuss two different physical processes that create localized high β plasma regions. One is nonlinear wave-steepening, generating magnetic decreases (MDs) by a ponderomotive force. The other is the mirror instability generating alternating high and low β plasma regions. It is demonstrated that MDs and mirror modes are observationally quite different structures. MDs spatially occur in interplanetary space and mirror modes primarily in planetary magnetosheaths. MDs are characterized by: 1) variable (exponentially decreasing number with increasing) angular changes, 2) variable (exponentially decreasing) thicknesses, and 3) no characteristic inter-event spacings. In sharp contrast, mirror modes are characterized by: 1) little or no angular changes across the structures, 2) a characteristic scale size, and 3) are quasiperiodic in nature. Arguments are presented for the recently observed magnetic dips in the heliosheath being mirror mode structures. The sources of free energy for instability are discussed. Both structures are important for energetic particle transport in astrophysical and heliospheric plasmas.

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Discussion session on star formation, molecular clouds and the interstellar medium

Astrophysics and Space Science ◽

10.1007/bf00990060 ◽

1994 ◽

Vol 217 (1-2) ◽

pp. 227-230

Author(s):

Karen M. Strom ◽

Lennart Nordh ◽

Eli Dwek

Keyword(s):

Star Formation ◽

Interstellar Medium ◽

Molecular Clouds ◽

Discussion Session

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Understanding biases in measurements of molecular cloud kinematics using line emission

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2432 ◽

2020 ◽

Vol 498 (2) ◽

pp. 2440-2455

Author(s):

Yuxuan (宇轩) Yuan (原) ◽

Mark R Krumholz ◽

Blakesley Burkhart

Keyword(s):

Magnetic Field ◽

Molecular Cloud ◽

Molecular Clouds ◽

Velocity Dispersion ◽

Signal To Noise Ratio ◽

Line Emission ◽

Molecular Line ◽

Velocity Dispersions ◽

Cloud Kinematics ◽

Measurement Biases

ABSTRACT Molecular line observations using a variety of tracers are often used to investigate the kinematic structure of molecular clouds. However, measurements of cloud velocity dispersions with different lines, even in the same region, often yield inconsistent results. The reasons for this disagreement are not entirely clear, since molecular line observations are subject to a number of biases. In this paper, we untangle and investigate various factors that drive linewidth measurement biases by constructing synthetic position–position–velocity cubes for a variety of tracers from a suite of self-gravitating magnetohydrodynamic simulations of molecular clouds. We compare linewidths derived from synthetic observations of these data cubes to the true values in the simulations. We find that differences in linewidth as measured by different tracers are driven by a combination of density-dependent excitation, whereby tracers that are sensitive to higher densities sample smaller regions with smaller velocity dispersions, opacity broadening, especially for highly optically thick tracers such as CO, and finite resolution and sensitivity, which suppress the wings of emission lines. We find that, at fixed signal-to-noise ratio, three commonly used tracers, the J = 4 → 3 line of CO, the J = 1 → 0 line of C18O, and the (1,1) inversion transition of NH3, generally offer the best compromise between these competing biases, and produce estimates of the velocity dispersion that reflect the true kinematics of a molecular cloud to an accuracy of $\approx 10{{\ \rm per\ cent}}$ regardless of the cloud magnetic field strengths, evolutionary state, or orientations of the line of sight relative to the magnetic field. Tracers excited primarily in gas denser than that traced by NH3 tend to underestimate the true velocity dispersion by $\approx 20{{\ \rm per\ cent}}$ on average, while low-density tracers that are highly optically thick tend to have biases of comparable size in the opposite direction.

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