scholarly journals Synthetic linemaps for hierarchial clouds

1991 ◽  
Vol 147 ◽  
pp. 496-497
Author(s):  
L. G. Stenholm
Keyword(s):  
Molecular Clouds ◽  
Special Problem ◽  
Theoretical Modelling ◽  
Random Component ◽  
Velocity Correlation ◽  
Internal Motions ◽  
Cloud Models ◽  
Spherical Cloud ◽  
Fundamental Formulation

Molecular clouds are characterized by large density contrasts and pronounced internal motions. Some information on their internal physics has been acquired through relatively simple data analysis and through theoretical modelling, mainly hydrodynamical calculations. More detailed radiative transfer models have been limited to spherical cloud models.A related but less recognized problem is the classification of molecular clouds, or otherwise stated, can we compress the information in the observed two-dimensional maps for various line parameters in such a way that the physical differences are retained, while the random component has been removed? This is a more fundamental formulation of the special problem of comparing the appearance of two molecular clouds, or of the problems of quantifying the velocity correlation within the clouds.

scholarly journals Synthetic linemaps for hierarchial clouds

1991 ◽  
Vol 147 ◽  
pp. 496-497
Author(s):  
L. G. Stenholm
Keyword(s):  
Molecular Clouds ◽  
Special Problem ◽  
Theoretical Modelling ◽  
Random Component ◽  
Velocity Correlation ◽  
Internal Motions ◽  
Cloud Models ◽  
Spherical Cloud ◽  
Fundamental Formulation

Molecular clouds are characterized by large density contrasts and pronounced internal motions. Some information on their internal physics has been acquired through relatively simple data analysis and through theoretical modelling, mainly hydrodynamical calculations. More detailed radiative transfer models have been limited to spherical cloud models.A related but less recognized problem is the classification of molecular clouds, or otherwise stated, can we compress the information in the observed two-dimensional maps for various line parameters in such a way that the physical differences are retained, while the random component has been removed? This is a more fundamental formulation of the special problem of comparing the appearance of two molecular clouds, or of the problems of quantifying the velocity correlation within the clouds.

scholarly journals Massive core/star formation triggered by cloud–cloud collision: Effect of magnetic field

2020 ◽  
Author(s):  
Nirmit Sakre ◽  
Asao Habe ◽  
Alex R Pettitt ◽  
Takashi Okamoto
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Strong Magnetic Field ◽  
Molecular Clouds ◽  
Weak Magnetic Field ◽  
Dense Cores ◽  
Cloud Models ◽  
Low Mass ◽  
Magnetohydrodynamic Simulations

Abstract We study the effect of magnetic field on massive dense core formation in colliding unequal molecular clouds by performing magnetohydrodynamic simulations with sub-parsec resolution (0.015 pc) that can resolve the molecular cores. Initial clouds with the typical gas density of the molecular clouds are immersed in various uniform magnetic fields. The turbulent magnetic fields in the clouds consistent with the observation by Crutcher et al. (2010, ApJ, 725, 466) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 μG. The collision speed of 10 km s−1 is adopted, which is much larger than the sound speeds and the Alfvén speeds of the clouds. We identify gas clumps with gas densities greater than 5 × 10−20 g cm−3 as the dense cores and trace them throughout the simulations to investigate their mass evolution and gravitational boundness. We show that a greater number of massive, gravitationally bound cores are formed in the strong magnetic field (4.0 μG) models than the weak magnetic field (0.1 μG) models. This is partly because the strong magnetic field suppresses the spatial shifts of the shocked layer that should be caused by the nonlinear thin shell instability. The spatial shifts promote the formation of low-mass dense cores in the weak magnetic field models. The strong magnetic fields also support low-mass dense cores against gravitational collapse. We show that the numbers of massive, gravitationally bound cores formed in the strong magnetic field models are much larger than in the isolated, non-colliding cloud models, which are simulated for comparison. We discuss the implications of our numerical results on massive star formation.

scholarly journals Classification of Filament Formation Mechanisms in Magnetized Molecular Clouds

2021 ◽  
Vol 916 (2) ◽  
pp. 83
Author(s):  
Daisei Abe ◽  
Tsuyoshi Inoue ◽  
Shu-ichiro Inutsuka ◽  
Tomoaki Matsumoto
Keyword(s):  
Molecular Clouds ◽  
Formation Mechanisms ◽  
Filament Formation

Active Prominences

1979 ◽  
Vol 44 ◽  
pp. 214-225 ◽  
Author(s):  
J. McKim Malville
Keyword(s):  
Magnetic Fields ◽  
High Energy ◽  
Low Energy ◽  
Necessary And Sufficient Condition ◽  
Quiescent Prominence ◽  
Internal Motions ◽  
Active Prominence ◽  
Necessary And Sufficient ◽  
Energy Side

Those prominences which are identified as active lie near the middle of a large group of objects found in the low corona bordered on the high energy side by flares and on the low energy side by quiescent prominences. Known by descriptive terms such as eruptive, surge, spray, tornado, and loop, active prominences typically have shorter lifetimes, broader line widths, larger internal motions, and stronger internal magnetic fields than quiescent prominences (Tandberg-Hanssen, 1974). When dealing with specific examples, however, it is often difficult to establish a necessary and sufficient condition for classification of such an object as an active prominence. The ambiguity at the low energy end involves “hybrid” objects which possess features of both quiescent and active prominences. For example, the active region filament may have a lifetime of several days, have large internal motions and relatively strong magnetic fields. The stable hedgerow quiescent prominence may contain small regions with large widths and large velocities. On the average, a quiescent will typically erupt every five to eight days, (Serio, et al., 1978; Bryson and Malville, 1978) and at those times a prominence is transformed from the quiescent to the active state. For these objects something other than morphology, velocity fields, or even magnetic fields is necessary to specify their condition; something less symptomatic and more fundamental is required. That necessary parameter may be, I shall suggest, the total current, J, flowing in the structure.

scholarly journals Statistical mass function of prestellar cores from the density distribution of their natal clouds

2020 ◽  
Vol 635 ◽  
pp. A88
Author(s):  
S. Donkov ◽  
T. V. Veltchev ◽  
Ph. Girichidis ◽  
R. S. Klessen
Keyword(s):  
Power Law ◽  
Molecular Clouds ◽  
Mass Density ◽  
Mass Function ◽  
Initial Mass Function ◽  
High Mass ◽  
Core Mass ◽  
Density Relationship ◽  
Prestellar Cores ◽  
Spherical Cloud

The mass function of clumps observed in molecular clouds raises interesting theoretical issues, especially in its relation to the stellar initial mass function (IMF). We propose a statistical model of the mass function of prestellar cores (CMF), formed in self-gravitating isothermal clouds at a given stage of their evolution. The latter is characterized by the mass-density probability distribution function (ρ-PDF), which is a power-law with slope q. The different molecular clouds are divided into ensembles according to the PDF slope and each ensemble is represented by a single spherical cloud. The cores are considered as elements of self-similar structure typical for fractal clouds and are modeled by spherical objects populating each cloud shell. Our model assumes relations between size, mass, and density of the statistical cores. Out of these, a core mass-density relationship ρ ∝ mx is derived where x = 1∕(1 + q). We find that q determines the existence or nonexistence of a threshold density for core collapse. The derived general CMF is a power law of slope − 1 while the CMF of gravitationally unstable cores has a slope (−1 + x∕2), comparable with the slopes of the high-mass part of the stellar IMF and of observational CMFs.

scholarly journals Magnetic Field Generation within Molecular Clouds

1993 ◽  
Vol 157 ◽  
pp. 429-430
Author(s):  
A. Lazarian
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Molecular Complexes ◽  
Magnetic Field Generation ◽  
Field Problem ◽  
Field Generation ◽  
Internal Motions ◽  
Turbulent Dynamo ◽  
Seed Field

Magnetic field generation in molecular (atomic) clouds at the early stages of galactic evolution is considered. It is shown that if there is no internal motions immersed the cloud, battery mechanisms (Lazarian 1992a) can account for the generation of thin magnetic shells around clouds insides in plasma with temperature gradients. If turbulent motions are present, the dynamo can be essential. The operation of α — ω, α2 and turbulent dynamos within molecular clouds is discussed. It is shown that the turbulent dynamo leads to generation of magnetic fields in the trace behind the cloud. These magnetic fields within the molecular clouds and in their vicinity are important for the solution of the galactic seed field problem (see Lazarian 1992b) and the formation of structures in clumpy molecular complexes.

scholarly journals Comparison of cloud models for Brown Dwarfs

2007 ◽  
Vol 3 (S249) ◽  
pp. 173-177 ◽  
Author(s):  
Ch. Helling ◽  
A. Ackerman ◽  
F. Allard ◽  
M. Dehn ◽  
P. Hauschildt ◽  
...  
Keyword(s):  
Cloud Model ◽  
Dust Cloud ◽  
Brown Dwarfs ◽  
Theoretical Modelling ◽  
Spectral Energy ◽  
Test Case ◽  
Energy Distributions ◽  
Cloud Models ◽  
Case Comparison ◽  
Cloud Modelling

AbstractA test case comparison is presented for different dust cloud model approaches applied in brown dwarfs and giant gas planets. We aim to achieve more transparency in evaluating the uncertainty inherent to theoretical modelling. We show in how far model results for characteristic dust quantities vary due to different assumptions. We also demonstrate differences in the spectral energy distributions resulting from our individual cloud modelling in 1D substellar atmosphere simulations.

scholarly journals A Morphological Classification of 18,190 Molecular Clouds Identified in 12CO Data from the MWISP Survey

2021 ◽  
Vol 257 (2) ◽  
pp. 51
Author(s):  
Lixia Yuan ◽  
Ji Yang ◽  
Fujun Du ◽  
Xunchuan Liu ◽  
Shaobo Zhang ◽  
...  
Keyword(s):  
Molecular Clouds ◽  
Milky Way ◽  
Systematic Difference ◽  
Line Of Sight ◽  
Physical Parameters ◽  
Emission Flux ◽  
Morphological Classification ◽  
Integrated Intensity ◽  
Co Emission

Abstract We attempt to visually classify the morphologies of 18,190 molecular clouds, which are identified in the 12CO(1–0) spectral line data over ∼450 deg2 of the second Galactic quadrant from the Milky Way Imaging Scroll Painting project. Using the velocity-integrated intensity maps of the 12CO(1–0) emission, molecular clouds are first divided into unresolved and resolved ones. The resolved clouds are further classified as nonfilaments or filaments. Among the 18,190 molecular clouds, ∼25% are unresolved, ∼64% are nonfilaments, and ∼11% are filaments. In the terms of the integrated flux of 12CO(1–0) spectra of all 18,190 molecular clouds, ∼90% are from filaments, ∼9% are from nonfilaments, and the remaining ∼1% are from unresolved sources. Although nonfilaments are dominant in the number of the discrete molecular clouds, filaments are the main contributor of 12CO emission flux. We also present the number distributions of the physical parameters of the molecular clouds in our catalog, including their angular sizes, velocity spans, peak intensities of 12CO(1–0) emission, and 12CO(1–0) total fluxes. We find that there is a systematic difference between the angular sizes of the nonfilaments and filaments, with the filaments tending to have larger angular scales. The H2 column densities of them are not significantly different. We also discuss the observational effects, such as those induced by the finite spatial resolution, beam dilution, and line-of-sight projection, on the morphological classification of molecular clouds in our sample.

scholarly journals Observational studies of the formation and evolution of dense cores

2015 ◽  
Vol 11 (S315) ◽  
pp. 95-102
Author(s):  
Mario Tafalla
Keyword(s):  
Molecular Clouds ◽  
Large Scale ◽  
Gravitational Instability ◽  
Continuum Emission ◽  
Star Forming ◽  
Dense Cores ◽  
Internal Motions ◽  
Formation And Evolution ◽  
End Stage ◽  
Kinematic Information

AbstractDense cores are the simplest star-forming sites. They represent the end stage of the fragmentation hierarchy that characterizes molecular clouds, and they likely control the efficiency of star formation via their relatively low numbers. Recent dust continuum observations of entire molecular clouds show that dense cores often lie along large-scale filamentary structures, suggesting that the cores form by some type of fragmentation process in an approximately cylindrical geometry. To understand the formation mechanism of cores, additional kinematic information is needed, and this requires observations in molecular-line tracers of both the dense cores and their surrounding cloud material. Here I present some recent efforts to clarify the kinematic structure of core-forming regions in the nearby Taurus molecular cloud. These new observations show that the filamentary structures seen in clouds are often more complex than suggested by the maps of continuum emission, and that they consist of multiple fiber-like components that have different velocities and sonic internal motions. These components likely arise from turbulent fragmentation of the large-scale flows that generate the filamentary structures. While not all these fiber-like components further fragment to form dense cores, a small group of them does so, likely by gravitational instability. This fragmentation produces characteristic chain-like groups of dense cores that further evolve to form stars.

scholarly journals Spiral arm triggering of star formation

2006 ◽  
Vol 2 (S237) ◽  
pp. 344-350
Author(s):  
Ian A. Bonnell ◽  
Clare L. Dobbs
Keyword(s):  
Star Formation ◽  
Molecular Clouds ◽  
Simple Explanation ◽  
Giant Molecular Clouds ◽  
Subsequent Formation ◽  
Internal Motions ◽  
Low Efficiency ◽  
Galactic Spiral ◽  
Size Relation ◽  
Spiral Arm

AbstractWe present numerical simulations of the passage of clumpy gas through a galactic spiral shock, the subsequent formation of giant molecular clouds (GMCs) and the triggering of star formation. The spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. In addition to triggering the star formation process, the clumpy gas passing through the shock naturally generates the observed velocity dispersion size relation of molecular clouds. In this scenario, the internal motions of GMCs need not be turbulent in nature. The coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Globally unbound molecular clouds provides a simple explanation of the low efficiency of star formation. While dense regions in the shock become bound and collapse to form stars, the majority of the gas disperses as it leaves the spiral arm.

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