scholarly journals The Effects of Rotation during Star Formation

1981 ◽  
Vol 93 ◽  
pp. 5-26
Author(s):  
Peter Bodenheimer
Keyword(s):  
Star Formation ◽  
Spatial Resolution ◽  
Molecular Clouds ◽  
Three Dimensional ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Magnetic Effects ◽  
Show Evidence ◽  
Hydrodynamic Calculations ◽  
Effects Of Rotation

Observations of molecular clouds show evidence of rotation and of fragmentation of subregions of the clouds into multiple stellar or protostellar systems. This review concentrates on the effects that rotation and pressure gradients have in a self-gravitating cloud to cause it to undergo the crucial process of fragmentation. Recent two-dimensional and three-dimensional numerical hydrodynamic calculations have made progress in determining these effects. In most cases the calculations are performed with modest spatial resolution and are limited to isothermal clouds with neglect of viscous and magnetic effects. The combined results of several calculations strongly suggest that rotating clouds that are unstable to collapse are also unstable to fragmentation.

scholarly journals Bipolar H II regions

2018 ◽  
Vol 617 ◽  
pp. A67 ◽  
Author(s):  
M. R. Samal ◽  
L. Deharveng ◽  
A. Zavagno ◽  
L. D. Anderson ◽  
S. Molinari ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Galactic Plane ◽  
Three Dimensional ◽  
Young Stellar Objects ◽  
Formation Processes ◽  
Two Dimensional ◽  
Stellar Objects ◽  
Flat Structures ◽  
Exciting Star

Aims. We aim to identify bipolar Galactic H II regions and to understand their parental cloud structures, morphologies, evolution, and impact on the formation of new generations of stars. Methods. We use the Spitzer-GLIMPSE, Spitzer-MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar H II regions and to examine their morphologies. We search for their exciting star(s) using NIR data from the 2MASS, UKIDSS, and VISTA surveys. Massive molecular clumps are detected near these bipolar nebulae, and we estimate their temperatures, column densities, masses, and densities. We locate Class 0/I young stellar objects (YSOs) in their vicinities using the Spitzer and Herschel-PACS emission. Results. Numerical simulations suggest bipolar H II regions form and evolve in a two-dimensional flat- or sheet-like molecular cloud. We identified 16 bipolar nebulae in a zone of the Galactic plane between ℓ ± 60° and |b| < 1°. This small number, when compared with the 1377 bubble H II regions in the same area, suggests that most H II regions form and evolve in a three-dimensional medium. We present the catalogue of the 16 bipolar nebulae and a detailed investigation for six of these. Our results suggest that these regions formed in dense and flat structures that contain filaments. We find that bipolar H II regions have massive clumps in their surroundings. The most compact and massive clumps are always located at the waist of the bipolar nebula, adjacent to the ionised gas. These massive clumps are dense, with a mean density in the range of 105 cm−3 to several 106 cm−3 in their centres. Luminous Class 0/I sources of several thousand solar luminosities, many of which have associated maser emission, are embedded inside these clumps. We suggest that most, if not all, massive 0/I YSO formation has probably been triggered by the expansion of the central bipolar nebula, but the processes involved are still unknown. Modelling of such nebula is needed to understand the star formation processes at play.

scholarly journals Two-Dimensional Radiation-Hydrodynamics Calculations of the Formation of 0-B Associations in Dense Molecular Clouds

1980 ◽  
Vol 58 ◽  
pp. 275-282
Author(s):  
Richard I. Klein ◽  
Maxwell T. Sandford II ◽  
Rodney W. Whitaker
Keyword(s):  
Star Formation ◽  
Formation Process ◽  
Molecular Cloud ◽  
Molecular Clouds ◽  
Radiation Hydrodynamics ◽  
Combined Effects ◽  
Two Dimensional ◽  
Density Enhancement

AbstractTwo-dimensional calculations of ionization-shockwave propagation into a curved molecular cloud are presented. Density enhancement occurs due to the combined effects of cloud curvature and radiation flow. The star formation process is expected to be enhanced near the edges of irregularly shaped molecular clouds.

Effect of Adverse Pressure Gradients on Turbulent Boundary Layers in Axisymmetric Conduits

1957 ◽  
Vol 24 (2) ◽  
pp. 191-196
Author(s):  
J. M. Robertson ◽  
J. W. Holl
Keyword(s):  
Boundary Layer ◽  
Flow Parameter ◽  
Three Dimensional ◽  
Turbulent Boundary Layers ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Flow Conditions ◽  
Straight Pipe ◽  
Outer Flow ◽  
Dimensional Boundary

Abstract The development of the three-dimensional boundary layer in a diffuser with several discharge arrangements has been studied for air flow, in continuation of the work of Uram (1). The flow conditions in a diffuser when followed by a straight pipe, an additional length of the diffuser, or a jet, are compared. Extension of the method of analysis developed by Ross for two-dimensional layers is presented. In some cases the use of three-dimensionally defined parameters leads to different results. Ross’s (2) unique outer-flow parameter is found to be no longer satisfactory. Other outer parameters are presented as possible substitutes.

scholarly journals MHD turbulence-Star Formation Connection: from pc to kpc scales

2010 ◽  
Vol 6 (S274) ◽  
pp. 333-339 ◽  
Author(s):  
E. M. de Gouveia Dal Pino ◽  
R. Santos-Lima ◽  
A. Lazarian ◽  
M. R. M. Leão ◽  
D. Falceta-Gonçalves ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Flux ◽  
Numerical Simulations ◽  
Magnetic Reconnection ◽  
Turbulent Flows ◽  
Molecular Clouds ◽  
Three Dimensional ◽  
Mhd Turbulence ◽  
Star Forming

AbstractThe transport of magnetic flux to outside of collapsing molecular clouds is a required step to allow the formation of stars. Although ambipolar diffusion is often regarded as a key mechanism for that, it has been recently argued that it may not be efficient enough. In this review, we discuss the role that MHD turbulence plays in the transport of magnetic flux in star forming flows. In particular, based on recent advances in the theory of fast magnetic reconnection in turbulent flows, we will show results of three-dimensional numerical simulations that indicate that the diffusion of magnetic field induced by turbulent reconnection can be a very efficient mechanism, especially in the early stages of cloud collapse and star formation. To conclude, we will also briefly discuss the turbulence-star formation connection and feedback in different astrophysical environments: from galactic to cluster of galaxy scales.

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

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.

Paper 1: Centrifugal Compressors for Gas Turbines

1968 ◽  
Vol 183 (14) ◽  
pp. 1-10 ◽  
Author(s):  
T. B. Ferguson
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Three Dimensional ◽  
Boundary Layer Theory ◽  
Pressure Gradients ◽  
Two Dimensional ◽  
Centrifugal Compressors ◽  
Blade Loading ◽  
Actual Flow ◽  
High Mach

The trends in fluid mechanical development of centrifugal compressors are discussed. The main developments in the impeller are the application of quasi-three-dimensional isentropic methods together with some separation criteria based on two-dimensional turbulent boundary layer theory. Diffusers are sometimes designed on a simplified two-dimensional basis but channel diffusers still appear to be preferred especially at high Mach numbers. Recent visualization studies have shown how far the actual flow in impellers may depart from the actual model and there is a lack of systematic quantitative experimental work on limiting blade loading and pressure gradients both in impellers and diffusers. A summary of gas turbine centrifugal compressors is also made.

A Length-Scale Model For Developing Turbulent Flow in a Rectangular Duct

1977 ◽  
Vol 99 (2) ◽  
pp. 347-356 ◽  
Author(s):  
F. B. Gessner ◽  
A. F. Emery
Keyword(s):  
Reynolds Stress ◽  
Three Dimensional ◽  
Scale Model ◽  
Length Scale ◽  
Pressure Gradients ◽  
Mixing Length ◽  
Two Dimensional ◽  
Rectangular Duct ◽  
Corner Flows ◽  
Theory And Experiment

A three-dimensional mixing length model is proposed for modeling local Reynolds stress behavior in rectangular ducts of arbitrary aspect ratio. The model is applicable to both developing and fully-developed flows, and can be applied to other 90-degree corner flows with mild streamwise pressure gradients. Comparisons between theory and experiment show that all components of the Reynolds stress tensor are modeled reasonably well, both in the vicinity of a corner and in two-dimensional regions away from the corner.

scholarly journals Smoothness correction for better SOFI imaging

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Siewert Hugelier ◽  
Wim Vandenberg ◽  
Tomáš Lukeš ◽  
Kristin S. Grußmayer ◽  
Paul H. C. Eilers ◽  
...  
Keyword(s):  
Statistical Analysis ◽  
Fluorescence Imaging ◽  
Spatial Resolution ◽  
Three Dimensional ◽  
Super Resolution ◽  
Cellular Structures ◽  
Stationary Processes ◽  
Two Dimensional ◽  
Computationally Efficient ◽  
Statistical Analysis Methods

AbstractSub-diffraction or super-resolution fluorescence imaging allows the visualization of the cellular morphology and interactions at the nanoscale. Statistical analysis methods such as super-resolution optical fluctuation imaging (SOFI) obtain an improved spatial resolution by analyzing fluorophore blinking but can be perturbed by the presence of non-stationary processes such as photodestruction or fluctuations in the illumination. In this work, we propose to use Whittaker smoothing to remove these smooth signal trends and retain only the information associated to independent blinking of the emitters, thus enhancing the SOFI signals. We find that our method works well to correct photodestruction, especially when it occurs quickly. The resulting images show a much higher contrast, strongly suppressed background and a more detailed visualization of cellular structures. Our method is parameter-free and computationally efficient, and can be readily applied on both two-dimensional and three-dimensional data.

scholarly journals Stratified Kelvin-Helmholtz turbulence of compressible shear flows

2018 ◽  
Author(s):  
Romit Maulik ◽  
Omer San
Keyword(s):  
Power Density ◽  
Spatial Resolution ◽  
Shear Flows ◽  
Incompressible Flows ◽  
Three Dimensional ◽  
Structure Functions ◽  
Compressible Turbulence ◽  
Power Density Spectrum ◽  
Two Dimensional ◽  
Density Spectrum

Abstract. We study the scaling laws and structure functions of stratified shear flows by performing high-resolution numerical simulations of inviscid compressible turbulence induced by Kelvin-Helmholtz instability. An implicit large eddy simulation approach is adapted to solve our conservation laws for both two-dimensional (with a spatial resolution of 16,3842) and three-dimensional (with a spatial resolution of 5123) configurations utilizing different compressibility characteristics such as shocks. For three-dimensional turbulence, we find that both kinetic energy and density-weighted energy spectra follow the classical Kolmogorov k−5/3 inertial scaling. This phenomenon is observed due to the fact that the power density spectrum of three-dimensional turbulence yields the same k−5/3 scaling. However, we demonstrate that there is a significant difference between these two spectra in two-dimensional turbulence since the power density spectrum flattens to k−1/4. This flattening may be assumed to be a reason for the k−7/3 scaling observed in the two-dimensional density-weight kinetic every spectra for high compressibility as compared to the k−3 scaling traditionally assumed with incompressible flows. Further inquiries are made to validate the statistical behavior of the various configurations studied through the use of second and third order velocity structure functions where it is noticed that scaling behavior differs between the two- and three-dimensional cases wherein only the latter is seen to follow trends from K41 theory.

scholarly journals Reconstructing three-dimensional densities from two-dimensional observations of molecular gas

2021 ◽  
Vol 502 (4) ◽  
pp. 5997-6009
Author(s):  
Zipeng Hu ◽  
Mark R Krumholz ◽  
Christoph Federrath ◽  
Riwaj Pokhrel ◽  
Robert A Gutermuth
Keyword(s):  
Star Formation ◽  
Recent Observation ◽  
Three Dimensional ◽  
Free Fall ◽  
Volume Density ◽  
Mean Value ◽  
Two Dimensional ◽  
Density Estimates ◽  
Fall Time ◽  
Dimensional Measurements

ABSTRACT Star formation has long been known to be an inefficient process, in the sense that only a small fraction ϵff of the mass of any given gas cloud is converted to stars per cloud free-fall time. However, developing a successful theory of star formation will require measurements of both the mean value of ϵff and its scatter from one molecular cloud to another. Because ϵff is measured relative to the free-fall time, such measurements require accurate determinations of cloud volume densities. Efforts to measure the volume density from two-dimensional projected data, however, have thus far relied on treating molecular clouds as simple uniform spheres, while their real shapes are likely to be filamentary and their density distributions far from uniform. The resulting uncertainty in the true volume density is likely to be one of the major sources of error in observational estimates of ϵff. In this paper, we use a suite of simulations of turbulent, magnetized, radiative, self-gravitating star-forming clouds in order to examine whether it is possible to obtain more accurate volume density estimates and thereby reduce this error. We create mock observations from the simulations, and show that current analysis methods relying on the spherical assumption likely yield ∼0.26 dex underestimations and ∼0.51 dex errors in volume density estimates, corresponding to a ∼0.13 dex overestimation and a ∼0.25 dex scatter in ϵff, comparable to the scatter in observed cloud samples. We build a predictive model that uses information accessible in two-dimensional measurements – most significantly, the Gini coefficient of the surface density distribution – to produce estimates of the volume density with ∼0.3 dex less scatter. We test our method on a recent observation of the Ophiuchus cloud, and show that it successfully reduces the ϵff scatter.

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