Nearby Molecular Clouds

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Chemistry : back to top. Turbulence in molcular clouds : back to top. Molcular Clouds Caution!!

The Milky Way in Molecular Clouds: A New Complete CO Survey

Sgr B2: Saggittarius B2 Complex. Cooling Curve : Cooling rate of gas as a function of temperature. Results : database with 1,, spectra. They showed Iint maps, channel maps, p-v diagrams, grand average spectrum, etc. Stil et al. Results : database with pointing. Results : It is found that sufficiently large gas sphere is graviattionally unstable, which is comparable to the Jean's formula of gravitational instability.

Tomisaka, PASJ Motte et al. It was the first time to get the compact circumstellar dusty structures around YSOs and the extended emission from dense cores and ambient clouds detected simultaneously. The diffuse clouds are fragmented into 58 starless cores, with 19 of them showing flat intensity profile, 10 of them showing sharp peak.

Ballesteros-Paredes et al. Results : They found that the clouds are usually trancient objects without well-defined cloud edge. Neither the thermal equilibrium nor the turbulent pressure is the cloud confining agent. Quasi-hydrostatic configurations can not be produced by turbulence. Results : They found that the Larson's density-size relation is an artifact due to limited dynamical range in column density in the old observation, while the velocity dispersion-size relation is true in both physical and observed clumps.

They also confirmed that the clump mass histrogram follows a log-normal distribution. They concluded that successful fitting of BE profile to an observed cloud core does not guarantee hydrostatic equilibrium , and the fitted parameters mass, central density, density contrast, temperature, and radial profile of the BE sphere may differ significantly from the actual values in the cores.

Results : 1 He argued that a turbulence eventually becomes unimportant in star formation because it is dissipative and decay rapidly to smaller scales; b magnetic field can be dissipated through ambipolar diffusion and the anisotropic magnetic field lines do not prevent matter from accumulating along the field lines. In contrast, thermal pressure work at all size scales and becomes increasingly important during the contraction of the interstellar clouds. Therefore, the Jeans mass determined by the ballance between thermal pressure and gravity is universal in the star formation processes, irrespective of what happened during the early stage of the star formation.

During the cloud contraction processes, the atomic cooling rates increase with density in low density gas, thus the cloud temperature decreases with the progress of contraction. But when the density is higher than a certain threshold, the atomic cooling rates do not increase anymore due to opacity effect , the cloud temperature reaches a minimum of about 5K Larson and then turns to rise up due to the more and more efficient gravitational collapse heating.

Therefore, the Jeans mass will decrease with further process of contraction. Consequently, the Jeans mass will decrease thus with more active fragmentation during the cold low-density contraction phase, whilst it will increase thus with fragmentation suppressed during the warmer high-density contraction phase. The critical state between the low-density and high-density contraction phases thus determines the final properties of the cloud fragmentation.

Nearby Molecular Clouds

This critical density is determined by the cooling properties, and thus nature and status of the material, of the clouds. Thus the mass of the first stars is largely determined by the intrinsic atomic and molecular properties of the H2.

This is important at some stages when most polar molecules have been frozen out onto grains, because then the gas is thermally coupled with dust grains and the dust thermal emission becomes the major cooling agent. This limit is 0. Simulations support that the Jeans mass or Bonnor-Ebert mass at the critical density for the temperature minimum determines the peak mass of the IMF. He did not mention: why? In dense clouds, the cloud core can only collapse to form spherical structure.

Molecular Clouds

The thin sheets in the circumstellar discs can maintain for a long enough time to fragment to form smaller stars or planets, but it does not alter the overall shape of the IMF. Ballesteros-Paredes, et al. Results : 1 They proposed that GMCs are formed by large-scale gravitational instability induced by the stellar spiral arms in the galactic disc mid-plane, whilst the more scattered smaller-scale MCs distributed as turbulent atomic gas are formed by the compression of turbulence produced by SNe, superbubbles or HII regions.

The PDF is usually a log-normal function, with the logarithmic density being in a Gaussian distribution.

Molecular Cloud

If, as suggested by some researchers, the lifetime of MCs is comparable or even shorter than the free-fall time constantly destroyed by star formation activities , the turbulence in MCs may never have enough time to decay. This is expected because clouds with much lower column density may not be molecular , while clouds with much higher column density may collapse quickly. MCs represent the tip of iceburg of the neutral stable gas distribution. However, both larger and smaller beta have been determined by other work. The index of compressible flows is uncertain yet. It could be several times of free-fall time, say, several Myr.

Thus, angular momentum must be lost at all stages of star formation.

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The Astrophysics Spectator: Molecular Clouds

The dynamics of multiple system can have effect to the formation of cluster. The more massive stars tend to move slower and thus sink to the center of the cluster. Accretion should be the major parth of star growth, because stellar encounter has very low possibility, unless it's in a very dense proto-cluster. Solomon, et al. Myers et al.

Densities are estimated with the visual cloud sizes. The typical C18O sources have properties as below. They concluded that most of clouds should be supported by turbulence ; more than half of the sources show non-Gaussina profiles and blue screwed profile dominate, indication infall motions.

Strong sources are also mapped. Sanders et al. Results : 1 the total mass of H2 3. Winnewisser, M. Frosts in the p Oph molecular cloud. Zinnecker, A. Websterl, T. Observations of 12 CO data and models for the dark cloud L Vedi, I. Williams, L. Avery, G. White, N. Dense cores and star formation in nearby dark clouds. IRAS observations of star formation in nearby molecular clouds.


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Beichman, J. Emerson, R. Jennings, S. Harris, B. Baud, E.


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