Fragmentation Of Filamentary Clouds Research Articles

Collapse of Primordial Filamentary Clouds under Far-Ultraviolet Radiation

The collapse and fragmentation of primordial filamentary clouds under isotropic dissociation radiation was investigated with one-dimensional hydrodynamical calculations. We investigate the effect of the dissociation photon on the filamentary clouds by calculating non-equilibrium chemical reactions. With the external radiation assumed to turn on when the filamentary cloud forms, a filamentary cloud with low initial density ($n_0$$\le$ 10$^{2}\ $ cm$^{-3}$ ) suffers a photodissociation of hydrogen molecules. In such a case, since the main coolant is lost, the temperature increases adiabatically enough to suppress the collapse. As a result, the filamentary cloud fragments into very massive clouds ($\sim$ 10$^{5}\ M_\odot$ ). On the other hand, the evolution of the filamentary clouds with high initial density ($n_0$$>$ 10$^{2}\ $ cm$^{-3}$ ) is hardly affected by the external radiation. This is because the filamentary cloud with high initial density shields itself from the external radiation. It is found that the fragment mass increases owing to the external radiation. This result is consistent with previous results with one-zone models. It is also found that the fragment mass decreases owing to the external dissociation radiation in the case with a sufficiently large line mass.

Fragmentation of Primordial Filamentary Clouds under Far-Ultraviolet Radiation

The collapse and fragmentation of uniform filamentary clouds under isotropic far-ultraviolet external radiation are investigated. Especially, the impact of the photodissociation of hydrogen molecules during collapse is considered. The dynamical and thermal evolutions of collapsing filamentary clouds are calculated by solving the virial equation and the energy equation while taking into account non-equilibrium chemical reactions. It is found that thermal evolution is hardly affected by external radiation if the initial density is high ($ n_0$$ \gt$ 10$ ^{2}$ cm$ ^{-3}$ ). On the other hand, if the line mass of the filamentary cloud is moderate and the initial density is low ($ n_0$$ \le$ 10$ ^{2}$ cm$ ^{-3}$ ), the thermal evolution of the filamentary cloud tends to be adiabatic, owing to the effect of external dissociation radiation. In this case, the collapse of the filamentary cloud is suppressed, and the filamentary cloud fragments into very massive clouds ($ \sim$ 10$ ^{4-5}\ M_\odot$ ) in the early stage of collapse. The analytic criterion for the filamentary clouds to fragment into such massive clouds is discussed. We also investigate the collapse and fragmentation of the filamentary clouds with an improved model. This model can partly capture the effect of run-away collapse. Also, in this model filamentary clouds with low initial density ($ n_0$$ \le$ 10$ ^{2}$ cm$ ^{-3}$ ) fragment into massive clouds ($ \sim$ 10$ ^{4}\ M_\odot$ ) owing to the effect of external radiation.

Decay of Alfven Waves in a Filamentary Cloud

The decay of Alfven waves in a filamentary molecular cloud is investigated through three-dimensional numerical simulations. We have considered a filamentary molecular cloud supported in part by the Alfven wave against the self-gravity. Our attention has been focused on the basic physical mechanism for the decay. The decay rate is obtained as a function of the wavelength and amplitude. It is found that when the wave is circularly polarized, the decay e-folding timescale is several times the fast wave crossing timescale for the filament and independent of the wavelength, whereas when the wave is linearly polarized, the amplitude of the wave decreases inversely proportional to time. It is also found that the decay of Alfven waves induces rotation and shear flow in the filamentary cloud. The propagation of two Alfven waves in the medium results in the excitation of daughter waves due to nonlinear coupling between mother waves. The wavenumber of the daughter waves is the sum or difference between those of the mother waves, and below a critical wavenumber of the daughter wave, the filamentary cloud fragments as a result of Jeans instability. The fragments collapse to form high-density rotating magnetized disks. In contrast, below a critical wavenumber of the mother wave, the cloud becomes a dense helical filament after the decay of the Alfven waves. The present models are compared with previous simulations and observations with regard to the rotation, fragmentation, and helical structure of filamentary clouds.

The Dependence of Fragmentation of Metal-Deficient Gas Clouds on H2and HD Cooling and the Initial Mass Function of First Generation Stars

Recent threee-dimensional numerical simulations have shown that a filamentary structure is one of the most prominent features in the structure formation of the early universe. Fragmentation of the filamentary clouds is expected to lead to the formation of first generation stars. In this context, we have studied the gravitational collapse and fragmentation of filamentary clouds with axisymmetric hydrodynamic simulations coupled with non-equilibrium chemistry of H 2 and HD. In this paper we review our recent results and show some supplemental numerical result Special attention is paid to the dynamical stability of the filamentary clouds and the role of HD cooling which becomes more dominant than H 2 cooling in some intergalactic environments. Our simulations suggest that the first generation stars are deficient in sub-solar mass stars. According to these simulations, a typical mass of low-mass first stars should be of order of 1M O .,and a typical mass of massive first stars should be around 10 2 M O .or 10M O ..depending on the H 2 fractional abundance, which determines the efficiency of HD cooling. We predict that the stellar IMF is bimodal with two peaks at 1- 2M O .and 10M O .or 10M O ..

Gravitational Contraction of Rotating Clouds: Formation of Self‐similarly Collapsing Disks

We investigate the isothermal gravitational collapse of rotating interstellar clouds with axisymmetric numerical simulations. The simulations show that a filamentary cloud fragments owing to the gravitational instability and the fragment evolves into a dynamically contracting disk. The disk contraction is followed until the central density increases by a factor of 1016 at most. The disk evolution shows similarity: the disk structure at a given time is similar to that at another time except for the scale. We construct various models, in which we change the wavelength of the perturbation and the initial rotation velocity, and study the dependences of the disk evolution on these model parameters. The surface density of the disk is proportional to the square of the sound speed, Σ∝c2s and almost independent of the wavelength of the perturbation imposed, i.e., the mass contained in the fragment. It indicates that the mass of the gravitationally contracting disk is independent of the parent cloud mass. When the initial cloud rotates slowly, the dense part of the fragment is nearly spherical in the early contraction phase and evolves into a disk. When the initial cloud rotates fast, the fragment has a disk shape from the early contraction phase. In the late contraction phase, the surface density and the rotation velocity do not depend strongly on the initial rotation velocity and depend weakly on it when it is small. Although the disk evolution is well understand by similarity collapse, it shows an oscillation around similarity collapse. A new shock wave forms each cycle of oscillation.