Helical Fields and Filamentary Molecular Clouds (original) (raw)
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Gravitational Collapse of Filamentary Magnetized Molecular Clouds
The Astrophysical Journal, 2003
We develop models for the self-similar collapse of magnetized isothermal cylinders. We find solutions for the case of a fluid with a constant toroidal flux-to-mass ratio (Γ φ = constant) and the case of a fluid with a constant gas to magnetic pressure ratio (β = constant). In both cases, we find that a low magnetization results in density profiles that behave as ρ ∝ r −4 at large radii, and at high magnetization we find density profiles that behave as ρ ∝ r −2. This density behaviour is the same as for hydrostatic filamentary structures, suggesting that density measurements alone cannot distinguish between hydrostatic and collapsing filaments-velocity measurements are required. Our solutions show that the self-similar radial velocity behaves as v r ∝ r during the collapse phase, and that unlike collapsing self-similar spheres, there is no subsequent accretion (i.e. expansion-wave) phase. We also examine the fragmentation properties of these cylinders, and find that in both cases, the presence of a toroidal field acts to strengthen the cylinder against fragmentation. Finally, the collapse time scales in our models are shorter than the fragmentation time scales. Thus, we anticipate that highly collapsed filaments can form before they are broken into pieces by gravitational fragmentation.
Helical fields and filamentary molecular clouds -- I
Monthly Notices of the Royal Astronomical Society, 2000
We study the equilibrium of pressure truncated, filamentary molecular clouds that are threaded by rather general helical magnetic fields. We first apply the virial theorem to filamentary molecular clouds, including the effects of non-thermal motions and the turbulent pressure of the surrounding ISM. When compared with the data, we find that many filamentary clouds have a mass per unit length that is significantly reduced by the effects of external pressure, and that toroidal fields play a significant role in squeezing such clouds. We also develop exact numerical MHD models of filamentary molecular clouds with more general helical field configurations than have previously been considered. We examine the effects of the equation of state by comparing`isothermal' filaments, with constant total (thermal plus turbulent) velocity dispersion, with equilibria constructed using a logatropic equation of state. Our theoretical models involve three parameters: two to describe the mass loading of the toroidal and poloidal fields, and a third that describes the radial concentration of the filament. We thoroughly explore our parameter space to determine which choices of parameters result in models that agree with the available observational constraints. We find that both equations of state result in equilibria that agree with the observational results. Moreover, we find that models with helical fields have more realistic density profiles than either unmagnetized models or those with purely poloidal fields; we find that most isothermal models have density distributions that fall off as r 21.8 to r 22 , while logatropes have density profiles that range from r 21 to r 21.8. We find that purely poloidal fields produce filaments with steep radial density gradients that are not allowed by the observations.
Gravitational instability of filamentary molecular clouds, including ambipolar diffusion
Monthly Notices of the Royal Astronomical Society, 2016
The gravitational instability of a filamentary molecular cloud in non-ideal magnetohydrodynamics is investigated. The filament is assumed to be in hydrostatic equilibrium. We add the effect of ambipolar diffusion to the filament which is threaded by an initial uniform axial magnetic field along its axis. We write down the fluid equations in cylindrical coordinates and perform linear perturbation analysis. We integrate the resultant differential equations and then derive the numerical dispersion relation. We find that, a more efficient ambipolar diffusion leads to an enhancement of the growth of the most unstable mode, and to increase of the fragmentation scale of the filament.
Monthly Notices of the Royal Astronomical Society, 2019
We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of th...
Classification of Filament Formation Mechanisms in Magnetized Molecular Clouds
The Astrophysical Journal, 2021
Recent observations of molecular clouds show that dense filaments are the sites of present-day star formation. Thus, it is necessary to understand the filament formation process because these filaments provide the initial condition for star formation. Theoretical research suggests that shock waves in molecular clouds trigger filament formation. Since several different mechanisms have been proposed for filament formation, the formation mechanism of the observed star-forming filaments requires clarification. In the present study, we perform a series of isothermal magnetohydrodynamics simulations of filament formation. We focus on the influences of shock velocity and turbulence on the formation mechanism and identified three different mechanisms for the filament formation. The results indicate that when the shock is fast, at shock velocity v sh ; 7 km s −1 , the gas flows driven by the curved shock wave create filaments irrespective of the presence of turbulence and self-gravity. However, at a slow shock velocity v sh ; 2.5 km s −1 , the compressive flow component involved in the initial turbulence induces filament formation. When both the shock velocities and turbulence are low, the self-gravity in the shock-compressed sheet becomes important for filament formation. Moreover, we analyzed the line-mass distribution of the filaments and showed that strong shock waves can naturally create high-line-mass filaments such as those observed in the massive star-forming regions in a short time. We conclude that the dominant filament formation mode changes with the velocity of the shock wave triggering the filament formation.
The magnetic field structure in molecular cloud filaments
Monthly Notices of the Royal Astronomical Society, 2018
We explore the structure of magnetic field lines in and around filaments in simulations of molecular clouds undergoing global, multi-scale gravitational collapse. In these simulations, filaments are not in a static equilibrium, but are long-lived flow structures that accrete gas from their environment and direct it toward clumps embedded in the filament or at the nodes at the conjunction with other filaments. In this context, the magnetic field is dragged by the collapsing gas, so its structure must reflect the flow that generates the filament. Around the filament, the gas is accreted onto it, and the magnetic lines must then be perpendicular to the filament. As the gas density increases, the gas flow changes direction, becoming almost parallel to the filament, and magnetic lines also tend to align with it. At the spine of the filament, however, magnetic lines become perpendicular again since they must connect to lines on the opposite side of the filament, resulting in"U"-shaped magnetic structures, which tend to be stretched by the longitudinal flow along the filament. Magnetic diffusive processes, however, allow the gas to continue to flow. Assuming a stationary state in which the ram pressure of the flow balances the magnetic tension, the curvature of the field lines is determined by the diffusion rate. We derive an expression relating the curvature of the field lines to the diffusive coefficient, which may be used to observationally determine the nature of the diffusive process.
On the Stability of Self-Gravitating Filaments
Filamentary structures are very common in astrophysical environments and are observed at various scales. On a cosmological scale, matter is usually distributed along filaments, and filaments are also typical features of the interstellar medium. Within a cosmic filament, matter can possibly contract and form galaxies, whereas an interstellar gas filament can clump into a series of bead-like structures which can then turn into stars. To investigate the growth of such instabilities and the properties of the resulting substruc-tures, we consider idealized self-gravitating filaments and derive the dispersion relation for perturbations within them. We assume no specific density distribution, treat matter as a fluid, and use hydrodynamics to derive the linearized equations that govern the growth of perturbations. Assuming small local perturbations leads to a dispersion relation analogous to the spherical Jeans case: perturbations of size higher than the Jeans length collapse and asymmetrie...
A New Model for Filamentary Molecular Clouds
We develop a theory for filamentary molecular clouds including the effects of ordered magnetic fields, and external pressure. We first derive a new virial equation appropriate for filamentary clouds. By comparing with observational results collected from the literature, we find that the fields are likely helical. Secondly, we construct numerical, MHD models of filamentary clouds that agree with the observational constraints. We find that our models produce more realistic density profiles r ∼ r −1.8 to −2 than previous models, where the density falls off as r −4 .
Filamentary flow and magnetic geometry in evolving cluster-forming molecular cloud clumps
Monthly Notices of the Royal Astronomical Society
We present an analysis of the relationship between the orientation of magnetic fields and filaments that form in 3D magnetohydrodynamic simulations of cluster-forming, turbulent molecular cloud clumps. We examine simulated cloud clumps with size scales of L ∼ 2-4 pc and densities of n ∼ 400-1000 cm −3 with Alfvén Mach numbers near unity. We simulated two cloud clumps of different masses, one in virial equilibrium, the other strongly gravitationally bound, but with the same initial turbulent velocity field and similar mass-to-flux ratio. We apply various techniques to analyse the filamentary and magnetic structure of the resulting cloud, including the DISPERSE filament-finding algorithm in 3D. The largest structure that forms is a 1-2 parsec-long filament, with smaller connecting sub-filaments. We find that our simulated clouds, wherein magnetic forces and turbulence are comparable, coherent orientation of the magnetic field depends on the virial parameter. Sub-virial clumps undergo strong gravitational collapse and magnetic field lines are dragged with the accretion flow. We see evidence of filament-aligned flow and accretion flow on to the filament in the sub-virial cloud. Magnetic fields oriented more parallel in the sub-virial cloud and more perpendicular in the denser, marginally bound cloud. Radiative feedback from a 16 M star forming in a cluster in one of our simulation's ultimately results in the destruction of the main filament, the formation of an H II region, and the sweeping up of magnetic fields within an expanding shell at the edges of the H II region.
Fragmentation and Collapse of Turbulent Molecular Clouds
We performed simulations of self-gravitating hydrodynamic turbulence to model the formation of filaments, clumps and cores in molecular clouds. We find that when the mass on the initial computational grid is comparable to the Jeans mass, turbulent pressure is able to prevent gravitational collapse. When the turbulence has damped away sufficiently, gravitational collapse can occur, and the resulting structure closely resembles the pre-singularity collapse of an isothermal sphere of . If several Jeans masses are initially placed on the grid, turbulence may not be sufficient to prevent collapse before turbulence can be significantly damped. In this case, the cores have density structures which are considerably shallower than expected for an isothermal gas, and resemble the solutions for a logatropic equation of state.