Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userAn empirical model for protostellar collapse
48
0
0.0
(
0
)
Added by
Derek Ward-Thompson
Publication date
2000
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We propose a new analytic model for the initial conditions of protostellar collapse in relatively isolated regions of star formation. The model is non-magnetic, and is based on a Plummer-like radial density profile as its initial condition. It fits: the observed density profiles of pre-stellar cores and Class 0 protostars; recent observations in pre-stellar cores of roughly constant contraction velocities over a wide range of radii; and the lifetimes and accretion rates derived for Class 0 and Class I protostars. However, the model is very simple, having in effect only 2 free parameters, and so should provide a useful framework for interpreting observations of pre-stellar cores and protostars, and for calculations of radiation transport and time-dependent chemistry. As an example, we model the pre-stellar core L1544.
rate research
Read More
We present the result of an empirical model for elastic $pp$ scattering at LHC which indicates that the asymptotic black disk limit ${cal R}=sigel/sigtotrightarrow1/2$ is not yet reached and discuss the implications on classical geometrical scaling behavior. We propose a geometrical scaling law for the position of the dip in elastic $pp$ scattering which allows to make predictions valid both for intermediate and asymptotic energies.
For a previously published study of the titanium hcp (alpha) to omega (omega) transformation, a tight-binding model was developed for titanium that accurately reproduces the structural energies and electron eigenvalues from all-electron density-functional calculations. We use a fitting method that matches the correctly symmetrized wavefuctions of the tight-binding model to those of the density-functional calculations at high symmetry points. The structural energies, elastic constants, phonon spectra, and point-defect energies predicted by our tight-binding model agree with density-functional calculations and experiment. In addition, a modification to the functional form is implemented to overcome the collapse problem of tight-binding, necessary for phase transformation studies and molecular dynamics simulations. The accuracy, transferability and efficiency of the model makes it particularly well suited to understanding structural transformations in titanium.
The formation and collapse of a protostar involves the simultaneous infall and outflow of material in the presence of magnetic fields, self-gravity, and rotation. We use self-similar techniques to self-consistently model the anisotropic collapse and outflow by a set of angle-separated self-similar equations. The outflow is quite strong in our model, with the velocity increasing in proportion to radius, and material formally escaping to infinity in the finite time required for the central singularity to develop. Analytically tractable collapse models have been limited mainly to spherically symmetric collapse, with neither magnetic field nor rotation. Other analyses usually employ extensive numerical simulations, or either perturbative or quasistatic techniques. Our model is unique as an exact solution to the non-stationary equations of self-gravitating MHD, which features co-existing regions of infall and outflow. The velocity and magnetic topology of our model is quadrupolar, although dipolar solutions may also exist. We provide a qualitative model for the origin and subsequent evolution of such a state. However, a central singularity forms at late times, and we expect the late time behaviour to be dominated by the singularity rather than to depend on the details of its initial state. Our solution may, therefore, have the character of an attractor among a much more general class of self-similarity.
A new numerical code, called SFUMATO, for solving self-gravitational magnetohydrodynamics (MHD) problems using adaptive mesh refinement (AMR) is presented. A block-structured grid is adopted as the grid of the AMR hierarchy. The total variation diminishing (TVD) cell-centered scheme is adopted as the MHD solver, with hyperbolic cleaning of divergence error of the magnetic field also implemented. The self-gravity is solved by a multigrid method composed of (1) full multigrid (FMG)-cycle on the AMR hierarchical grids, (2) V-cycle on these grids, and (3) FMG-cycle on the base grid. The multigrid method exhibits spatial second-order accuracy, fast convergence, and scalability. The numerical fluxes are conserved by using a refluxing procedure in both the MHD solver and the multigrid method. The several tests are performed indicating that the solutions are consistent with previously published results.
Through the magnetic braking and the launching of protostellar outflows, magnetic fields play a major role in the regulation of angular momentum in star formation, which directly impacts the formation and evolution of protoplanetary disks and binary systems. The aim of this paper is to quantify those phenomena in the presence of non-ideal magnetohydrodynamics effects, namely the Ohmic and ambipola r diffusion. We perform three-dimensional simulations of protostellar collapses varying the mass of the prestellar dense core, the thermal support (the $alpha$ ratio) and the dust grain size-distribu tion. The mass mostly influences the magnetic braking in the pseudo-disk, while the thermal support impacts the accretion rate and hence the properties of the disk. Removing the grains smaller than 0. 1 $mu$m in the Mathis, Rumpl, Nordsieck (MRN) distribution enhances the ambipolar diffusion coefficient. Similarly to previous studies, we find that this change in the distribution reduces the magnet ic braking with an impact on the disk. The outflow is also significantly weakened. In either case, the magnetic braking largely dominates the outflow as a process to remove the angular momentum from t he disk. Finally, we report a large ionic precursor to the outflow with velocities of several km s$^{-1}$, which may be observable.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
