Magnetic Braking, Ambipolar Diffusion, and the Formation of Cloud Cores and Protostars. II. A Parameter Study
Magnetic Braking, Ambipolar Diffusion, and the Formation of Cloud Cores and Protostars. II. A Parameter Study
复制标题
磁制动、双极扩散以及云核和原恒星的形成。
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
T. Mouschovias
中科院分区：
文献类型：
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作者：
S. Basu;T. Mouschovias
s of recently accepted papers Vortices in Circumstellar Disks Fred C. Adams and Richard Watkins Physics Department, University of Michigan, Ann Arbor, MI 48109, USA We discuss the physics of vortices in the circumstellar disks associated with young stellar objects. We elucidate the basic physical properties of these localized storm systems. In particular, we consider point vortices, linear vortices, the effects of self-gravity, magnetic fields, and nonlinear aspects of the problem. We find that these vortices can exist in many different forms in the disks of young stellar objects and may play a role in the formation of binary companions and/or giant planets. Vortices may enhance giant planet formation via gravitational instability by allowing dust grains (heavy elements) to settle to the center on a short time scale; the gravitational instability itself is also enhanced because the vortices also create a larger local surface density in the disk. In addition, vortices can enhance energy dissipation in disks and thereby affect disk accretion. Finally, we consider the possibility that vortices of this type exist in molecular clouds and in the disk of the galaxy itself. On all of these size scales, vortices can produce long-lived structures which may correspond to observed structures in these systems. Accepted by The Astrophysical Journal Magnetic Braking, Ambipolar Diffusion, and the Formation of Cloud Cores and Protostars: II. A Parameter Study Shantanu Basu and Telemachos Ch. Mouschovias 1 Physics and Astronomy Department, Michigan State University, East Lansing, MI, 48824, USA 2 Departments of Physics and Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA E-mail contact: tchm@astro.uiuc.edu or basu@msupa.pa.msu.edu The formulation of the problem of the formation of protostellar cores in self-gravitating, magnetically supported, rotating, isothermal model molecular clouds was presented in a previous paper, where detailed numerical simulations for two different model clouds were also discussed. In this paper, we study the effect of varying five dimensionless free parameters: the ratio ρ̃ of external density and central density in a reference state (which is related simply to an initial equilibrium state), the initial radial length scale l̃ref of the column density of the cloud, the central angular velocity of the reference state Ω̃c,ref , the central neutral-ion collision time in the reference state τ̃ni,ref (which is inversely proportional to the collapse retardation factor νff ≡ τff/τni), and the exponent k in the relation between the ion and neutral densities ni ∝ nn. In addition to the models previously presented, seven more models are investigated here. Different values (1/1000 − 1/100) of the initial magnetic-braking efficiency parameter ρ̃(> 0) do not significantly affect the evolution; magnetic braking remains effective during the quasistatic phase, and ineffective during the (dynamic) collapse of the magnetically and thermally supercritical core. The initially very effective magnetic braking also means that the solution is insensitive to values of Ω̃c,ref . Different values of l̃ref yield qualitatively similar evolution, with smaller cloud sizes leading to slightly smaller core sizes. Increasing the value of τ̃ni,ref leads to more rapid evolution and larger, more rapidly rotating cores. A smaller k leads to relatively more rapid evolution in the core and a better coreenvelope separation. We also give an analytical explanation of the previously presented result, that the gravitational field acting on an infalling mass shell in the central region of a nonhomologously contracting thin disk increases as 1/r m, where rm is the Lagrangian radius of the shell. Accepted by The Astrophysical Journal, Oct 10 issue
