Turbulent Protoplanetary Disks Research Articles

Kinetic effects on evaporation and condensation of Mg-silicate dust particles moving in the turbulent protoplanetary disk

Magnesium and Silicon are the most abundant rock forming elements in the terrestrial planets, and Mg/Si fractionation among planets and meteorites is believed to have occurred in the early solar nebula. In order to evaluate Mg/Si fractionation in the solar nebula, evaporation and condensation of magnesium silicates moving vertically in the protoplanetary disk are simulated based on experimental kinetics and an astrophysical model. Evaporation of forsterite dust particles is shown to be a rapid process and be regarded as an equilibrium reaction during the vertical drift. Evaporation of enstatite is controlled by diffusion through the forsterite layer that is formed as an evaporation residue. Since the diffusion distance under kinetic conditions during the vertical drift is about 1μm, evaporation of enstatite grains would be nearly equilibrium for grains of 1μm and non-equilibrium for grains of 10μm. Condensation of forsterite is simulated as grain growth after heterogeneous nucleation on pre-existed refractory grains. It is found that condensation of forsterite proceeds with keeping equilibrium. Enstatite is formed in the nebula via a reaction between forsterite and gas, and the formation reaction is controlled by diffusion through the enstatite layer. Formation of enstatite would be kinetically inhibited due to its slow diffusion kinetics. The present result implies that the temperature range, in which forsterite is a dominant solid component, would be widened to lower temperature than at equilibrium, and Mg/Si fractionation, which has been explained by extraction of materials with a high Mg/Si ratio such as forsterite from the system, would occur more easily than that in the equilibrium model.

Magnetic and vertical shear instabilities in accretion discs

ABSTRA C T The stability properties of magnetized discs rotating with angular velocity Q o Q﷿s; zfi, dependent on both the radial and the vertical coordinates s and z, are considered. Such a rotation law is adequate for many astrophysical discs (e.g., galactic and protoplanetary discs, as well as accretion discs in binaries). In general, the angular velocity depends on height, even in thin accretion discs. A linear stability analysis is performed in the Boussinesq approximation, and the dispersion relation is obtained for short-wavelength perturbations. Any dependence of Q on z can destabilize the flow. This concerns primarily small-scale perturbations for which the stabilizing effect of buoyancy is strongly suppressed due to the energy exchange with the surrounding plasma. For a weak magnetic field, instability of discs is mainly associated with vertical shear, whilst for an intermediate magnetic field the magnetic shear instability, first considered by Chandrasekhar and Velikhov, is more efficient. This instability is caused by the radial shear which is typically much stronger than the vertical shear. Therefore the growth time for the magnetic shear instability is much shorter than for the vertical shear instability. A relatively strong magnetic field can suppress both these instabilities. The vertical shear instability could be the source of turbulence in protoplanetary discs, where the conductivity is low.

Coagulation and settling of dust in a turbulent protoplanetary disk

We apply our analytic model for the dust diffusivity to calculate the vertical structure of the dust sub-disk in a turbulent protoplanetary nebula. We present a numerical solution of a vertical dust settling equationand a coagulation equation for dust grains covering four orders of magnitude in time and grain size.

Magnetically-aided evolution of protoplanetary disks

We investigate the global evolution of a turbulent protoplanetary disk incorporating the effects of Maxwell stress due to a large-scale magnetic field permeating the disk. A magnetic field is produced continuously by an αΩ dynamo and the resultant Maxwell stress assists the viscous stress in p roviding the means for disk evolution. The most striking feature of magnetized disk evolution is the presence of the surface density bulge located in the “magnetic gap”, the region of the disk where the degree of ionization is too low to allow for coupli ng between the magnetic field and the gas. The bulge persists for a time of the order of 105–106 yr. The presence and persistence of the surface density bulge may have important implications for the process of planet formation and the overall characteristics of resultant planetary systems.

Evolution of magnetized protoplanetary disks

view Abstract Citations (25) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Magnetized Protoplanetary Disks Reyes-Ruiz, Mauricio ; Stepinski, Tomasz F. Abstract We investigate the global evolution of a turbulent protoplanetary disk in its viscous stage, incorporating the effects of Maxwell stress due to a large-scale magnetic field permeating disk. We assume that the viscous stress is given by an alpha model. A magnetic field is produced contemporaneously by an alpha omega dynamo mechanism and the resultant Maxwell stress assists the viscous stress in providing the means for disk evolution. The aim of this work is to compare the evolution of magnetized and nonmagnetized disks driven by turbulent viscosity of the same magnitude and thus assess the effects of a self-generated magnetic field on the structure and dynamical evolution of protoplanetary disks. Two illustrative examples corresponding to two different initial conditions are considered: a high-mass case that starts with a disk of 0.245 solar mass and angular momentum of 5.6 x 1052g sq cm/s, and a low-mass that case starts with a disk of 0.11 solar mass and angular momentum of 1.8 x 1052g sq cm/s. For each of these two cases the radial development of a disk is calculated numerically assuming a fiducial value of the dimensionless viscosity parameter alphass = 0.01, as well as alphass = 2 x 10-3. In all cases the central star has a mass equal to 1 solar mass. The most striking feature of magnetized disk evolution is the presence of the surface density bulge located in the region of the disk where the dynamo mechanism cannot support a magnetic field. The bulge persists for a time of the order of 105-106 yr. The presence and persistence of the surface density bulge may have important implications for the process of planet formation and the overall characteristics of resultant planetary systems. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175120 Bibcode: 1995ApJ...438..750R Keywords: Accretion Disks; Magnetic Fields; Planetary Evolution; Protoplanets; Viscosity; Finite Difference Theory; Ionization; Planetary Systems; Turbulence; Astrophysics; ACCRETION; ACCRETION DISKS; MAGNETOHYDRODYNAMICS: MHD; STARS: PLANETARY SYSTEMS full text sources ADS |

Evolution of Protoplanetary Disks with Condensation and Coagulation

We investigate the global evolution of the viscous stage of a turbulent protoplanetary disk. The time-dependent radial development of the nebula is calculated with the help of analytical one-zone models of the vertical structure that relate optical properties of grains to turbulent viscosity. Special attention is given to possible growth of dust particles that cause significant opacity and hence viscosity changes. In the framework of a simple two-component model for the dust size population we follow the evolution of the total surface density of the disk. Assuming that grain growth is especially rapid in the main condensation regions (e.g., due to the enhanced sticking probabilities) we find significant surface density enhancements in these radially confined regions. Moreover, a lower turbulence strength in these regions (e.g., caused by the suppression of thermal convection) leads to a further increase in the density. Possible consequences for planet formation are briefly outlined.

Are cosmic rays effective for ionization of protoplanetary disks?

view Abstract Citations (26) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Are Cosmic Rays Effective for Ionization of Protoplanetary Disks? Dolginov, Arkady Z. ; Stepinski, Tomasz F. Abstract The principal uncertainty in studying the magnetic properties of protoplanetary disks concerns the ionization levels of the disk's gas. The low gas temperature precludes thermal ionization, leaving cosmic rays as the dominant source of ionization. It has been shown that the resulting electrical conductivity is just high enough for a MHD dynamo to produce contemporaneously a magnetic field in most of the extended parts of a turbulent protoplanetary disk. Here we argue that the effectiveness of cosmic rays to ionize the bulk of the gas is impaired by the influence of the generated magnetic field on the propagation of cosmic rays within a disk. Cosmic rays scatter on magnetic inhomogeneities, and, as a result, their penetration depth decreases to only a fraction of the disk half-thickness, resulting in a severe depletion of free charge from the midplane regions of a disk. That, in turn, undercuts the dynamo mechanism, so extended parts of the disk are free from a dynamically significant magnetic field. We also point out that any additional, even small, in situ source of ionization, such as radioactive Al-26, may again make a dynamo a viable regeneration process capable of producing a dynamically important magnetic field. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/174146 Bibcode: 1994ApJ...427..377D Keywords: Accretion Disks; Cosmic Rays; Magnetohydrodynamic Waves; Spatial Resolution; Star Formation; Stellar Magnetic Fields; T Tauri Stars; Coriolis Effect; Electrical Resistivity; Molecular Clouds; Planetary Magnetospheres; Astrophysics; ISM: COSMIC RAYS; MAGNETOHYDRODYNAMICS: MHD; STARS: FORMATION; STARS: MAGNETIC FIELDS full text sources ADS |