Chondrule formation by current sheets in protoplanetary disks
- AMNH
We numerically explore where, when and if magnetorotational instability in protoplanetary disks can produce current sheets that could form chondrules. Results are tested against astronomical, meteoritic, thermodynamic, and experimental evidence. Theories of protoplanetary disk evolution require that the viscosity of the differentially rotating disk (the resistance of the disk to shear forces) be sufficient for stellar accretion on timescales of 106 years. With only molecular (frictional) viscosity, accretion takes 103 times longer. Vertical turbulent convection cannot provide the needed viscosity. The leading mechanisms for disk viscosity are (a) gravitational instability, which would drive density waves in the disk, and (b) coupling of the disk rotation to its magnetic field. In cold disk regions with a high mass density, gravitational instabilities could occur, and drive chondrule-forming shocks. Alternatively, magnetorotational instability (MRI) is predicted to occur in regions of the disk where the gas is ionized enough to couple to magnetic fields. Like spiral density waves, MRI effectively transfers angular momentum outward in the disk. The MRI also produces magnetic field gradients. In weakly ionized regions where neutral particles can slip through ions (i.e.-where ambipolar diffusion occurs), magnetic field gradients are predicted to grow steeper with time, producing sheets of strong electrical current. We propose that these current sheets could melt chondrule precursors. Unlike mechanisms involving accumulation and dissipation of charge (nebular 'lightning', e.g.-[11]), the MRI is driven by the abundant energy of the differential rotation of the disk itself. Furthermore, current sheets are predicted to occur in different regions over the lifetime of the disk. Which disk regions make current sheets, when, and where might they form chondrules? Because current sheet thermal profiles are qualitatively similar to those of shocks, our results are testable against meteoritic evidence by techniques analogous to those used by [6].
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1008908
- Resource Relation:
- Conference: 35th Lunar and Planetary Science Conference;March 15-19, 2004;Houston, Texas
- Country of Publication:
- United States
- Language:
- ENGLISH
Similar Records
MINERAL PROCESSING BY SHORT CIRCUITS IN PROTOPLANETARY DISKS
Linear stability of magnetized massive protoplanetary disks