Research On Episodic Accretion In Magnetically Layered Protoplanetary Discs

The Solar Nebula hypothesis is the most popular theory explaining the origin of Solar System. This theory is described that interstellar gas cloud could collapse inside-out because of the gravitation collapse or the stress from the adjacent supernova explosion. Molecular cloud cores collapse into protostar. In order to conserve angular momentum, the rotating nebula clouds around protostar circumvolve faster and the nebula configuration become flat due to falling masses to mid-plane. Finally, the system of protostar-disc formed. Actually, the Solar Nebula is such model. The central protostar which gains its mass through accretion from the disc evolve gradually into the current Solar. The bulks of gas-dust fall and accumulate in the disc, which form Planets and small Solar System bodies around the Sun. The Solar Nebula is also called protoplanetary disc, since this disc structure affords the formation of Planets.Actually, protoplanetary disc is an accretion disc around the Star. On the base of theories of accretion disc, much more practical model of protoplanetary disc could be established to research and explore the origin of Solar System. Through numerical simulation of evolvement of that disc, we can examine the implications of existing observations and also predict the future phenomenon on Solar System. To calculate numerically the evolvement of protoplanetary disc, those primary parameters (such as surface density, accretion rate, viscosity coefficient, temperature, the Q parameter and so on) must be confirmed. The purposes of this thesis are researches on accretion rate in magnetically layered protoplanetary disc by numerical simulation and explanations of some interesting astronomical phenomena.The viscosity on previous viscous disc is uniform, which seems be not exact. It would miss useful information when we study the evolution of the Solar System with constantαon viscous disc. Different from the viscous disc, the magnetically layered disc were mentioned firstly by Jin (1996). It assuming that in the intermediate region (InR) the gas near the mid-plane can not be ionized powerfully, because the temperature is too low to thermal ionization and the surface density is too high for the cosmic rays to penetrate. The magneto-rotation instability (MRI) can not survive in this arena namely quiescent layer. In fact, the surface gas in the InR and the whole gas in the out region (OR) can be ionized by cosmic rays to support MRI. The MRI can survive in the inner region (IR) due to the high temperature. These suggest that the viscosity is not uniform and theαparameter is volatile.Episodic accretion in layered protoplanetary discs has been studied by Armitage on 2001. It is pointed out that the local self-gravity instability (GI) in quiescent layer drives the angular momentum transport (AMT). The numerical calculations survey that accretion rate onto the ptotostar occur outburst, with a peak value M? 10?5M⊙yr-1 in ¹04yr. Repeated episodes of strong mass outflow could occur due to such outburst in young stellar objects. This accretion model appears at an early phase (t<<1Myr) of the protoplanetary evolution. Actually, this model also predicts that many young pre-main-sequence stars have lower average accretion rate. After a few Myr, the size of disc is much more massive than the minimum-mass nebular; and the layered disc mass is ten times the weight of the viscous disc. Most of that discrepant mass accumulate in the quiescent layer at 1^AU.The innovations of this paper are two points. 1) Global self-gravity instability in quiescent layer drives the angular momentum transport (AMT). GI will occur as long as the surface density at any radius exceeds the critical value. And then the whole disc will be a high viscosity state, with a largeαvalue. This mechanism increase the probability of occurrence of GI, compared with the local mechanism. 2) A new model of viscosity is expressed in this paper. When GI is on, the parameterαwhich depends on the local surface density in the local mechanism is taken as 0.02 in this paper. When GI is off,αis much complicated expression, not the simple constant value adopted by Armitage. It leads that the lowest accretion rate is mildly changed other than the stable value.In this thesis, the disc is taken as evolvement from zero mass until either a steady state or a limit cycle with a constant mass flux out of the disc in-falling into the disc. Adopting the global GI mechanism and the new viscosity model, we will numerically calculate the accretion rate to protostar ( (?)at r_in). The results survey that steady accretion is not possible for infall rate in the rage 5×10^-10 <(?)_infall<2×10^-6M_⊙yr^-1. The accretion rate appears outbursts periodically: peak value 3×10^-6M_⊙yr^-1 with time scale 10³yr, low value 10^₁0M_⊙yr^-1 lasting 102^yr or more. This outburst time is lower than the result in the local mechanism. Compared with the observation about outflow of FU Orionis, the stat gained from our numerical calculation is much better. Additionally, the migration rate of low-mass planets is predictably reduced in the quiescent layer. Massive planets allowed to interact with disc have determinate eccentricity.