The Impact Of Different Interstellar Medium Structures On The Dynamical Evolution Of Young Type Ia Supernova Remnants

A supernova remnant(SNR)is a relic of the catastrophic explosion of a star at its end of evolution.When the supernova explodes,it will cast all the envelope media and inject energy of about 1051 ergs in form of kinetic energy into the ambient interstellar medium(ISM).The ejecta expand at a supersonic speed,and the subsequent forward shock wave sweeps up the ambient ISM,compresses and heats them to high tempera-ture,resulting in thermal radiation.Besides,the forward shock wave can also accelerate the charged particles to extreme high energy and then emit non-thermal photons through various physical processes.Therefore SNRs are generally believed to be the main source of galactic cosmic rays(GCRs).Recently,plenty of data from observations have con-firmed that SNRs can accelerate CRs to extreme high energy,and suggested that the morphologies of SNRs are closely related to the distributions of background ISM,the progenitor properties,the structure of stellar wind and so on.In this thesis,assuming a young type la SNR evolves in the form of cylinder system,we investigate dynamical properties of the SNR in various ambient interstellar media.In our simulations,three cases of different interstellar medium structures are considered:the uniform medium,the turbulent medium and the cloudy medium.Large-scale density and magnetic fluctuations are calculated and mapped into the computational domain before simulations and the clouds are set by random distribution in advance.Main results in this study are as follows:(1)The evolving morphologies of the SNR in the turbulent and the cloudy medium structures will deviate from symmetry compared to that in the uniform medium.As time evolves,three waves are formed in the uniform case:forward shock(F-S),contact discontinuity(CD),and reverse shock(RS),the Rayleigh-Taylor(RT)instability is especially triggered between shocked ejecta and shocked ISM,and the remnant is basically symmetric.The turbulent medium causes rippling of the shock front,and deforms RT unstable region as well as the remnant morphology;finally the interactions between the clouds and the shock wave are dominated by the Richtmyer-Meshkov(RM)instability.As time evolves,however,the RT unsta-ble region and the finger-like structures will be strongly suppressed and distorted by direct impact with the clouds,so the interactions between the shock wave and dense clouds eventually lead to the clouds' fragmentation and a slight deviation from symmetry.(2)The magnetic field structures in the SNR are different in three kinds of medium structures.The magnetic field is generally amplified near the RT unstable region,but the width of amplification region is quite different among the three cases.The extension of amplification region in the uniform medium case is smaller compared with those in other two cases.The region extends both outward and inward in the cloudy medium case mostly contributed by the clouds in the intershock region and the clouds reaching the RT unstable region,respectively.While in the turbulent medium case,the amplification region extends outward and the peak appears at the immediate downstream of the forward shock.Moreover the magnetic-field amplification is closely related to the clouds' density,and the maximum magnetic-field strength displays a sharp increase in the cloudy medium case as the clouds approach close enough to the RT unstable region.(3)In 2D simulations of the SNR,although most of the qualitative simulation results in 2D cylindrical symmetry can be reproduced in 2D Cartesian coordinates,the quantitative results are different.On the one hand,in 2D Cartesian coordinates,the values of density as well as the intensity of the magnetic-field amplification in the intershock region will increase substantially especially in the RT unstable region,however,this is not the case in 2D cylindrical symmetry.On the other hand,the forward shock radius at a given time in 2D cylindrical symmetry is much smaller than that in 2D Cartesian coordinates,the former evolves as r ? t2/5 but the latter evolves as r ? t1/2.The thesis is organized as follows.In Chapter one,we briefly introduce an overview of the supernova's explosion mechanism and spectroscopy classification,the background knowledge of the morphological classification,the general process of the dynamical evo-lution,the characteristics of dynamical environments and magnetic fields of SNRs.In Chapter two,we introduce the PLUTO code that applied in our work,the dynamical equations of the shock waves in supernova remnants,and three kinds of fluid instabil-ities which are common in MHD simulations,namely the Rayleigh-Taylor instability,Kelvin-Helmholtz instability and Richtmyer-Meshkov instability.In Chapter three,we present main results of our study.Finally we give our conclusions and prospect.