A Study On The Mechanisms Of Thermal Composite Supernova Remnants

The study of supernova remnants（SNRs）is essential to understand the proper-ties and the evolution of the interstellar medium.With extremely high temperature and low density,SNRs can serve as a laboratory for some high energy physics ex-periments.Among all kinds of SNRs,there is a new class with a radio shell and a centrally brightened thermal X-ray emission called thermal composite SNR or Mixed-morphology SNR that was characterized about twenty years ago.Currently,the phys-ical mechanisms for causing the unusual morphology of thermal X-ray emission are still controversial.The standard SNR theories are not able to reproduce thermal X-ray emission with its centroid in the interior.Moreover,an overionization（recombination of the nonequilibrium ionization plasmas）scenario in thermal X-ray emission is found in some of this kind recently.It is again not consistent with the classical SNR evolu-tion schemes.Usually,an SNR shock is believed to heat and ionize the ISM and the ejecta.The overionization implies that there must be some rapid decline in the temper-ature.Therefore,the overionization provides another clue for the physical mechanisms in thermal composite SNRs.To understand the thermal composite SNRs,we first have revisited the archival XMM-Nexwton and Chandra data of the thermal composite SNR Kesteven41（Kes 41 or G337.8-0.1）and performed a millimeter observation towards this source with ¹²CO（J = 1-0）,¹³CO（J= 1-0）,and C¹⁸O（J = 1-0）lines.The X-ray emission,mainly concentrated toward the southwestern part of the SNR,is characterized by distinct S and Ar He-like lines in the spectra.The X-ray spectra can be fitted with an absorbed non-equilibrium ionization plasma model at a temperature 1.3-2.6 keV and an ionization timescale 0.1-1.2 x 10¹²cm^-3 s.The metal species S and Ar are over-abundant,with 1.2-2.7 and 1.3-3.8 solar abundances,respectively,which strongly indicate the presence of a substantial ejecta component in the X-ray emitting plasma of this SNR.Kes 41 is found to be associated with a giant molecular cloud at a systemic LSR velocity-50 km s^-1 and confined in a cavity,delineated by a north-ern molecular shell,a western concave MC that features a discernible shell,and an HI cloud was seen toward the southeast of the SNR.The birth of the SNR in a pre-existing molecular cavity implies a mass>18 M（?）for the progenitor if it is not in a binary sys-tem.Thermal conduction and cloudlet evaporation seem to be feasible mechanisms to interpret the X-ray thermal composite morphology,while the scenario of gas-reheating by the shock reflected from the cavity wall is quantitatively consistent with the obser-vations.We have also presented an updated list of thermal composite SNRs in this paper.To probe the different mechanisms in thermal composite SNRs,we need to run some hydrodynamic simulations.However,it is common to assume the plasmas are in collisional ionization equilibrium throughout current hydrodynamic or magnetohy-diodynamic simulations.We thus developed an improved non-equilibrium ionization method that we have developed as an optional module for the FLASH magnetohy-drodynamic simulation code in this paper.The method employs an eigenvalue ap-proach rather than the earlier iterative ordinary differential equation approach to solve the stiff differential equations involved in nonequilibrium ionization calculations.The new code also allows the atomic data to be easily updated from the AtomDB database.We compare both the updated atomic data and the methods separately.The ncew atomic data are shown to make a significant difference in some circumstances,although the general trends remain the same.Additionally,the new method also allows simultaneous calculation of the non-equilibrium radiative cooling,which is not included in the orig-inal method.The eigenvalue method improves the calculation efficiency overall with no loss of accuracy.We explore some common ways to present the non-equilibrium ionization state with a sample simulation and find that using the average ionic charge difference from the equilibrium tends to be the clearest method.A clue to the nature of the thermal composite SNRs is the presence of regions that show X-ray evidence of recombining plasmas.Recent calculations of remnant evo-lution in a cloudy interstellar medium that included thermal conduction but not non-equilibrium ionization showed promise in explaining observed surface brightness dis-tributions but could not determine if recombining plasmas were present.In this paper,we present numerical hydrodynamical models of thermal composite SNRs in 2D and 3D including explicit calculation of nonequilibrium ionization effects.Both the spatial ionization distribution and temperature-density diagrams show that recombination oc-curs inside the simulated thermal composite SNR and that both adiabatic expansion and thermal conduction cause recombination,albeit in different regions.Features created by the adiabatic expansion stand out in the spatial and temperature-density diagrams,but thermal conduction also plays a role.Thus thermal conduction and adiabatic expan-sion both contribute significantly to the cooling of high-temperature gas.Realistic ob-servational data are simulated with both spatial and spectral input from various regions.We also discuss the possibility of analyzing the sources of recombination and dom-inant hydrodynamical processes in observations using temperature-density diagrams and spatial maps.It is important for the upcoming X-ray instruments（like Athena and XRISM）.In addition,we have also done a series of works on the resonant scattering of emission lines in X-ray regime（See appendix B）.