Fullerenes And Interstellar Extinction

The Milky Way’s stars and other galaxies are filled with gaseous ions,atoms and molecules（interstellar gas）and tiny dust particles（interstellar dust）.Interstellar dust is dispersed in the interstellar gas,which together with interstellar gas is known as the interstellar medium.The interstellar medium plays an important role in the evolution of galaxies.They are capable of radiating energy,so their presence can be proven by observing their characteristic spectrum of radiation;They can absorb and scatter starlight,causing it to dim,a phenonon known as interstellar extinction,which also reflects the presence of interstellar medium.Interstellar carbon dust is an important component of the interstellar medium,including graphene,fullerenes,nanodiamonds,etc.In recent years,with the development of observation equipment and technology,scientists have detected the infrared vibrational spectra of interstellar C₆₀ and C₇₀.Fullerenes,an important carbon dust,have also been confirmed by observations.The extinction produced by dust is related to the properties of the dust itself（size,shape,optical properties）and its abundance in the universe.We studied the extinction of monolayer and multilayer fullerenes.For monolayer fullerenes,we focused on C₆₀,C₇₀ and C₈₄.We calculate their absorption cross sections and extinction in the visible ultraviolet band using dielectric functions obtained by electron energy loss spectroscopy（EELS）experiments;For multilayer fullerenes,we focused on C₆₀@C₂₄₀and C₂₀@C₆₀.Using the absorption cross sections of C₆₀@C₂₄₀ and C₂₀@C₆₀ calculated by TDDFT,we calculated their extinction。In addition,in order to construct the new dust model,we also calculated the extinction of graphite and silicate using the Mie theory combined with the size distribution function.Interstellar extinction curves have a large number of spectral properties,which may be related to some special dust or small molecular carriers.Therefore,we must build a dust model,and obtain the theoretical extinction curve by estimating the total extinction amount of the dust,and then fit it with the average Galactic interstellar extinction curve（Fitzpatrick1999,Rv=3.1）,so as to determine the contribution of dust to interstellar extinction and the characteristics and abundance of dust.We employed two methods to obtain information on the contribution of fullerenes to interstellar extinction and its abundance in the interstellar medium.The first method is based on the Li&Draine model,without changing the abundance and the size distribution of the four components in the model,directly adding fullerenes to the model to obtain model extinction curve,which is then compared with the average Galactic interstellar extinction curve.Under the condition of not exceeding the observational uncertainties of the average Galactic interstellar extinction curve,the upper limit of the abundance of fullerenes is finally determined(C₆₀,C₇₀,C₈₄,C₆₀@C₂₄₀,C₂₀@C₆₀ are 5 ppm,7 ppm,5 ppm,2 ppm,12 ppm).The second method is to establish a new dust model,reconsider the size and abundance of dust in the model,and comprehensively consider the contributions of these dust components to extinction,not only can the upper limit of fullerene abundance be determined(C₆₀,C₇₀,C₈₄,C₆₀@C_240,C₂₀@C₆₀are 15 ppm,20 ppm,17 ppm,4 ppm,15 ppm,respectively),but also the abundance and parameters related to size of other dust components can be determined.