Optical Properties Of Interstellar Iron Needle-shaped Dust

Interstellar dust,existed in interstellar medium,was gradually known to people for phenomena such as interstellar extinction and interstellar reddening at the beginning of last century.Although its mass only about1% of the total mass of the interstellar medium in the galaxy,the farinfrared radiation from interstellar dust accounts for about 12% of the total electromagnetic radiation of the interstellar medium.it significantly influences human exploration and research of the universe,thus the research on interstellar dust has therefore long been an important field of study in disciplines such as astrophysics.Iron has long been thought to be present in the interstellar medium mainly as silicate dust,but recent studies suggest that iron is also present in non-silicate forms,such as the needle-like form of single iron crystals.It is believed that metallic whiskers can be formed in supernova ejecta or can be formed quickly from metallic iron grains in the solar nebula,and the above studies imply that needle-shaped iron interstellar dust may have some abundance in the interstellar medium.Current studies on the optical properties of infinitely long needle-shaped iron dust have revealed that iron needle-shaped dust has ”exotic optical properties”: small differences in extinction at different wavelengths(called ”gray” extinction),weak reddening,large far-infrared and milli-metre-wave emission coefficients,low thermal equilibrium temperature,and the dust opacity is unusually large from the UV to mm-wave range.However,the optical properties of finite needle-shaped iron dusts have not been studied before this study.In this thesis,the optical cross section,reddening and equilibrium temperature of finite-length needle-like iron dust with different long-axis ratios and different base radii are studied for the first time by the DDA method,and the optical cross section of finite-length needle-like iron dust grows in power with wavelength in the UV band,then the optical cross section rises rapidly and maintains high oscillations,and finally the optical cross section starts from the last peak at several microns to approximately power The model of inverse ”U”-like variation of the optical cross section with decreasing wavelength is obtained.A special pattern is also obtained for the variation of the optical cross-section with shape and size: except for the bottom radius of 0.1?m where the mass absorption coefficient curve is flattened,the peak position,curve trend and relative peak height are similar for the same length-axis ratio;the upper limit of the wavelength of the optical cross-section oscillation becomes larger as the length-axis ratio increases.We also investigate the reddening effect of dusts of different shapes and sizes.We find that the reddening is weakest for dusts with a length-axis ratio of 5 and a bottom radius of not more than 0.1?m,and the average reddening of interstellar dusts decreases the most and the extinction curve is the flattest for needle-shaped iron dusts with a bottom radius of 0.03 ?m.Finally,we also calculate the thermal equilibrium temperature of the larger dust sizes,and we obtain that the thermal equilibrium temperature of the dust with 0.1?m bottom radius is below 10 K near the Sun,which is ”very cold dust”.