星际介质中的纳米尘埃

星际尘埃,作为一种在宇宙中普遍存在的重要成分,在天文学研究中起着重要的作用.星际介质中的尘埃其尺寸分布涵盖从几个埃到几个亚微米的范围.对于有这样大的尺寸差异的星际尘埃,尘埃的热辐射机制差别很大.较大的尘埃颗粒,有比较大的吸收和发射截面,因而其吸收光子的速率和发射的速率也比较大,可以从吸收和发射能量的平衡来得出热平衡温度,并用热平衡温度和黑体辐射来计算其光谱.对于大小为纳米尺度或者更小的尘埃,由于其很小的尺寸,这类尘埃的热容量非常小.当这种尘埃吸收一个与其热容量相当或者更大能量光子的时候,尘埃就会经历一个非常明显的温度涨落:尘埃吸收一个紫外光子瞬间,其温度迅速上升,到达顶点后,由于尚没有外来能量的影响(因吸收截面小,吸收光子几率也非常小),尘埃开始通过热辐射降温,直到吸收另一个光子开始新的循环,这就是单光子加热模型(Single-Photon Heating Model).显然,热平衡和单光子加热是两个明显不同的过程.对于单光子加热,由于尘埃颗粒的温度涨落作用,尘埃不会处于一个稳定的平衡温度状态,因而不能用单一温度来描述其热辐射,必须计算出尘埃在温度涨落过程中的温度分布函数,然后计算其辐射谱.本文主要介绍纳米颗粒在星际空间中的存在证据,单光子加热模型,以及处于温度涨落中纳米颗粒的辐射特征.

Nano-grains in the interstellar medium

Interstellar grains, an essential component of the universe, play an important role in modern astrophysics. Their sizes range from several angstroms to several submicrometers; consequently, their thermal properties also vary significantly. For large, submicrometer-sized grains, when exposed to the interstellar radiation field, they are expected to attain equilibrium temperatures determined from the balance between the absorption of starlight and emission of longer-wavelength photons. Their emission spectra are simply the Planck function at the equilibrium temperature multiplied by the dust emission efficiencies. For nanometer-sized （or even smaller） grains, due to their small heat capacities, a single starlight photon would heat them to high temperatures and the grains would then rapidly cool down by radiating longer-wavelength photons. Because of their small absorption cross sections, it takes much time for them to absorb another starlight photon--before encountering another starlight photon, the grains have already radiated away the absorbed photon energy. Therefore, for nano-sized grains, we need to consider the stochastical-beating process and calculate their temperature probability distribution functions. They will not attain an equilibrium temperature, instead, they will undergo ＂temperature fluctuation＂. Their emission spectra are obtained by integrating over the temperature probability distribution function.