Nano-grains in the interstellar medium

Abstract

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-heating 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.