Modelling photoluminescence from small particles. II. Implications for dust rotation and the Extended Red Emission

The Extended Red Emission (ERE) has been observed in virtually all possible environments in which dust is present, ranging from HII regions to carbon rich planetary nebulae, to reflection nebulae, to dense interstellar clouds, to the diffuse interstellar medium. One problem in comparing ERE with laboratory photoluminescence (PL) measurements is given by the fact that laboratory samples are usually macroscopic in size, or very thin films, while interstellar dust particles are usually of sizes comparable to, or smaller than, the wavelength of visible light. We here apply a newly developed general recipe to extrapolate the expected PL of small, homogeneous spheres starting from the available laboratory results obtained for bulk samples, and infer previously unsuspected consequences for ERE-like PL phenomena.