On the carbonaceous carriers of infrared plateau and continuum emission

Abstract

This study explores the molecular origins of plateaus and continua underlying the infrared and far-infrared (far-IR) bands emitted by compact nebulae, especially protoplanetary nebulae. Computational organic chemistry codes are used to deliver the vibrational integrated band intensities of various large, typical carbonaceous structures. These spectra are composed of a rather continuous distribution of weak modes from which emerge the fingerprints. The 6–18 μm region is interspersed with a great many weak lines, to which the plateaus are assigned. Similarly, the far-IR spectrum is ascribed to the phonon (skeletal) spectrum which is readily identified beyond 18 μm. The absorptivities and absorption cross-sections per interstellar H atom deduced from these spectra are comparable with those of laboratory dust analogues and astronomical measurements, respectively. Moreover, the 5–35 μm spectra of two typical protoplanetary nebulae were reasonably well simulated with combinations of molecules containing functional groups which carry the 21- and 30-μm bands, and molecules devoid of these but carrying strong phonon spectra. These results may help understand the emergence of plateaus, the origin of continua underlying far-IR bands, as well as the composition of circumstellar dust.