On the carbonaceous carriers of IR plateau and continuum emission

TL;DR

This paper investigates the molecular origins of IR plateau and continuum emissions in proto-planetary nebulae using computational chemistry to identify the vibrational spectra of carbonaceous structures, linking them to observed astronomical features.

Contribution

It introduces a computational approach to identify the molecular carriers of IR plateaus and continua, connecting laboratory, computational, and astronomical data.

Findings

Weak vibrational modes explain IR plateaus.

Far IR spectra are linked to phonon modes.

Simulations match observed spectra of proto-planetary nebulae.

Abstract

This study explores the molecular origins of plateaus and continuum underlying IR and FIR bands emitted by compact nebulae, especially proto-planetary 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 to 18-mu 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 mu. The absorptivities and absorption cross-sections per interstellar H atom deduced from these spectra are comparable with those of laboratory dust analogs and astronomical measurements, respectively. Moreover, the 5-35 mu spectra of two typical Proto-Planetary Nebula were reasonably well simulated with combinations of molecules containing functional groups which carry the 21- and 30-mu 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 FIR bands, as well as the composition of circumstellar dust.