Excitation of unidentified infrared bands by H atom impacts

Abstract

A model was developed for the excitation of the unidentified infrared bands (UIBs) by H atom impacts in the interstellar medium. It builds upon the fact that, in the presence of far‐ultraviolet (far‐UV) radiation and hydrocarbon grains, the hydrogen gas will be partially dissociated and the grain surface will be partially hydrogenated and partially covered with free C bonds. Under such a statistical equilibrium, H atoms from the gas will recombine with C atoms at the grain surface at some rate. At each recombination, the H atom deposits an energy of about 5 eV in the grain. Half of this is directly converted into vibrational excitation, always distributed in the same way among the most tightly coupled vibration modes of the grain. Absent frequent grain–grain collisions, the only outlet for this energy is infrared re‐emission, part of it in the UIBs, provided the chemical structure of the grains is adequate, and the other part in the continuum. The partition only depends upon the grain size, all grains being assumed to have the same constitution. Only a fraction, about 0.25, of the grains (among the smallest ones) will contribute significantly to the UIBs. It is shown quantitatively that H atom impacts are generally more efficient excitation agents than UV absorption because of the overwhelming abundance of H relative to UV photons. Only very close to young bright stars is this no longer true because photon flux then largely exceeds H atom flux. Thus, H atom impacts and far‐UV absorption are both necessary to understand the variety of observed UIB spectra. The model translates into a small number of equations enabling a quantitative comparison of its predictions with available astronomical observations, which have become exquisitely rich and accurate in the last two decades.

Spontaneous formation and protonation of dicyanofuran isomers under physical conditions found in interstellar space: quantum chemical insights into thermodynamics and spectroscopy

“…According to the literature, the C–O vibrational mode contributes to the UIR between 6 and 6.5 μm in the case of an O-bridge between two aromatic groups, 52 and at 6.2, 7.7 and 8.6 μm in cases of C–O functional groups in molecules composed of C, H, O, N and S atoms. 53,54 A careful analysis of the C–O stretching modes for studied compounds showed that their contribution fell between 7.4 and 9.2 μm, which compares well with the literature. The agreement between the observational characteristics of UIR features and molecular vibrational contributions reported herein implied that dicyanofurans and their protonated forms had a high chance to be detected in the ISM.…”

The contribution of CHONS particles to the diffuse high-Galactic-latitude IR emission

“…The molecules and clusters that are too small to reach an equilibrium temperature can be shown to emit, through a different mechanism, a spectrum which differs from a graybody spectrum (see Papoular 2012 and. This spectrum extends over the phonon continuum but is weaker than the gray-body spectrum modeled above.…”

“…We consider here the possibility that part of it might be due to grain heating by capture of wandering H atoms. This heating agent was studied in detail in Papoular (2012), where it was shown to be effective in the case of hydrocarbon grains with dangling carbon bonds, i.e. not capped with a hydrogen atom.…”

“…For high Galactic latitudes low velocity (local) thin clouds, G=1 and nH may reach 100 cm −3 , giving a 10-fold advantage to H-capture excitation. At very high galactic latitudes, the effect is enhanced by the decrease of G. Papoular (2012) argued that the global Galactic FIR excess, estimated by Mezger (1978) at a factor 8, could be explained in this way.…”

On the detections of C60 and derivatives in circumstellar environments

“…Some alternative mechanisms, such as the release of chemical energy (Duley and Williams, 2011) and atom impacts (Papoular, 2012), have been introduced to explain the excitation of AIBs. It remains unknown whether these mechanisms play a role on the excitation of C 60 IR bands.…”