RESUMO
The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard the Rosetta spacecraft has measured molecular oxygen (O2) in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) in surprisingly high abundances. These measurements mark the first unequivocal detection of O2 in a cometary environment. The large relative abundance of O2 in 67P/C-G despite its high reactivity and low interstellar abundance poses a puzzle for its origin in comet 67P/C-G, and potentially other comets. Since its detection, there have been a number of hypotheses put forward to explain the production and origin of O2 in the comet. These hypotheses cover a wide range of possibilities from various in situ production mechanisms to protosolar nebula and primordial origins. Here, we review the O2 formation mechanisms from the literature, and provide a comprehensive summary of the current state of knowledge of the sources and origin of cometary O2.
RESUMO
The puzzling difference between the IR spectra of polycyclic aromatic hydrocarbons (PAHs) and those of the corresponding positive ions (PAHs(+)) is a well documented fact, although the basic reason for it is far from clear. In this report, the CH fragment, in its neutral and ionized forms is taken as a case study for investigating the collapse of the CH stretching vibration with ionization. A comprehensive study of the dipole moment function around the equilibrium geometries of the fragments using large scale configuration interaction and quantum Monte Carlo methods shows very different variations with the CH distance: a marked decrease for neutral CH((2)Pi) and a perfect stability for ionized CH(+)((1)Sigma(+)). These results are consistent with strong/weak intensities of the CH vibrations in the neutral/ionized PAHs, the key point being the presence, or not, of a hole in the pi shell. A topological analysis of the electronic densities shows that the collapse of the CH stretching with ionization is directly linked to the compensation between the internal charge transfer contribution and the distortion of the electronic density within the CH bond.
