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Seminars at FGG

Quantum chemistry (+modelling) in support of research on the Diffuse Interstellar Bands

Speaker: Giacomo Mulas (INAF-OSS. Astronomico di Cagliari)

Date and time: 2013-11-15 11:30

Diffuse Interstellar Bands are one of the longest standing unsolved enigmas in astronomy and, broadly speaking, in spectroscopy. The most current catalogues of DIBs count them in about ~400 unidentified absorption features. So far, none of them is yet firmly identified, with only very few plausible candidate carriers for very few of them, still awaiting confirmation or rejection. DIBs are so numerous, and hence so dense in the visible range of the spectrum, that reliable spectroscopic identification mandates 1) a very accurate match in position between astronomical observations and laboratory/theoretical counterparts and 2) a simultaneous acceptable match of several features, for all properties which can be compared (i.e. position, intensity, spectral profile). Requirement 1), i.e. very accurate absolute spectral position of transitions, is mostly out of reach with purely theoretical methods for polyatomic organic species. However, relative positions and intensities of bands in vibronic progressions may be computed with sufficient accuracy to provide a valid constraint. Also, computed spectral properties can be used to model the expected rotational envelopes of specific transitions of prospective DIB carriers in well-defined interstellar environments, again providing additional constraints. In short, theory alone will hardly discover the real DIB carriers, but can probably help a lot in rejecting candidates, reducing the burden on laboratory spectroscopists to fewer, most promising candidates. In this talk we will review what theoretical spectroscopy and quantum chemistry can possibly provide in this quest.