Search

We report the results of a search for emission features from interstellar deuterated polycyclic aromatic hydrocarbons (PAHs) in the 4 μm region with the Infrared Camera (IRC) on board AKARI. No significant excess emission is seen in 4.3-4.7 μm in the spectra toward the Orion Bar and M17 after the subtraction of line emission from the ionized gas. A small excess of emission remains at around 4.4 and 4.65 μm, but the ratio of their intensity to that of the band emission from PAHs at 3.3-3.5 μm is estimated as 2%-3%. This is an order of magnitude smaller than the values previously reported and also those predicted by the model of deuterium depletion onto PAHs. Since the subtraction of the ionized gas emission introduces an uncertainty, the deuterated PAH features are also searched for in the reflection nebula GN 18.14.0, which does not show emission lines from ionized gas. We obtain a similar result that excess emission in the 4 μm region, if present, is about 2% of the PAH band emission in the 3 μm region. The present study does not find evidence for the presence of the large amount of deuterated PAHs that the depletion model predicts. The results are discussed in the context of deuterium depletion in the interstellar medium. [ABSTRACT FROM AUTHOR]

Using a large collection of near-infrared spectra (2.5-5.4 μm) of Galactic H II regions and H II region-like objects, we perform a systematic investigation of astronomical polycyclic aromatic hydrocarbon (PAH) features. Thirty-six objects were observed using the infrared camera on board the AKARI satellite as a part of a director's time program. In addition to the well known 3.3-3.6 μm features, most spectra show a relatively weak emission feature at 5.22 μm with sufficient signal-to-noise ratios, which we identify as the PAH 5.25 μm band (previously reported). By careful analysis, we find good correlations between the 5.25 μm band and both the aromatic hydrocarbon feature at 3.3 μm and the aliphatic hydrocarbon features at around 3.4-3.6 μm. The present results give us convincing evidence that the astronomical 5.25 μm band is associated with C-H vibrations, as suggested by previous studies, and show its potential to probe the PAH size distribution. The analysis also shows that the aliphatic-to-aromatic ratio of I3.4-3.6 μm/I3.3 μm decreases against the ratio of the 3.7 μm continuum intensity to the 3.3 μm band, Icont, 3.7 μm/I3.3 μm, which is an indicator of the ionization fraction of PAHs. The midinfrared color of I9 μm/I18 μm also declines steeply against the ratio of the hydrogen recombination line Brα at 4.05 μm to the 3.3 μm band, IBrα/I3.3 μm. These facts indicate possible dust processing inside or at the boundary of ionized gas. [ABSTRACT FROM AUTHOR]