Search for the Infrared Emission Features From Deuterated Interstellar Polycyclic Aromatic Hydrocarbons

Abstract

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) onboard 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.

Theoretical study of deuteronated PAHs as carriers for IR emission features in the ISM

“…PADs show distinct features at 4.4 and 4.65 µm that have been observed towards Orion Nebula and M17 (Peeters et al 2004). These observations have not been confirmed by AKARI observations (Onaka et al 2014). These emission bands are characteristics of C − D stretching modes in PADs (Peeters et al 2004).…”

Quantum tunneling observed without its characteristic large kinetic isotope effects

“…We suggest that interstellar aromatic hydrocarbons including C 6 H 6 can be hydrogenated or deuterated by the tunneling of H or D atoms at low temperatures. The deuteration of interstellar aromatic hydrocarbons to form deuterated aliphatic structures is of particular interest (12)(13)(14)(15)19), because such materials could represent a major carrier of deuterium enrichment (D/H = 10 −4 to 10 −2 ) beyond levels expected from the elemental D/H ratio in space (1.5 × 10 −5 ) observed in carbonaceous meteorites and interplanetary dust particles (17,20). They may carry signatures of the survival of interstellar materials within the solar system, because the deuterium enrichment is most noticeable in cold, dense interstellar regions (e.g., molecular clouds), where deuterated species are thermodynamically more stable than their hydrogenated counterparts owing to the zero-point energy difference of several tens of kelvin (18).…”

“…The present study is also the first report to our knowledge of the nonenergetic deuteration of aromatic hydrocarbons at low temperature. We discuss the importance of our findings for astrochemistry and geochemistry in relation to the origin of deuterium enrichment observed in extraterrestrial materials such as interstellar aromatic/aliphatic hydrocarbons, carbonaceous meteorites, and interplanetary dust particles (12)(13)(14)(15)(16)(17), the chemistry of which influences our understanding of interstellar physicochemical processes, including the formation of the solar system (18)(19)(20). .…”

Processing of Interstellar Medium as Divulged by Akari

“…With the low spectral resolution of the IRC spectroscopy it is difficult to make unambiguous identification of those faint features and the remaining excess gives us an upper limit for the PAD to PAH band ratio of 3%. The difference in the band oscillator strength is compensated by the difference in the excitation conditions and the band ratio is nearly equal to the abundance ratio under the standard conditions (Onaka et al, 2014). We further look for the PAD features in a larger sample and find that the largest excess emission detected in this spectral range is 6% relative to the 3.3-3.5 µm band intensities.…”

“…ISO observations detected these bands at a marginal level towards the Orion bar and M17 (Peeters et al, 2004). However, recent observations with AKARI IRC could not confirm the presence of the features from deuterated PAHs (PADs) (Onaka et al, 2014). As indicated in Figure 2, there is excess emission remaing in 4.4-4.6 µm, some of which can be attributed to known species.…”