Deuterated interstellar polycyclic aromatic hydrocarbons

We report infrared spectral evidence of deuterated interstellar polycyclic aromatic hydrocarbons (PAHs). Two bands are detected in the infrared emission from the ionization bar in Orion at 4.4 and 4.65 mum. The one at 4.65 mum is present at the 4.4 sigma level, while the one at 4.4 mum is more tentative, with a sigma equal to 1.9. An emission band at 4.65 mum is also detected in the infrared emission from M17 by Verstraete and coworkers at the 4.4 sigma level. These wavelengths are highly characteristic of C-D stretching modes of deuterated PAHs and fall in a featureless region of the interstellar spectrum. The IR spectroscopic properties of deuterated PAHs are summarized, and an estimate of the deuterium fractionation between the aromatic and aliphatic carbons associated with the PAHs is given. High deuterium fractionation is implied; this is discussed in terms of interstellar PAH fractionation mechanisms.