Deuterated Interstellar Polycyclic Aromatic Hydrocarbons

Abstract

We report infrared spectral evidence of deuterated interstellar polycyclic aromatic hydrocarbons (PAHs). Two\rbands are detected in the infrared emission from the ionization bar in Orion at 4.4 and 4.65
m. The one at 4.65
m\ris present at the 4.4 level, while the one at 4.4
m is more tentative, with a equal to 1.9. An emission band at\r4.65
m is also detected in the infrared emission from M17 by Verstraete and coworkers at the 4.4 level. These\rwavelengths are highly characteristic of C-D stretching modes of deuterated PAHs and fall in a featureless region\rof the interstellar spectrum. The IR spectroscopic properties of deuterated PAHs are summarized, and an estimate of\rthe deuterium fractionation between the aromatic and aliphatic carbons associated with the PAHs is given. High\rdeuterium fractionation is implied; this is discussed in terms of interstellar PAH fractionation mechanisms.