Discovery of the 11.2 micron polycyclic aromatic hydrocarbon band in absorption toward Monoceros R2 IRS 3

Interpretation of the infrared emission bands is difficult because these ba nds likely arise from a mixture of ionized and neutral polycyclic aromatic hydrocarbon (PAH) molecules. Modeling the emission process is also difficul t because the size distribution of the molecules, their ionization state an d excitation, and the geometry of the emitting regions are generally unknow n. If these molecular bands could be found in absorption, many of these fac tors would be eliminated, making it much easier to interpret the data. We h ave discovered an absorption band in Monoceros R2 IRS 3 centered at 11.25 m um that we identify with a C-H out-of-plane vibrational mode of PAH molecul es. The shape and position of the band are very similar to that seen in emi ssion in planetary nebulae, H II regions, and the interstellar medium.