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 bands likely arise from a mixture of ionized and neutral polycyclic aromatic hydrocarbon (PAH) molecules. Modeling the emission process is also difficult because the size distribution of the molecules, their ionization state and excitation, and the geometry of the emitting regions are generally unknown. If these molecular bands could be found in absorption, many of these factors would be eliminated, making it much easier to interpret the data. We have discovered an absorption band in Monoceros R2 IRS 3 centered at 11.25 mum that we identify with a C-H out-of-plane vibrational mode of PAH molecules. The shape and position of the band are very similar to that seen in emission in planetary nebulae, H II regions, and the interstellar medium.