Spatial variations of the 3 micron emission features within UV-excited nebulae: photochemical evolution of interstellar polycyclic aromatic hydrocarbons.

We have obtained 3 microns spectra at several positions in the Orion Bar region and in the "Red Rectangle," the nebula surrounding HD 44179. The recently discovered weak emission features at 3.40, 3.46, 3.51, and 3.57 microns (2940, 2890, 2850, and 2800 cm-1) are prominent in the Orion Bar region. The 3.40 microns and 3.51 microns features increases in intensity relative to the dominant 3.29 microns (3040 cm-1) feature when going from the ionized to the neutral zone across the Orion Bar. However, only a weak and rather broad 3.40 microns feature is present at the position of HD 44179. These spectra demonstrate that some of the 3 microns emission components vary independently of each other and in a systematic way within UV-excited nebulae. This spatial variation is discussed in terms of the UV excitation and photochemical evolution of polycyclic aromatic hydrocarbons and related molecular structures. The spatial behavior of the weak emission features can be understood qualitatively in terms of hot bands of the CH stretch and overtones and combination bands of other fundamental vibrations in simple PAHs. An explanation in terms of emission by molecular sidegroups attached to the PAHs is less straightforward, particularly in the case of the Red Rectangle and other evolved mass-losing objects. We estimate PAH sizes of 20-50 carbon atoms based on the susceptibility of PAHs to destruction by the far ultraviolet fields present in the Orion Bar and the Red Rectangle; the size range is similar to independent estimates made previously.