The Unidentified Infrared (UIR) Bands and The PAH Hypothesis

	The discovery of an unexpected infrared emission feature from two planetary nebulae by Gillett, Forrest, and Merrill in 1973 marked the beginning of an exciting chapter of modern astrophysics.[F.C. Gillett, W.J. Forrest, and K.M. Merrill, Ap.J., 183 (1973) 87] Gillett et al. recognized that this band near 885 cm-1 (11.3 µm) was associated with interstellar dust and that its identification could give important insight into dust formation and evolution through the latter stages of the stellar life cycle. Moreover, this feature could not be associated with graphite grains, long thought to be produced in intense circumstellar outflows from late-type, carbon-rich stars. Subsequent pioneering observations showed this was just one part of a now well-known spectrum of features emitted from a wide variety of very different astronomical objects.

		This spectrum - dubbed the Unidentified Infrared Bands because at the time of their discovery, the material from which they originated was unknown - is characterized by dominant features near 3030, 1610, 1280, 1150, and 890 cm-1 (3.3, 6.2, 7.7, 8.6, and 11.2 µm, see image at left) as well as a number of more minor bands and underlying continua. The brightest emission is observed from dusty regions exposed to intense ultraviolet radiation. Surprisingly, the age and history of the material seem to be relatively unimportant. Similar emission spectra are observed from objects which span the lifecycle of matter in the interstellar medium (ISM) - from objects only a few thousand years in age (late carbon star ejecta and planetary nebulae shells) to those millions of years in age (H II regions, reflection nebulae, and the diffuse ISM). Understanding the source of this unexpected, yet widespread, phenomenon has become an important problem in astrophysics.

	The idea now gaining acceptance - that stochastically-heated, gas phase, polycyclic aromatic hydrocarbons (PAHs) are the band carriers - was first put forth over a decade ago. This attribution is based on several pieces of "circumstantial" evidence which point to these species. First, the interstellar emission is non-thermal in nature. The banded (rather than continuous) nature of the spectrum, the typically large feature/continuum ratio, and the close association with ultraviolet radiation all indicate that the emission is due to infrared fluorescence from gas phase molecules excited by the absorption of single ultraviolet and visible photons rather than thermal emission from a solid material. Second, careful observations of planetary nebulae have established that the fraction of the total infrared energy that is emitted through these features is closely correlated with the amount of available carbon, indicating that the gas-phase molecular carriers are carbon-rich. Third, since the carbon-rich carriers must survive under remarkably harsh conditions, they must also be extremely stable. Finally, although there are variations among the relative band intensities, the features are correlated, implying that a single class of chemical species is responsible. Of course, any proposed carrier must have an IR spectrum consistent with the positions and intensities of these bands. As a molecular class, PAHs readily accommodate all these constraints. The figure below contains en example comparison between the infrared emission of the Orion Bar (indicated in the image below) and the spectrum of a mixture of PAHs whose spectra were each measured in the lab.