PAHlution in interstellar space.

PAHlution in interstellar space 

Over the last decade and a half, observers have identified numerous chemical compounds in the clouds that pervade interstellar space. Starting from single elements and the simplest compounds, the list has grown by the addition of more and more complicated compounds. Now three California researchers suggest the presence of a mixture of very complicated compounds, polycyclic aromatic hydrocarbons (PAHs).

This science of astrochemistry proceeds by examining the spectra of objects in the sky for emissions or absorptions of radiation characteristic of different chemical substances, either fairly sharp wavelengths (lines) or short ranges of wavelengths (bands). Many such spectral features have been detected in the sky long before they were identified with particular substances through comparison with spectra obtained in the laboratory.

L. J. Allamandola and A. G. G. M. Tielens of NASA's Ames Research Center at Moffett Field, Calif., and J. R. Barker of SRI International in Menlo Park, Calif., consider the so-called unidentified infrared emission bands, eight bands ranging in wavelength from 3.3 to 11.3 micrometers, as well as a continuum of radiation over the range 1 to 5 micrometers. Other astrophysicists have suggested that PAHs carrying fewer hydrogen atoms than the total number of sites in their structure where hydrogen might be bound could be responsible for these unidentified infrared features. Allamandola, Tielens and Barker manage to identify the features with energy associated with the vibrations of various parts of the structure of such compounds, provided a mixture of PAHs is present--or, in their own words in the March 1 ASTROPHYSICAL JOURNAL LETTERS: "auto exhaust along the Milky Way.'