1) Chemistry: A family of → organic molecules composed
of carbon and hydrogen atoms (→ hydrocarbons) in which
→ carbon atoms appear in multiple loops (polycyclic)
with strong chemical → bonds that exist between them (aromatic).
PAHs are formed during the incomplete burning of coal, oil and gas, garbage, or other
organic substances like tobacco or charbroiled meat. As a pollutant, they are of
concern because some compounds (benzo(a)pyrene) have been identified as tending to
2) Astro.: Strong → infrared emission features at
3.3, 6.2, 7.7, 8.6, and 11.2 microns, which are attributed to PAH molecules in the form of
→ dust grains with radii smaller than 10 Å.
PAHs are common toward various sources characterized
by a strong → ultraviolet radiation field
(such as → active galaxies,
→ reflexion nebulae,
→ planetary nebulae, → H II regions).
In particular, they trace the presence of
→ massive stars. The PAH features are thought to
result from the fluorescent emission process of PAH molecules and ions excited by
ultraviolet radiation of nearby stars. PAHs are likely
produced by → photoevaporation of
→ very small grains.