The phenomenon whereby the → photodissociation
transitions of a molecule in interstellar
clouds become → optically thick, so that the molecule
in question is “shielded” by
other molecules against dissociating stellar → far-ultraviolet (FUV)
photons. In the case of → molecular hydrogen (H2),
when the → column density exceeds 1014 cm-2,
the UV absorption bands become optically thick, and H2 undergoes
self-shielding. More specifically, all of the photons that could lead to
UV photodissociation are absorbed by H2 in the outer
layers of the cloud, hence protecting the H2 within the cloud. Self-shielding occurs
in → diffuse interstellar clouds exposed to the interstellar
→ radiation field or in → molecular clouds
in proximity to sources of UV photons. Dust can also absorb UV photons,
further limiting the photodissociation, but it dominates only
when the local UV radiation field is unusually intense relative to the
density of the cloud.
See also: → self-; → shield;
→ -ing.
