HII regions slides

Forbidden linesForbidden line: arises when an electron is excited by a collision into a‘metastable state’.When densities are higher (e.g. our atmos. greater than about 10 8 percm 3 ), electron would almost immediately be knocked out of metastable stateby collision and not be given time to emit a photon (collisional de-excitation =no photon).But in a low density nebula, the time between collisions ~ 10 10,000seconds(v. long time) allowed time to radiate spontaneouslyIn a nebular environment, practically every ion goes to the ground state byforbidden radiationThe term ‘Forbidden’ refers to quantum mechanical rules - a little misleading!More intuative name: Collisionally excited lines (CELs) - line emissionfollowing collision with electron