HII regions slides

Thermal balance in ionized nebulaePhotoionization adds kinetic energy (KE) into the H cloud since it creates hot, fastphotoelectrons.T can increase indefinitely though… Energy must be lost somehowEnergy can be lost through photons: H recombination means KE of e photonwhich can escapeOn average, heat lost per recombination = 3/2 k T eso Cooling rate is:L = (3/2 k T e ) N R [J m -3 s -1 ]Energy input, or the Heating rateG = N PI Q [J m -3 s -1 ]Q = the heating energy injected into the gas per photoionization (in Joules)in equilibrium, G = L (and N R = N PI ) T e = (2/3)Q/k.