Week 6 Photoionisation Regions Flashcards
what determines the energy of a photon
ionisation potential and electron momentum
what is the recombination rate equation
N =βnenp = βne^2
where β is the recombination coefficient
ne is the free electron number density
np is the free photon density
what is a fossil HII region
ionised cloud of hydrogen where the ionising source has disappeared
what is the recombination time equation
τrec = 1/βne
when is ionisation balance achieved
when the ionisation fraction is constant and the rate of recombination and rate of ionisation are equal
what is the ionisation balance equation
x^2/(1-x) = σJ/βn
where x is the ionisation fraction x = ne/n
what happens to the ionisation fraction in a HII region
it drops abruptly at a certain point
what is the ionisation distance equation
R = (3/4π 1/β S/n^2)^1/3
how is thermal balance maintained in a gas
recombination cools the gas
what is expected if a gas is in thermal and ionisation equilibrium
temperature of the gas should be comparable to the effective temperature of the star producing ionising radiation
describe the first phase of expansion during photoionisation (Stromgren sphere phase)
ionisation front moves outwards from the centre with supersonic velocity however the gas is static so there is no shock front
describe the second phase of expansion during the photoionisation
starts when the ionisation front reaches Mach 2
