Abstract
The magnitude of the temperature fluctuations ( t
2 ) required to explain the observed inconsistencies between metallicities inferred from recombination lines and from forbidden lines cannot be attained by steady-state equilibrium photoionization models. If, on the other hand, the nebular ionizing source were variable, the temperature fluctuations t
2 would be significantly larger. We investigate the time-dependent response of the nebular ionization and temperature structure when photoionized by a periodically varying source. We study how the asymptotic mean value,
<t
2> , behaves as a function of the period or amplitude of the source variability. We find that the temperature fluctuations occur only in the outer section of the nebula, close to the ionization front, within a zone corresponding to 8-20% of the ionized layer's thickness. We conclude that the amplitude of the exciting star variations required to achieve a <t
2>= 0.025 (as in the Orion nebula) is unacceptably large. Source variability is therefore not a viable mechanism to explain the observed values of t
2 . We reach a similar conclusion from studies of the temporal variability resulting from intermittent shadows behind opaque condensations. We find that photoionized nebulae are on average less massive but somewhat hotter in the case of cyclically variable ionizing sources.
