Physics Behind Process
The PNLF method is unbiased by metallicity. This is because oxygen is a primary nebular coolant; any drop in its concentration raises the plasma’s electron temperature and raises the amount of collisional excitations per ion. This compensates for having a smaller number of emitting ions in the PNe. Consequently, a reduction in oxygen density only lowers the emergent λ5007 emission line flux by approximately the square root of the difference in abundance. At the same time, the PNe’s core responds to metallicity the opposite way. In the case where the metallicity of the progenitor star is smaller, then the PNe’s central star will be a bit more massive and its emergent ultraviolet flux will be a bit larger. This added energy almost precisely accounts for the decreased emissions of the PNe. Consequently, the total λ5007 flux that is produced by a PNe is practically uncorrelated to metallicity. This beneficial negation is in agreement with more precise models of PNe evolution. Only in extremely metal-poor PNe does the brightness of the PNLF cutoff dim by more than a small percentage.
The relative independence of the PNLF cutoff with respect to population age is harder to understand. The 5007 flux of a PNe directly correlates to the brightness of its central star. Further, the brightness of its central star directly correlates to its mass. In a PNe, the central star's mass directly varies in relation to its progenitor's mass. However, by observation, it is demonstrated that reduced brightness does not happen.
Read more about this topic: Planetary Nebula Luminosity Function
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