Investigation of Carbon Abundance in the Crab Nebula

As part of a larger program to map elemental distributions in the Crab Nebula supernova remnant, we have carefully investigated the [CI] 9850 emission line as a means for measuring the gaseous carbon mass fraction.~~ Knowledge of the amount of carbon is necessary for understanding the progress of nucleosynthesis in the gas.~ Whereas this near-infrared line is very weak and not used for abundance determinations in most astronomical nebulae, it is ``anomalously'' strong in the Crab Nebula.~~ In order to determine whether or not [CI] 9850 is a reliable indicator of carbon abundance, we employed a numerical photoionization code to examine the dependence of [CI] 9850 emission on various factors, including carbon, helium, nitrogen, and oxygen abundances.~~ We also varied the ionization parameter and hydrogen density to gain insight regarding the huge difference in strength of this line in other types of objects.~ Our calculations show that [CI] 9850 is a robust indicator of carbon abundance in the Crab Nebula, where it is visible because of a high helium mass fraction and a low ionization parameter, which are not found in other nebulae.