On the Abundance Gradient of the Galactic Disk

Estimates of the gas temperature in planetary nebulae obtained from the [O III] emission line ratio and from the Balmer discontinuity indicate differences reaching up to 6000 K (Liu and Danziger 1993). The [O III] temperature is commonly used to obtain the ionic fractions of highly ionized ions, particularly the O and Ne ions when using the empirical method to calculate the elemental abundances of photoionized gas from the observed emission line intensities. However, if the gas temperature is overestimated, the elemental abundances may be underestimated. In particular, it may lead to an incorrect elemental abundance gradient for the Galaxy, usually used as a constraint for the chemical evolution models. Using Monte Carlo simulations, we calculate the systematic error introduced in the abundance gradient obtained from planetary nebulae by an overestimation of the gas temperature. The results indicate that the abundance gradient in the Galaxy should be steeper than previously assumed.