Abstract
For planetary nebulae, the electron temperature derived from the [O III] line ratio is usually higher than that obtained from the Balmer discontinuity. This discrepancy is generally explained in terms of temperature fluctuations. It is shown here that the [O III] line ratio could lead to a lower value of the temperature if high-density condensations are present in the gas. An analytical formalism is given for the correction factors for ionic abundances in the presence of such condensations. Illustrative clump filling factors are deduced for several planetary nebulae recently observed by Liu & Danziger. We discuss the effect of dense clumps on the ionic and chemical abundances, partly based on photoionization model calculations. Accurate observations of [O II] 7325/3727 Å and [S II] 4070/6725 Å line ratios provide a diagnostic of the presence of dense clumps.
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