DENSITY CONDENSATIONS IN PLANETARY-NEBULAE AND THE ELECTRON-TEMPERATURE

For planetary nebulae, the electron temperature derived from the [O II I] line ratio is usually higher than that obtained from the Balmer dis continuity. This discrepancy is generally explained in terms of temper ature fluctuations. It is shown here that the [O III] line ratio could lead to a lower value of the temperature if high-density condensation s are present in the gas. An analytical formalism is given for the cor rection factors for ionic abundances in the presence of such condensat ions. Illustrative clump filling factors are deduced for several plane tary nebulae recently observed by Liu and Danziger. We discuss the eff ect of dense clumps on the ionic and chemical abundances, partly based on photoionization model calculations. Accurate observations of [O II ] 7325/3727 Angstrom and [S II] 4070/6725 Angstrom line ratios provide a diagnostic of the presence of dense clumps.