THE EFFECT OF TEMPERATURE-FLUCTUATIONS ON THE DETERMINATION OF THE CARBON ABUNDANCE OF PLANETARY-NEBULAE

By combining the C III lambda lambda 1906 + 1909 with the C II lambda 4267 line intensities it is possible to determine T(C++); this temperature is, in general, considerably smaller than T(O++), the temperature derived from the O III lambda 4363 to lambda 5007 intensity ratio. We study possible causes for this difference. We show that in the presence of spatial temperature fluctuations T(C++) < T(O++). We find that the objects with the highest T(O++) - T(C++) values are those that show large velocities and complex velocity fields, therefore we suggest that the deposition of mechanical energy by the stellar winds of PNe is the main responsible for the temperature differences. Based on these arguments and the similar T(C++), T(Bac) and T(He I) values, obtained for well observed objects, we propose that the N(C++)/N(H+) and N(O++)/N(H+) values derived from the ratio of collisionally excited lines to H beta should be based on T(C++) instead of T(O++); alternatively the abundance ratios derived from recombination line intensity ratios are almost independent of the adopted temperature, and consequently are more reliable.