Poster abstract details

It is well established that PNe are not thermally homogeneous, beyond what chemically homogeneous photoionization models predict. To discriminate between the different scenarios proposed in the literature, we have determined the temperature and density based on HeI lines for a dozen PNe. We compare these determinations with those derived from collisionally excited lines, CELs, and from the Balmer continua. We find high Balmer temperatures for very high ionization PNe (higher than those derived from HeI and CELs): this is due to the higher temperature in the He++ region. For PNe where He+ is the dominant helium species we find that the HeI temperatures agree with the Balmer temperatures and that both are lower than those derived from CELs; suggesting that most PNe are chemically homogeneous.