2008ApJ...674..954B

Good to excellent photoionization models based on the Cloudy code (from Ferland and coworkers) were obtained for 13 out of 19 spectra of planetary nebulae (PNs).
Result. are deficient when the slit is missing most of the in-depth extent of the ionized region. Features that can be associated to shock excitation and abnormally large helium abundances are found when this is the case. Models assume that the exciting star is a Rauch photosphere illuminating a static uniform density spherical shell. Model central stars lie close to a 2.5 M_☉post-AGB theoretical evolutionary track in the H-R diagram (from Vassiliadis and Wood), with implied PN ages between 1000 and 7000 yr. Observed and modeled nebular temperatures derived from [N II] λλ(6548+6584)/λ5755 agree within 10%, but models usually underestimate temperatures found from [O III] λλ(4959+5007)/λ4363, more so when the slit does not cover the in-depth extent of the ionized region. Helium, nitrogen, oxygen, neon, sulfur, chlorine, and argon model abundances are uncertain at the 15%, 15%, 10%, 7%, 30%, 5%, and 7% levels, respectively. The following relationships between chemical elements are found: He tends to be more abundant when N/O is large; there is a loose anticorrelation between N/O and O/H; N/C and C/H are anticorrelated; and the sum of the carbon, nitrogen, and oxygen abundances is larger in carbon-rich objects. Finally, it is shown that neon abundance in PNs has been overestimated, and an alternative ionization correction factor is provided.