2005A&A...443..115M

Spectroscopic observations obtained with the VLT of one planetary nebula (PN) in Sextan A and of five PNe in Sextan B and of several HII regions in these two dwarf irregular galaxies are presented. The extended spectral coverage, from 320.0 to 1000.0nm, and the large telescope aperture allowed us to detect a number of emission lines, covering more than one ionization stage for several elements (He, O, S, Ar). The electron temperature diagnostic [OIII] line at 436.3nm was measured in all six PNe and in several HII regions allowing for an accurate determination of the ionic and total chemical abundances by means of the Ionization Correction Factors method. For the time being, these PNe are the farthest ones where such a direct measurement of the electron temperature is obtained. In addition, all PNe and HII regions were also modelled using the photoionization code CLOUDY (Ferland et al., 1998PASP..110..761F). The physico-chemical properties of PNe and HII regions are presented and discussed. A small dispersion in the oxygen abundance of HII regions was found in both galaxies: 12+log(O/H)=7.6±0.2 in Sextan A, and 7.8±0.2 in Sextan B. For the five PNe of Sextan A, we find that 12+log(O/H)=8.0±0.3, with a mean abundance consistent with that of HII regions. The only PN known in Sextan A appears to have been produced by a quite massive progenitor, and has a significant nitrogen overabundance. In addition, its oxygen abundance is 0.4dex larger than the mean abundance of HII regions, possibly indicating an efficient third dredge-up for massive, low-metallicity PN progenitors. The metal enrichment of both galaxies is analyzed using these new data.