2009A&A...505.1027H

We aim to derive the chemical behavior of a significant sample of PNe and HII regions in the irregular galaxy NGC6822. The selected objects are distributed in different zones of the galaxy. Our purpose is to obtain the chemical abundances of the present interstellar medium (ISM), represented by HII regions, and the corresponding values at the time of formation of PNe. With these data the chemical homogeneity of NGC6822 were tested and the abundance pattern given by HII regions and PNe used as an observational constraint for computing chemical evolution models to infer the chemical history of NGC 6822. Due to the faintness of PNe and HII regions in NGC6822, to gather spectroscopic data with large telescopes is necessary. We obtained a well suited sample of spectra by employing VLT-FORS2 and Gemini-GMOS spectrographs. Ionic and total abundances were calculated for the objects where electron temperatures could determined through the detection of [OIII]λ4363 or/and [NII]λ5755 lines. A ``simple'' chemical evolution model was developed and the observed data were used to compute a model for NGC6822 in order to infer a preliminary chemical history in this galaxy. Confident determinations of He, O, N, Ne, S and Ar abundances were derived for a sample of 11 PNe and one HII region. We confirm that the present ISM is chemically homogeneous, at least in the central 2kpc of the galaxy, showing a value 12+logO/H=8.06±0.04. From the abundance pattern of PNe, we identified two populations: a group of young PNe with abundances similar to HII regions and a group of older objects with abundances a factor of two lower. A pair of extreme Type I PNe were found. No third dredge-up O enrichement was detected in PNe of this galaxy. The abundance determinations allow us to discuss the chemical behavior of the present and past ISM in NGC6822. Our preliminary chemical evolution model predicts that an important gas-mass loss occurred during the first 5.3Gyr, that no star higher than 40M_☉ was formed, and that 1% of all 3-15M_☉ stars became binary system progenitors of SNIa.