Chemical behavior of the Dwarf Irregular Galaxy NGC 6822. Its PN and HII region abundances

Abstract

We aim to derive the chemical behavior of a significant sample of PNe and HII regions in the irregular galaxy NGC 6822 The selected objects are distributed in different zones of the galaxy. Due to the faintness of PNe and HII regions in NGC 6822, to gather spectroscopic data with large telescopes is necessary. We obtained a well suited sample of spectra by employing VLT-FORS 2 and Gemini-GMOS spectrographs. Ionic and total abundances are calculated for the objects where electron temperatures can be determined through the detection of [OIII] \lambda 4363 or/and [NII] \lambda 5755 lines. A "simple" chemical evolution model has been developed and the observed data are used to compute a model for NGC 6822 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 2 kpc of the galaxy, showing a value 12+log O/H = 8.06$\pm$0.04. From the abundance pattern of PNe, we identified two populations: a group of young PNe with abundances similar to HII regions and a group of older objects with abundances a factor of two lower. A couple of extreme Type I PNe were found. No third dredge-up O enrichement was detected in PNe of this galaxy. The abundance determinations allow us to discuss the chemical behavior of the present and past ISM in NGC 6822. Our preliminary chemical evolution model predicts that an important gas-mass lost occurred during the first 5.3 Gyr, that no star higher than 40 M$_\odot$ was formed, and that 1% of all 3-15 M$_\odot$ stars became binary systems progenitors to SNIa.Comment: 15 pages, 3 figures and 4 tables. A&A, Accepted 13/06/200