First Stellar Abundances in NGC 6822 from VLT‐UVES and Keck‐HIRES Spectroscopy

We have obtained the first high resolution spectra of individual stars in thedwarf irregular galaxy, NGC 6822. The spectra of the two A-type supergiantswere obtained at the VLT and Keck Observatories, using UVES and HIRES,respectively. A detailed model atmospheres analysis has been used to determinetheir atmospheric parameters and elemental abundances. The mean iron abundancefrom these two stars is $<$[Fe/H]$>=-$0.49 $\pm$0.22 ({\it $\pm$0.21}, with Cryielding a similar underabundance, $<$[Cr/H]$>=-$0.50 $\pm$0.20 ({\it$\pm$0.16}). This confirms that NGC 6822 has a metallicity that is slightlyhigher than that of the SMC, and is the first determination of the present-dayiron-group abundances in NGC 6822. The mean stellar oxygen abundance,12+log(O/H)=8.36 $\pm$0.19 ({\it $\pm$0.21}),is in good agreement with thenebular oxygen results. Oxygen has the same underabundance as iron,$<$[O/Fe]$>$=+0.02 $\pm$0.20 ({\it $\pm$0.21}). This O/Fe ratio is very similarto that seen in the Magellanic Clouds, which supports the picture that chemicalevolution occurs more slowly in these lower mass galaxies, although the O/Feratio is also consistent with that observed in comparatively metal-poor starsin the Galactic disk. Combining all of the available abundance observations forNGC 6822 shows that there is no trend in abundance with galactocentricdistance. However, a subset of the highest quality data are consistent with aradial abundance gradient.Comment: Accepted by ApJ (Sept 13, 2000) 13 pages, 7 tables, 8 figure