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

Abstract

We have obtained the first high resolution spectra of individual stars in the dwarf irregular galaxy, NGC 6822. The spectra of the two A-type supergiants were obtained at the VLT and Keck Observatories, using UVES and HIRES, respectively. A detailed model atmospheres analysis has been used to determine their atmospheric parameters and elemental abundances. The mean iron abundance from these two stars is $<$[Fe/H]$>=-$0.49 $\pm$0.22 ({\it $\pm$0.21}, with Cr yielding a similar underabundance, $<$[Cr/H]$>=-$0.50 $\pm$0.20 ({\it $\pm$0.16}). This confirms that NGC 6822 has a metallicity that is slightly higher than that of the SMC, and is the first determination of the present-day iron-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 the nebular 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 similar to that seen in the Magellanic Clouds, which supports the picture that chemical evolution occurs more slowly in these lower mass galaxies, although the O/Fe ratio is also consistent with that observed in comparatively metal-poor stars in the Galactic disk. Combining all of the available abundance observations for NGC 6822 shows that there is no trend in abundance with galactocentric distance. However, a subset of the highest quality data are consistent with a radial abundance gradient.