Chemical abundances of planetary nebulae in the Sagittarius dwarf elliptical galaxy

Spectrophotometry and imaging of the two planetary nebulae He 2-436 and Wray 16-423, recently discovered to be in the Sagittarius dwarf elliptical galaxy, are presented. Wray 16-423 is a high-excitation planetary nebula (PN) with a hot central star. In contrast, He 2-436 is a high-density nebula with a cooler central star and evidence of local dust, the extinction of which exceeds that for Wray 16-423 by EB-V = 0.28. The extinction to Wray 16-423 (EB-V = 0.14) is consistent with the line-of-sight extinction to the Sagittarius Dwarf. Both PNs show Wolf-Rayet features in their spectra, although the lines are weak in Wray 16-423. Images in [O III] and H alpha + [N II], although affected by poor seeing, yield a diameter of 1.'' 2 for Wray 16-423 after deconvolution, while He 2-436 was unresolved. He 2-436 has a luminosity about twice that of Wray 16-423, and its size and high density suggest a younger nebula. In order to reconcile the differing luminosity and nebular properties of the two nebulae with similar-age progenitor stars, it is suggested that they are on He-burning tracks An abundance analysis is presented for both PNs using empirical abundance determinations. The abundance pattern is very similar in both nebulae, and both show an oxygen depletion of -0.4 dex with respect to the mean oxygen abundance of Galactic planetary nebulae and [O/H] = -0.6. The Sagittarius PN progenitor stars are representative of the higher metallicity tail of the Sagittarius population. The pattern of abundance depletion is similar to that in the only other planetary nebula in a dwarf galaxy companion of the Milky Way, that in Fornax, for which new spectra are presented. However, the abundances are larger than for Galactic halo PNs, suggesting a later formation age. The oxygen abundance of the Sagittarius galaxy deduced from its PN shows similarities with that of dwarf ellipticals around M31, advancing the notion that this galaxy was a dwarf elliptical before its interaction with the Milky Way.