Chemical composition of planetary nebulae in the Large and Small Magellanic Clouds

The technique of nebular-gas diagnostics was used to find electronic temperatures T _e , concentrations n _e , relative ion concentrations n(A⁺ⁱ )/n(H⁺), and chemical abundances A/H for planetary nebulae in the Large and Small Magellanic Clouds. This analysis took into account inhomogeneities of the nebular-gas density in the nebulae. We determined the pre-galactic helium abundance Y _p and its rate of enrichment dY/dZ for the envelopes of nine nebulae in the Large Magellanic Cloud. Taken together with the Galaxy’s planetary nebulae and HII regions in blue compact dwarf galaxies, Y _p = 0.248 ± 0.002, dY/dZ = 2.31 ± 0.48 and Y _p = 0.247 ± 0.002, dY/dZ = 3.03 ± 0.56, respectively, when macroinhomogeneities and macro/microinhomogeneities of the gas density in galactic nebulae are taken into account.