THE HALO PLANETARY-NEBULAE M 2-29 AND BB-1

Abstract

Optical spectrophotometric data are presented for the Population II planetary nebulae (PN) M 2-29 and BB-1. For M 2-29 we derived T(e) (O III) = 24,000 K N(e) (S II) = 3000 cm-3, and chemical abundance of 12 + log He/H = 10.97, 12 + log N/H = 6.98, 12 + log O/H = 7.31, 12 + log Ne/H = 6.72, 12 + log Ar/H = 5.26, and 12 + log S/H = 5.91. For BB-1, we derived T(e) (O III) = 14,500 K, N(e) (S II) = 3000 cm-3, and a chemical composition of 12 + log He/H = 11.02, 12 + log C/H = 9.16, 12 + log N/H = 7.94, 12 + log O/H = 7.68, 12 + log Ne/H = 7.76, 12 + log Ar/H = 4.74, and 12 + log S/H = 5.80. For this object we also analyze available IUE spectra. The chemical composition of these objects is compared to that of the known Type IV PN. It is concluded that M 2-29 shows the lowest O/H ratio in PNs and that it belongs to the Ar- and S-rich Type IV PN group, while BB-1 is very C-, N-, and Ne-rich, and presents very low Ar and S abundances. Evidence supporting that O and Ne excesses in PN are produced by the progenitor stars is presented. The He+/H+ ratios for both objects were derived by taking into account the effect of the collisional excitation of the metastable 2(3)S level of He0. It is found that the proposed correction schemes seem to overestimate the correction for the lambda-5876 transition.