The carbon-poor halo planetary nebula DDDM-1.

Abstract

The authors have analysed ultraviolet and optical spectra of DDDM-1, a planetary nebula in the galactic halo. Abundances of N, O, S, Ne and Ar are all about a factor 6 lower than solar, while Fe and Mg are deficient by a further factor of 3. No carbon lines were seen in any spectra, and the authors obtain the upper limit C/H < 1.4×10^-5(C/O < 0.1). The relative strengths of several He I and Fe III nebular lines are compared with predicted strengths. A photoionization model for the nebula is presented. For the central star the authors find T_eff = 38000 (±6000)K. It is suggested that the progenitor of DDDM-1 had a sufficiently low core mass, when on the asymptotic giant branch, that few or no thermal pulses occurred before ejection of the nebula, so that the carbon abundance has not been enhanced. The authors suggest that the progenitor star formed from a fragment of the primitive galactic halo. A comparison is made between the composition of halo planetary nebulae and of old, metal-poor stars in the Galaxy. It is suggested that the giant CH stars are the progenitors of many of the halo planetaries.