THE CHEMICAL ABUNDANCES OF THE HYDROGEN-POOR PLANETARY-NEBULAE A30 AND A58

We present new CCD narrowband images and spatially resolved intermedia te-dispersion spectroscopy of the hydrogen-poor nebulae A30 and A58. T hese data have allowed us to obtain information on the physical proper ties and chemical abundances of the inner knots and outer envelopes of A30 and A58 as a basis for the discussion of their implications in th e evolutionary theories. In particular, hydrogen emission from the cen tral knots has been carefully measured, and, consequently, the rates o f conversion of hydrogen into helium have been established with accura cy. The rates found indicate that almost all the original hydrogen (be tween 75% and 95%) has been burned into helium. The chemical segregati on among the knots of A30 previously suggested by Jacoby & Ford is con firmed. The A58 inner knot, which is spatially resolved in two differe nt components, shows evidence of excitation by shocks. Helium abundanc es and the N/O ratio in the outer envelope of this nebula are very hig h, therefore indicating that the nebula formed as the result of the ev olution of an intermediate-mass star. An analysis of the chemical abun dances, including other objects of the same class, gives some hints th at He/H and O/H are anticorrelated. The time elapsed between the forma tion of the outer envelopes and the ejection of the central knots also seems to be related to the central knots' chemical abundances.