2003MNRAS.340..253W

We have analysed optical spectra of two of the hydrogen-deficient knots (J1 and J3) in the born-again planetary nebula Abell 30, together with ultraviolet (UV) spectra of knots J3 and J4. We determine electron temperatures in the knots based on several diagnostics. The [OIII] nebular-to-auroral transition ratio yields temperatures of the order of 17000 K. The weak temperature-dependence of the ratios of helium lines λ4471, λ5876 and λ6678 is used to derive a temperature of 8850 K for knot J3 and 4600 K for knot J1. Ratios of OII recombination lines, which directly measure the temperature in the coldest regions of the knots, are used to derive temperatures of 2500 K for knot J3, and just 500 K for knot J1. We calculate abundances both from collisionally excited lines and from the well-observed recombination spectra of C, N, O and Ne ions. The forbidden line abundances agree well with previous determinations, but the recombination line abundances are several hundred times higher. These results confirm the scenario proposed by Harrington & Feibelman, in which the knots contain a cold core highly enriched in heavy elements. Forbidden lines are almost entirely emitted by the hot outer part of the knot, while recombination lines are emitted predominantly from the cold core. The C/O ratios we derive imply that the knots are oxygen-rich, contrary to theoretical predictions for born-again nebulae.