%0 Journal Article
%@ 0035-8711
%A Liu, X-W
%A Barlow, MJ
%A Zhang, Y
%A Bastin, RJ
%A Storey, PJ
%D 2006
%F discovery:10095566
%I WILEY-BLACKWELL
%J Monthly Notices of the Royal Astronomical Society
%K ISM: abundances, planetary nebulae: individual: Hf 2-2
%N 4
%P 1959-1970
%T Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines
%U https://discovery.ucl.ac.uk/id/eprint/10095566/
%V 368
%X We present high-quality optical spectroscopic observations of the planetary nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination lines (ORLs) from heavy-element ions. Analysis of the H I and He I recombination spectrum yields an electron temperature of ∼900 K, a factor of 10 lower than given by the collisionally excited [O III] forbidden lines. The ionic abundances of heavy elements relative to hydrogen derived from ORLs are about a factor of 70 higher than those deduced from collisionally excited lines (CELs) from the same ions, the largest abundance discrepancy factor (adf) ever measured for a PN. By comparing the observed O IIλ4089/λ4649 ORL ratio to theoretical value as a function of electron temperature, we show that the O II ORLs arise from ionized regions with an electron temperature of only ∼630 K. The current observations thus provide the strongest evidence that the nebula contains another previously unknown component of cold, high-metallicity gas, which is too cool to excite any significant optical or ultraviolet CELs and is thus invisible via such lines. The existence of such a plasma component in PNe provides a natural solution to the long-standing dichotomy between nebular plasma diagnostics and abundance determinations using CELs on the one hand and ORLs on the other.
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