@article{discovery10095566,
          number = {4},
            year = {2006},
           month = {June},
         journal = {Monthly Notices of the Royal Astronomical Society},
       publisher = {WILEY-BLACKWELL},
            note = {This version is the version of record. For information on re-use, please refer to the publisher's terms and conditions.},
           title = {Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines},
           pages = {1959--1970},
          volume = {368},
          author = {Liu, X-W and Barlow, MJ and Zhang, Y and Bastin, RJ and Storey, PJ},
        keywords = {ISM: abundances, planetary nebulae: individual: Hf 2-2},
        abstract = {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 {$\sim$}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 II{\ensuremath{\lambda}}4089/{\ensuremath{\lambda}}4649 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 {$\sim$}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.},
            issn = {0035-8711},
             url = {https://doi.org/10.1111/j.1365-2966.2006.10283.x}
}