TY  - JOUR
TI  - Chemical abundances for Hf 2-2, a planetary nebula with the strongest-known heavy-element recombination lines
AV  - public
Y1  - 2006/06//
EP  - 1970
N2  - 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 II?4089/?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 ?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.
ID  - discovery10095566
PB  - WILEY-BLACKWELL
KW  - ISM: abundances
KW  -  planetary nebulae: individual: Hf 2-2
SP  - 1959
VL  - 368
IS  - 4
N1  - This version is the version of record. For information on re-use, please refer to the publisher?s terms and conditions.
UR  - https://doi.org/10.1111/j.1365-2966.2006.10283.x
SN  - 0035-8711
JF  - Monthly Notices of the Royal Astronomical Society
A1  - Liu, X-W
A1  - Barlow, MJ
A1  - Zhang, Y
A1  - Bastin, RJ
A1  - Storey, PJ
ER  -