Physical conditions and chemical abundances of the variable planetary nebula IC 4997

Abstract

The planetary nebula (PN) IC 4997 is one of a few rapidly evolving objects with variable brightness and nebular emission around a hydrogen-deficient star. In this study, we have determined the physical conditions and chemical abundances of this object using the collisionally excited lines (CELs) and optical recombination lines (ORLs) measured from the medium-resolution spectra taken in 2014 July with the FIbre-fed Échelle Spectrograph on the Nordic Optical Telescope at La Palma Observatory. We derived electron densities of ≳ 3 × 104 cm-3 and electron temperatures of ≳ 14 000 K from CELs, whereas cooler temperatures of ~11 000 and ~7000 K were obtained from helium and heavy element ORLs, respectively. The elemental abundances deduced from CELs point to a metal-poor progenitor with [O/H] ≲ -0.75, whereas the ORL abundances are slightly above the solar metallicity, [O/H] ≍ 0.15. Our abundance analysis indicates that the abundance discrepancy factors (ADFs ≡ ORLs/CELs) of this PN are relatively large: ADF(O2+) ≳ 8 and ADF(N2+) ≳ 7. Further research is needed to find out how the ADFs and variable emissions are formed in this object and whether they are associated with a binary companion or a very late thermal pulse.