Abundance discrepancy factors in high-density planetary nebulae

Abstract

ABSTRACT From high-resolution spectra, chemical abundances from collisionally excited lines (CELs) and optical recombination lines (ORLs) have been determined for planetary nebulae (PNe) Cn 3-1, Vy 2-2, Hu 2-1, Vy 1-2 and IC 4997, which are young and dense objects. The main aim of this work is to derive their O+2/H+ abundance discrepancy factors (ADFs) between CELs and ORLs. He, O, N, Ne, Ar, S, and Cl abundances were obtained and our values are in agreement with those previously reported. We found that Cn 3-1, Hu 2-1, and Vy 1-2 have O abundances typical of disc PNe, while Vy 2-2 and IC 4997 are low O abundance objects ($\rm {12+log(O/H) \sim 8.2}$), which can be attributed to possible O depletion into dust grains. ADFs(O+2) of $4.30^{+1.00}_{-1.16}$, 1.85 ± 1.05, $5.34^{+1.27}_{-1.08}$ and $4.87^{+4.34}_{-2.71}$ were determined for Vy 2-2, Hu 2-1, Vy 1-2, and IC 4997, respectively. The kinematics of CELs and ORLs was analysed for each case to study the possibility that different coexisting plasmas in the nebula emit them. Expansion velocities of [O iii] and O ii are equal within uncertainties in three PNe, providing no evidence for these lines being emitted in different zones. Exceptions are Hu 2-1 and Vy 2-2, where ORLs might be emitted in different zones than CELs. For Vy 2-2 and IC 4997, we found that nebular and auroral lines of the same ion (S+, N+, Ar+2, Ar+3, O+2) might present different expansion velocities. Auroral lines show lower $\rm {V_{exp}}$, which might indicate that they are emitted in a denser and inner zone than the nebular ones.