2001ApJ...562..515D

We have identified a line at 3.625 µm in the planetary nebulae (PNs) NGC 7027 and IC 5117 as the ²D_3/2-²D_5/2 fine-structure transition of the 3d⁹ ion Zn⁺³. Zinc (N=30) is believed to be made primarily by explosive nucleosynthesis and r-process neutron capture reactions, and because Zn/H closely tracks Fe/H in metal-poor stars and is relatively undepleted into dust in the ISM, Zn/H has been used as a surrogate for Fe/H in the high-redshift universe. However, some isotopes of Zn are also produced as part of the s-process in asymptotic giant branch stars, the progenitors of PNs. Although there is currently no published calculation of the collision strength for [Zn IV] 3.625 µm, for plausible values the observed line fluxes do not require Zn/H greater than solar, suggesting that these two PNs, which have elevated abundances of other s-process elements and C, are not significantly self-enriched in Zn. Consequently, Zn/H determined from emission lines should be a good indicator of elemental Fe/H in PNs, where gas-phase Fe/H values cannot be used directly because of depletion effects, enabling comparisons between nebular and stellar Fe/H abundances. We also report an upper limit on the strength of the 1-0 P(2) line of HeH⁺ at 3.608 µm in NGC 7027.