The planetary nebula A39: An observational benchmark for numerical modeling of photoionized plasmas

Galactic nebulae are the main probes for the chemical evolution of the inte rstellar medium. Yet, recent observations have shown that chemical abundanc es determined from recombination and collisionally excited emission lines c an differ by as much as an order of magnitude in some planetary nebulae (PN s). Many PNs have complex geometries and morphological evidence for interac tions from stellar winds, and it is not clear to what extent winds, inhomog eneities, or shocked gas affect the observed spectrum. There currently is n o full explanation for this discrepancy, which brings into question whether we understand the physical state of these low-density plasmas at all. This paper presents new spectroscopy from the KPNO Mayall 4 m telescope and ima gery from the WIYN 3.5 m telescope of A39, a large PN with an exceptionally simple geometry. It appears to be a limb-brightened spherical shell, the s implest possible nebula. There is little evidence for external interactions , so this is the case in which photoionization simulations should be in nea r-perfect agreement with observation. We combine optical and UV spectroscop y to form a composite spectrum and compare this with photoionization models . No problems were encountered in reproducing the observed spectrum, althou gh even this simple object has two distinct emission-line regions and exhib its several anomalies. A39 was too faint to detect the crucial heavy-elemen t recombination lines in our data set, so it was not possible to compare co llisional and recombination abundances. We predict the spectrum over a broa d range of bandpasses to facilitate future observations that may probe deep er than our instrumentation allowed.