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G. Jacoby (WIYN, Inc), G. J. Ferland (University of Kentucky), K. T. Korista (Western Michigan University)
The large planetary nebula, Abell 39, appears on the sky as a beautiful, nearly perfect spherical shell, and photometric measurements support that interpretation (see: http://www.noao.edu/jacoby/a39hires.jpg). Its very simple geometry offers a special opportunity to test nebular photoionization models with one of the usual uncertainties removed.
Based on imaging in 0.7" seeing at [OIII] 5007 with the WIYN 3.5-m telescope, it has a diameter of 155", as defined by the peak intensity point on the rim, and is circular to within 0.5". The shell thickness that matches the observations best, when modeled with a thin spherical shell projected onto the sky, is 10". A faint, high excitation halo exists outside the main shell, extending the diameter to 190".
Optical spectra obtained at the KPNO Mayall 4-m telescope, combined with UV spectra from the Hubble Space Telescope and STIS, were used to constrain photoionization models (CLOUDY 94.01) to derive the parameters of the nebula and its central star. The star has Teff = 150,000K and a mass ~.6 solar masses. From the photoionization models, the nebula is found to be a factor of 3-4 lower than Solar in metallicity. This result is not unusual by itself, but it does serve as a caution for abundance determinations using the ``ionization correction factor'' (ICF) method, which erroneously predicts a nearly normal abundance. The discrepancy arises from the uniformly high ionization state of helium throughout the nebula; helium usually serves as an indicator of ionization states, but in this case, offers little leverage.
Abell 39 presents a number of additional intriguing puzzles that will be discussed. This research was supported in part by NASA grant GO-07284.01-96a.
The author(s) of this abstract have provided an email address for comments about the abstract: jacoby@noao.edu