Confirmation of SBS 1150+599A as an Extremely Metal-poor Planetary Nebula

SBS 1150+599A is a blue stellar object at high galactic latitude discoveredin the Second Byurakan Survey. New high-resolution images of SBS 1150+599A arepresented, demonstrating that it is very likely to be an old planetary nebulain the galactic halo, as suggested by Tovmassian et al (2001). An H-alpha imagetaken with the WIYN 3.5-m telescope and its "tip/tilt" module reveals thediameter of the nebula to be 9.2", comparable to that estimated from spectra byTovmassian et al. Lower limits to the central star temperature were derivedusing the Zanstra hydrogen and helium methods to determine that the star'seffective temperature must be > 68,000K and that the nebula is optically thin.New spectra from the MMT and FLWO telescopes are presented, revealing thepresence of strong [Ne V] lambda 3425, indicating that the central startemperature must be > 100,000K. With the revised diameter, new central startemperature, and an improved central star luminosity, we can constrainphotoionization models for the nebula significantly better than before. Becausethe emission-line data set is sparse, the models are still not conclusive.Nevertheless, we confirm that this nebula is an extremely metal-poor planetarynebula, having a value for O/H that is less than 1/100 solar, and possibly aslow as 1/500 solar.Comment: 19 pages, 6 figures. Accepted for publication in the Astronomical Journa