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

SBS 1150+599A is a blue stellar object at high Galactic latitude discovered in the Second Byurakan Survey. New high-resolution images of SBS 1150+599A are presented, demonstrating that it is very likely to be an old planetary nebula in the galactic halo, as suggested recently by Tovmassian et al. An Hα image taken with the WIYN 3.5 m telescope and its tip-tilt module reveals the diameter of the nebula to be 9.″2, comparable to that estimated from spectra by Tovmassian et al. Lower limits to the central star temperature were derived using the Zanstra hydrogen and helium methods to determine that the star's effective temperature must be higher that 68,000 K and that the nebula is optically thin. New spectra from the Multiple Mirror Telescope and Fred L. Whipple Observatory telescope are presented, revealing the presence of strong [Ne V] λ3425 lines, indicating that the central star temperature must be higher than 100,000 K. With the revised diameter, new central star temperature, and an improved central star luminosity, we can constrain photoionization models for the nebula significantly better than before. Because the emission-line data set is sparse, the models are still not conclusive. Nevertheless, we confirm that this nebula is an extremely metal-poor planetary nebula, having a value for O/H that is less than 1/100 solar, possibly as low as 1/500 solar.