Central star temperatures of low-excitation planetary nebulae

Abstract

New Zanstra temperatures of the central stars or low-excitation PNe and Kaler's (1987) Stoy method for determining the T(e)s of such stars are correlated here with the intensity of forbidden O III 5007 A for young PNE with no or weak He II 4686 A. Recent energy balance temperatures are shown to be unsatisfactory for this class of objects. Comparisons among the temperatures in cases were He II is detected show that PNe may start becoming optically thin for central star temperatures in the 40,000-50,000 K range and that the Zanstra discrepancy for this class of objects may be resolved by invoking low optical depth. The Stoy calibration seems to give the best estimate of central star temperatures from the 5007 A line. The correlations are valuable for determining effective temperatures in cases where no nuclei can be detected and are especially useful for extragalactic objects.