CENTRAL STAR TEMPERATURES OF LOW-EXCITATION PLANETARY-NEBULAE

We correlate Kaler's Stoy (energy balance) and new Zanstra temperatures of the central stars of low-excitation planetary nebulae with the intensity of [O III] lambda-5007, where all objects are small (young) and have no or weak He II lambda-4686 (intensity < 10). More recent energy balance temperatures are shown to be unsatisfactory for this class of objects. We find T(S) = 25,840 + 19.30I(lambda-5007) + 2.8229 x 10(-2) I2(lambda-5007) - 1.46115 x 10(-5) I3(lambda-5007), T(Z)(H) = 30,370 + 14.26I(lambda-5007), and T(Z) = 28,380 + 26.13I(lambda-5007), where for the last we substitute T(Z)(He II) for T(Z)(H) in cases where He II is detected. Comparisons among the temperatures show that planetaries may start becoming optically thin for central star temperatures in the 40,000 K-50,000 K range and that the Zanstra discrepancy for this class of objects may be resolved by invoking low optical depth; an excess in the stellar He+ Lyman continuum may also provide a contribution. The Stoy calibration seems to give the best estimate of central star temperatures from the lambda-5007 line. The correlations are valuable for determining effective temperatures in cases where no nuclei can be detected and are especially useful for extragalactic objects.