Central star temperatures of low-excitation planetary nebulae

We correlate Kaler's Stoy (energy balance) and new Zanstra temperatures of the central stars of lowexcitation planetary nebulae with the intensity of [O III] λ5007, where all objects are small (young) and have no or weak He II λ4686 (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(λ5007) + 2.8229 × 10^-2 I²(λ5007)-1.46115 × 10^-5I³(λ5007), T_z(H) = 30,370 + 14.26I(λ5007), and T_z. = 28,380 + 26.13I(λ5007), 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 λ5007 line. The correlations are valuable for determining effective temperatures in cases where no nuclei can be detected and are especially useful for extragalactic objects.