Abstract
The continuum spectrum of the planetary nebula Hf 2-2 close to the Balmer discontinuity is modeled in the context of the long standing problem of the abundance and temperature discrepancy found when analyzing optical recombination lines and collisionally excited forbidden lines in nebulae. Models are constructed using single and double Maxwell-Boltzmann distributions as well as kappa-distributions for the energies of the free electrons. New results for the necessary continuum and line emission coefficients are presented calculated with kappa-distributed energies. The best fit to the observed continuum spectrum is found to be a model comprising two components with dramatically different temperatures and with a Maxwell-Boltzmann distribution of electron energies. On the basis of a chi-squared analysis, this model is strongly favored over a model with kappa-distributed electron energies.
Highlights
The long standing problem in nebular physics related to the abundance and temperature discrepancy between the results obtained from optical recombination lines (ORL) and collisionally excited lines (CEL) has been visited by many researchers in the last few decades
One explanation is a multi-component model of planetary nebulae in which low temperature, high metallicity components produce the ORL and are embedded in a high temperature, relatively low metallicity, hydrogen-rich component that produces the CEL (Liu et al 2000)
The analysis of the Balmer continuum spectrum of Hf 2-2 seems to indicate the signature of a two-component nebula with two different temperatures
Summary
The long standing problem in nebular physics related to the abundance and temperature discrepancy between the results obtained from optical recombination lines (ORL) and collisionally excited lines (CEL) has been visited by many researchers in the last few decades. One explanation is a multi-component model of planetary nebulae in which low temperature, high metallicity components (clumps) produce the ORL and are embedded in a high temperature, relatively low metallicity, hydrogen-rich component that produces the CEL (Liu et al 2000) It has been suggested recently (Nicholls et al 2012) that this discrepancy may be largely explained by the assumption of a different electron energy distribution, a κ-distribution, rather than the Maxwell-Boltzmann (MB) distribution which is tradition-. The debate about the electron energy distribution in astronomical objects, including planetary nebulae, is relatively old and dates back to at least the 1940s when Hagihara Hagihara (1944) suggested that the distribution of free electrons in gaseous assemblies deviates considerably from the MB distribution This was refuted by Bohm & Aller (1947) who, on the basis of a detailed quantitative balance analysis, concluded that any deviation from the Maxwellian equilibrium distribution is very small.
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