TESTING THE EXISTENCE OF NON-MAXWELLIAN ELECTRON DISTRIBUTIONS IN HII REGIONS AFTER ASSESSING ATOMIC DATA ACCURACY

The classic optical nebular diagnostics [NII], [OII], [OII], [SII], [SIII], and [ArIII] are employed to search for evidence of non-Maxwellian electron distributions, namely kappa distributions, in a sample of well-observed Galactic HII regions. By computing new effective collision strengths for all these systems and A-values when necessary (e. g., SII), and by comparing with previous collisional and radiative data sets, we have been able to obtain realistic estimates of the electron-temperature dispersion caused by the atomic data, which in most cases are not larger than similar to 10%. If the uncertainties due to both observation and atomic data are then taken into account, it is plausible to determine for some nebulae a representative average temperature while in others there are at least two plasma excitation regions. For the latter, it is found that the diagnostic temperature differences in the high-excitation region, e.g., T-e(OIII), Te(SIII), and Te(ArIII), cannot be conciliated by invoking kappa distributions. For the low-excitation region, it is possible in some, but not all, cases to arrive at a common, lower temperature for [NII], [OII], and [SII] with kappa approximate to 10, which would then lead to significant abundance enhancements for these ions. An analytic formula is proposed to generate accurate kappa-averaged excitation rate coefficients (better than 10% for kappa >= 5) from temperature tabulations of the Maxwell-Boltzmann effective collision strengths.