Testing the existence of non-Maxwellian electron distributions in H II regions after assessing atomic data accuracy

C. Mendoza(1,2), M. A. Bautista (1)

(1) Department of Physics, Western Michigan University, Kalamazoo, MI 49008, USA, (2) Centro de Física, Instituto Venezolano de Investigaciones Científicas (IVIC), PO Box 20632, Caracas 1020A, Venezuela.

The classic optical nebular diagnostics [N II], [O II], [O III], [S II], [S III], and [Ar III] are employed to search for evidence of non-Maxwellian electron distributions, namely κ distributions, in a sample of well-observed Galactic H II regions. By computing new effective collision strengths for all these systems and A-values when necessary (e.g., S II), 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 ~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., Te (O III), Te (S III), and Te (Ar III), cannot be conciliated by invoking κ distributions. For the low-excitation region, it is possible in some, but not all, cases to arrive at a common, lower temperature for [N II], [O II], and [S II] with  κ ≈ 10, which would then lead to significant abundance enhancements for these ions. An analytic formula is proposed to generate accurate  κ-averaged excitation rate coefficients (better than 10% for  κ ≥ 5) from temperature tabulations of the Maxwell-Boltzmann effective collision strengths.

See complete preprint –> http://arxiv.org/abs/1402.4044

 

 

 

 

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