Testing the Existence of Non-Maxwellian Electron Distributions in H II Regions after Assessing Atomic Data Accuracy
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2014
- DOI:
- 10.1088/0004-637X/785/2/91
- arXiv:
- arXiv:1402.4044
- Bibcode:
- 2014ApJ...785...91M
- Keywords:
-
- atomic data;
- atomic processes;
- H II regions;
- Astrophysics - Instrumentation and Methods for Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 44 pages, 9 figures, accepted for publication in the Astrophysical Journal