Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons

David C. Nicholls (1), Michael A. Dopita (1,2) & Ralph S. Sutherland (1) ((1) Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd., Weston ACT 2611, Australia; (2) Astronomy Department, King Abdulaziz University, P.O.  Box 80203, Jeddah, Saudi Arabia)

The measurement of electron temperatures and metallicities in H II regions and Planetary Nebulae (PNe) has — for several decades — presented a problem: results obtained using different techniques disagree. What it worse, they disagree consistently. There have been numerous attempts to explain these discrepancies, but none has provided a satisfactory solution to the problem. In this paper, we explore the possibility that electrons in H II regions and PNe depart from a Maxwell-Boltzmann equilibrium energy distribution. We adopt a “kappa-distribution” for the electron energies. Such distributions are widely found in Solar System plasmas, where they can be directly measured. This simple assumption is able to explain the temperature and metallicity discrepancies in H II regions and PNe arising from the different measurement techniques. We find that the energy distribution does not need to depart dramatically from an equilibrium distribution. From an examination of data from H II regions and PNe it appears that kappa ~10 is sufficient to encompass nearly all objects. We argue that the kappa-distribution offers an important new insight into the physics of gaseous nebulae, both in the Milky Way and elsewhere, and one that promises significantly more accurate estimates of temperature and metallicity in these regions.

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2 Responses to Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons

  1. Grazyna Stasinska says:

    Very nice work!
    something really new and promising in the physics of ionized nebulae and abundance derivations. I look forward to seeing further developments of this – and there is a lot to do on many fronts!
    This means that perhaps atomic physicists should perhaps also provide effective collision strengths for such kappa electron distributions…

    In the paper it says: “…the recombination temperature, TB, is appreciably lower than the kinetic temperature TU” : how much do you think that this would affect nebular abundances derived from recombination lines ?
    I also wonder how much this would affect nebular cooling (by recombination, collisional excitation of fine structure lines and of optical and ultraviolet lines such as H Ly alpha)

    Best wishes,

    Grazyna Stasinska

  2. David Nicholls says:

    We are in the early stages of exploring the implications of the effects of kappa distributions, and are in the middle of re-vamping the Mappings III photoionization modelling code to take non-equilibrium effects into account. We hope to have the answers soon, and to publish initial results. However, it is already clear that recombination temperatures are systematically lowered, which means that recombination line abundances are most likely overestimated. The [OIII] Te is systematically raised, which means that the Te method delivers systematically lower abundances. The Strong Line methods are affected at the 10-30% level but UV lines will be much stronger than in standard models. All of this offers great promise in solving the abundance discrepancies between the different techniques.

    David Nicholls & Mike Dopita

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