Atomic physics underlying the spectroscopic analysis of massive stars and supernovae

Claudio Mendoza (IVIC/CeCalCULA)

John Hillier

Gary Ferland has kindly sent me the preprint arXiv:1101.5372v1 (astro-ph.HE) of the paper “The atomic physics underlying the spectroscopic analysis of massive stars and supernovae” by John Hillier. This paper was presented at the 8th International Conference on High Energy Density Laboratory Astrophysics (HEDLA 2010) held in Caltech last 15-18 March 2010. Hillier gives an account of the CMFGEN spectroscopic tool for the spectral modeling of massive stars and supernovae (SNe) which enables a time-dependent treatment of radiative transfer. The crucial importance of including all the relevant atomic processes and accurate atomic data for elements with Z  ≤ 28 in order to master the NLTE regime is discussed, particularly the deterrents brought about by the poor quality of the atomic parameters for some of the iron-group elements and the incomplete study of charge-exchange.

The CMFGEN code computes stellar parameters and abundances, allows a more formal treatment of the hydrodynamics of the stellar wind and provides radiation fields for nebular photoionization calculations, SNe diagnostics and fundamental data for further studies of starbursts and star formation in galaxies. Distances to Type II SNe may also be estimated by the expanding photosphere method. Since the LTE approach is not applicable in O stars, W-R stars and SNe, the statistical equations must be solved including all the processes that populate and depopulate the ionic states. In CMFGEN the following processes are taken into account:

  • Photoionization and radiative recombination
  • Low and high temperature dielectronic recombination
  • Bound-bound transitions
  • Electron collisional excitation and de-excitation
  • Collisional ionization and recombination
  • Auger ionization by high-energy photons
  • Charge exchange
  • Two-photon emission

In most situations, electron velocities are assumed to display a Mawellian distribution, but in SNe, collisional ionization and excitation with non-thermal, high-energy electrons must be considered.

Hillier goes over the difficulties in rendering the line formation process in NLTE spectroscopic analyses in order to obtain chemical abundances, and how LS-coupling gf-values are not really sufficient.  Also the importance of resonance positions in photoionization cross sections to determine accurate low-temperature recombination rates is emphasized, apart from the unavailability of cross sections for important iron-group elements such as Sc, Co and Ni. Particularly critical data sets are those containing collisional rates, which are currently poor for transitions involving high-lying levels, and charge exchange reactions with species different from H and He.

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