Up: Ionization balance for optically Ni
Subsections
To describe the ionization processes we refer to the work of Arnaud &
Rothenflug (1985) and to the updating for the Fe ions of Arnaud & Raymond
(1992) (hereafter AR85 and AR respectively).
The contributions to the ionization rates^{}
are given for all the ions of
H, He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca and Ni
from different atomic subshells separately.
The direct ionization rate coefficients
versus temperature T are given by:
 
(1) 

F(x_{j})= A_{j}[1x_{j}f_{1}(x_{j})]+

(2) 
+bj[1+x_{j}x_{j}(2+x_{j})f_{1}(x_{j})]
C_{j}f_{1}(x_{j})+D_{j}x_{j}f_{2}(x_{j}).
The summation is performed over the subshells j of the ionizing ion with
 
(3) 
and where kT and I_{j} are in eV.
The values parameters A_{j}, B_{j},
C_{j} and D_{j} are given in AR85 and AR.
Following AR85 and AR we also take into
to account the excitationautoionization (hereafter EA) contribution
of ions in the ground state. This is a good approximation for lowdensity
plasmas
(as in supernova remnants or clusters of galaxies) because the lifetime of
excited states is small as compared to the mean collision time.
The other ions not included in the AR85 work are calculated
by interpolation
or extrapolation along the isosequence.
For the radiative recombination rates^{1}
the calculations of
Shull & Van Steenberg (1982) (hereafter SV) were used for some of the most
abundant astrophysical elements (Mg, Si, S, Ar, Ca, Fe,
Ni).
They give the fitting parameters A and
for the following formula:
 
(4) 
with the electron temperature T in eV.
LM extrapolated these calculations also to other astrophysical less
abundant elements.
For Fe XVFe XXIV we used the formula of
AR:
 
(5) 
where the electron temperature T is in Kelvin and
the fitting parameters are given by AR
in tabular form.
For the Hlike, Helike, Lilike and Nalike isosequences we used the
new calculations, in the framework of the
opacity project, of Verner & Ferland (1996). They fit the data by
the following formula:
 
(6) 
with the electron temperature T in Kelvin, and
the fitting parameters A,b, T_{0},
T_{1} are given
for all the ions from H through Zn. This last formula is valid in a
wide range of temperatures, from eV to T=100 keV.
We also use the same formula
for C, N, O and Ne ions with the fitting parameters
of Péquignot et al. (1991).
Up: Ionization balance for optically Ni
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