3.5. Spatial temperature variations
At high metallicities, as explained above, large temperature gradients are
expected in ionized nebulae. Therefore, empirical methods based on
[O III]
4363/5007 will
underestimate the abundances of heavy elements, since the
[O III]
4363 line will be
essentially emitted in the high temperature
zones, inducing a strong overestimate of the average Te.
Therefore, although with very large telescopes it will now be possible
to measure [O III]
4363 even in high
metallicity giant H II regions, one should
refrain from exploiting this line in the usual way. Doing this, one would
necessarily find sub-solar oxygen abundances, even for giant
H II regions with metallicities well above solar (Fig. 3 of
Stasinska 2002).
High metallicity luminous PNe offer a much safer way to probe
the metallicity in central parts of galaxies
(see Sect. 5.3 for the relevance of PNe
as metallicity indicators of their environment). Indeed the higher
effective temperatures and the higher densities in luminous PNe
induce higher values of Te in the O++ zone
and a shallower
temperature gradient, leading to a negligible bias in the derived
abundances (see Fig. 4 of
Stasinska 2002).
While Te-based empirical methods are biased for metal
rich giant H II regions, tailored photoionization
modeling to reproduce the
distribution of the emission in the
H,
H
,
He I
5876,
[O II]
3727 and
[O III]
5007 lines are worth
trying. As suggested by
Stasinska (1980a),
at high metallicity, regions emitting strongly [O III]
5007 will be decoupled
from the regions emitting strongly in the recombination lines,
and would be almost cospatial with regions emitting most of [O
II]
3727.
While the models of
Stasinska (1980a)
were made under spherical
symmetry, the statement is more general, because it relies of the
principles of ionization and thermal balance outlined in
Sects. 1.1 and
1.2.