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Astron. Astrophys. 351, 1075-1086 (1999)
4. Extinction correction
The extinction correction has been performed by interpolating
Cardelli et al.'s (1989) -dependent
extinction law, with , the ratio of
visual extinction to colour-excess. The intensities measured for
[NII ] 5754 in
the blue and red spectral ranges have been used to bring the
H and
H lines to the same scale. [The same
correction factor has been applied to all the regions along each slit
position, the maximum value being of 10%.] The parameters
and
(the effective optical depth for
extinction at ) have been fixed
separately for each area by fitting the ratios
and
to their recombination values
(Hummer & Storey 1987) for suitable physical conditions. The
values of and
(listed in Table 2) are quite
sensitive to calibration errors, and values as high as
are used for several positions,
whereas Cardelli et al. (1989) derived their extinction law just for
. However, the values of the
extinction parameters have no critical effect on the final results. We
can obtain a highly conservative upper limit to the errors introduced
by the extinction correction by considering the area most affected by
extinction, M17-3, where and
have been used. If
had been used instead - with
implying
- a drastic change to
(the mean value for the diffuse
interstellar medium) and would lead
to a change of less than 50% (0.15 dex) in the ionic abundances
derived here.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999
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