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Astron. Astrophys. 351, 1075-1086 (1999)

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4. Extinction correction

The extinction correction has been performed by interpolating Cardelli et al.'s (1989) [FORMULA]-dependent extinction law, with [FORMULA], the ratio of visual extinction to colour-excess. The intensities measured for [NII[FORMULA]5754 in the blue and red spectral ranges have been used to bring the H[FORMULA] and H[FORMULA] 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 [FORMULA] and [FORMULA] (the effective optical depth for extinction at [FORMULA]) have been fixed separately for each area by fitting the ratios [FORMULA] and [FORMULA] to their recombination values (Hummer & Storey 1987) for suitable physical conditions. The values of [FORMULA] and [FORMULA] (listed in Table 2) are quite sensitive to calibration errors, and values as high as [FORMULA] are used for several positions, whereas Cardelli et al. (1989) derived their extinction law just for [FORMULA]. 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 [FORMULA] and [FORMULA] have been used. If [FORMULA] had been used instead - with [FORMULA] implying [FORMULA] - a drastic change to [FORMULA] (the mean value for the diffuse interstellar medium) and [FORMULA] would lead to a change of less than 50% (0.15 dex) in the ionic abundances derived here.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999