All Figures

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{4637f1.eps} \end{figure}$ Figure 1: Example of a "best fit''. The thin (red) lines represent the empirical spectrum (UM 137(W), taken on August, 18, 1998); the thicker black lines show the best-fitting spectra obtained using the different SSP libraries.
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In the text

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{4637f2.eps} \end{figure}$ Figure 2: Synthesised spectra over the whole wavelength range. The thin (red) lines represent the empirical spectrum (as in Fig. 1), the solid lines show the best-fitting spectra, and the dashed lines show the decompositions of the best-fitting spectra into young, intermediate, and old populations, respectively. The shaded regions represent the radiation shortward of the Lyman limit (912 Å), which is responsible for the ionization of the surrounding gas. Note that both axes are on a logarithmical scale, in contrast to Fig. 1.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{4637f3.eps} \end{figure}$ Figure 3: Illustration of how the H $\beta$ emission line equivalent width was measured through the example of UM137(CentE). In the left panel, the solid line represents the empirical spectrum, whereas the short-dashed line shows the best fit using the BC-2000 stellar library. The shaded region between these two lines (shaded with lines running from the upper left to the lower right) shows the area used to calculate the emission line strength, and the shaded region (lines running from the upper right to the lower left) between the solid line and zero level (the long-dashed line) shows the location of the continuum band. The right panel shows the emission line after subtracting the (rebinned) best fit. The shaded area corresponds to the shaded area between the spectrum and the best fit from the left panel. Here, too, the zero level is shown as a long-dashed line to show the quality of the subtraction.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{4637f4.eps} \end{figure}$ Figure 4: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the oxygen abundance from the gas.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{4637f8d.eps} \end{figure}$ Figure 5: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the gas reddening parameter ${C_{\rm H\beta }}$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{4637f5.eps} \end{figure}$ Figure 6: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the equivalent width of ${\rm H\beta }$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.7cm,clip]{4637f6.eps} %\end{figure}$ Figure 7: Monte Carlo simulation of the logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the oxygen abundance from the gas for optically thick nebulae.
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In the text

$\begin{figure} \par\includegraphics[width=8.7cm,clip]{4637f7.eps} %\end{figure}$ Figure 8: Monte Carlo simulation of the logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the equivalent width in ${\rm H\beta }$ for optically thick nebulae.
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In the text

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{4637f1.eps} \end{figure}$	Figure 1: Example of a "best fit''. The thin (red) lines represent the empirical spectrum (UM 137(W), taken on August, 18, 1998); the thicker black lines show the best-fitting spectra obtained using the different SSP libraries.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{4637f4.eps} \end{figure}$	Figure 4: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the oxygen abundance from the gas.
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{4637f8d.eps} \end{figure}$	Figure 5: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the gas reddening parameter ${C_{\rm H\beta }}$ .
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{4637f5.eps} \end{figure}$	Figure 6: Logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the equivalent width of ${\rm H\beta }$ .
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$\begin{figure} \par\includegraphics[width=8.7cm,clip]{4637f6.eps} %\end{figure}$	Figure 7: Monte Carlo simulation of the logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the oxygen abundance from the gas for optically thick nebulae.
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$\begin{figure} \par\includegraphics[width=8.7cm,clip]{4637f7.eps} %\end{figure}$	Figure 8: Monte Carlo simulation of the logarithmic ratio of the predicted number of ionizing photons to the number of recombinations as a function of the equivalent width in ${\rm H\beta }$ for optically thick nebulae.
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