All Figures

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f1.eps} \end{figure}$ Figure 1: Position in a $\log~(g)$ versus ${T}_{{\rm eff}}$ diagram of the Supergiant models and the Kurucz dwarf models used in this paper. See Fig. 2 for the line symbols. CoStar models are labeled according to the original naming convention. CMFGEN models have the same parameters than WM-Basic models and are not drawn here.
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In the text

$\begin{figure} \par\includegraphics[width=18cm,clip]{MS4059f2.eps} \end{figure}$ Figure 2: Comparison between the 6 stellar atmosphere models: CoStar (solid), WM-Basic (dotted), CMFGEN (dashed), TLUSTY (dash dot), Kurucz (dash dot dot) and the Blackbody (long dashes, left panels only), for the same ${T}_{{\rm eff}}$ of 35 kK (upper plots), except for CoStar model (see text), and 40 kK (lower plots). The left panels show the Spectral Energy Distribution and the right panels show, for any energy E (eV), the number Q_E of photons with energy greater than E, relative to the corresponding number for the Blackbody emission, all the spectra having the same value for Q_13.6. Vertical lines are plotted at 13.6 eV (solid) and 27.6, 35.0 and 41.1 eV (dotted), corresponding to the ionization potentials of the ions considered in this paper (Ar⁺, S⁺⁺, and Ne⁺ resp.).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f3.eps} \end{figure}$ Figure 3: Dereddened observed values for the excitation sensitive line ratio [Ar III/II] versus [S IV/III] (the corresponding ionization potentials are also given). Source with a galactocentric distance lower than 7 kpc are symbolized with a +, otherwise with an X. Results from the photoionization model grid are line plotted using the same codes as in Fig. 2. The plot have been done such as the lowest ionization potential (indicated in braces) is always on the y-axis. Models obtained with 35 and 40 kK stars are shown using filled diamonds and empty squares respectively (except for CoStar model at 32.2 kK, empty diamond, see text). The y=x line is also drawn.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f4.eps} \end{figure}$ Figure 4: Same as Fig. 3 for the excitation sensitive line ratio [Ar III/II] versus [Ne III/II].
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f5.eps} \end{figure}$ Figure 5: Same as Fig. 3 for the excitation sensitive line ratio [S IV/III] versus [Ne III/II].
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f6.eps} \end{figure}$ Figure 6: Increase of the excitation diagnostics [Ar III/II] versus [Ne III/II], when $\log(\bar{U})$ is increased from -2.6 to -0.8 (solid arrows), when the metallicity of the gas is increased from half solar to twice solar (dashed arrows), and when ${T}_{{\rm eff}}$ is increasing from 35 to 40 kK (dotted arrows). Upper set of arrows are for BlackBody models, and lower set for WM-Basic models. In the latter case the metallicity is changes coherently for both the gas and the star. Codes are the same than in Figs. 2, 3.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f7.eps} \end{figure}$ Figure 7: Correlation between He I 5876 Å/H $_\beta$ and [Ar III/II]. The line styles are the same as in Fig. 2. Arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f8.eps} \end{figure}$ Figure 8: Comparison between energy distribution (same as right panels of Fig. 2) between Supergiants (light curves) and Dwarfs (bold curves) of WM-Basic (dotted) and CMFGEN (dashed) models, all at 35 kK.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f9.eps} \end{figure}$ Figure 9: Comparison between excitation diagnostic [Ar III/II] obtained with Supergiants (light curves) and Dwarfs stars (bold curves), for WM-Basic (dotted) and CMFGEN (dashed) atmosphere models. Models at 35 and 40 kK are shown by filled diamonds and empty squares respectively.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f10.eps} \end{figure}$ Figure 10: Variation of the excitation diagnostics [Ne III/II] versus [Ar III/II] for the same atmosphere models (here WM-Basic and CMFGEN, dotted and dashed thin lines respectively), multiplying the effective recombination coefficient for only Ar⁺⁺ by 10 (bold lines).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f11.eps} \end{figure}$ Figure 11: Variation of the excitation diagnostic [Ar III/II] according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f12.eps} \end{figure}$ Figure 12: Variation of the optical excitation diagnostic He I 5876 Å/H $_\beta$ with ${T}_{{\rm eff}}$ for supergiant stars. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f13.eps} \end{figure}$ Figure 13: Variation of the optical excitation diagnostic [O III] 5007 Å/ [O II] 3727,29 Å with ${T}_{{\rm eff}}$ for supergiant stars. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f14.eps} \end{figure}$ Figure 14: Variation of the radiation softness parameter $\eta _{\rm Ar-Ne}$ versus [Ar III/II]. The codes are the same than in Figs. 2, 3. Note that the model predictions for both axis depend also on the assumed ionization parameter $\bar{U}$ . Arrows as in Fig. 6 for WM-Basic models.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f15.eps} \end{figure}$ Figure 15: Variation of the radiation softness parameter $\eta _{\rm Ar-Ne}$ according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. Arrows have the same meaning as in Fig. 6, for WM-Basic models.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f16.eps} \end{figure}$ Figure 16: Variation of the radiation softness parameter $\eta _{\rm S-O}$ (defined by Vilchez & Pagel 1988, see note 8), according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. Arrows have the same meaning as in Fig. 6, for WM-Basic models, for WM-Basic models only.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f17.eps} \end{figure}$ Figure 17: Variation of the radiation softness parameter $\eta _{\rm S-Ne}$ versus $\eta _{\rm Ar-Ne}$ , when changing $\bar{U}$ , ${T}_{{\rm eff}}$ or Z. Arrows codes are the same as in Fig. 6.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f18.eps} \end{figure}$ Figure 18: Average stellar effective temperature and dispersion as a function of time for metallicities Z=0.008 ( $1/2.5~Z_\odot$ , solid lines) and 0.04 ( $2~Z_\odot$ , dashed) predicted for an ensemble of single star H II regions for a Salpeter IMF with $M_{\rm up}=120~M_\odot$ and a minimum Lyman continuum flux $\log~({Q}_{\rm lim}) \ge 48.$ . Note the rather small differences in average ${T}_{{\rm eff}}$ and the large intrinsic dispersion for each given metallicity.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f19.eps} \end{figure}$ Figure 19: [Ar III/II] versus metallicity, measured here by the Ne abundance (see text). The line is a linear fit to the observations in log-log space. Arrows as in Fig. 6, for WM-Basic models.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f20.eps} \end{figure}$ Figure 20: [Ne III/II] versus metallicity, as in Fig. 19. A very similar plot is obtained for [S IV/III].
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f1.eps} \end{figure}$	Figure 1: Position in a $\log~(g)$ versus ${T}_{{\rm eff}}$ diagram of the Supergiant models and the Kurucz dwarf models used in this paper. See Fig. 2 for the line symbols. CoStar models are labeled according to the original naming convention. CMFGEN models have the same parameters than WM-Basic models and are not drawn here.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f4.eps} \end{figure}$	Figure 4: Same as Fig. 3 for the excitation sensitive line ratio [Ar III/II] versus [Ne III/II].
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f5.eps} \end{figure}$	Figure 5: Same as Fig. 3 for the excitation sensitive line ratio [S IV/III] versus [Ne III/II].
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f7.eps} \end{figure}$	Figure 7: Correlation between He I 5876 Å/H $_\beta$ and [Ar III/II]. The line styles are the same as in Fig. 2. Arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f8.eps} \end{figure}$	Figure 8: Comparison between energy distribution (same as right panels of Fig. 2) between Supergiants (light curves) and Dwarfs (bold curves) of WM-Basic (dotted) and CMFGEN (dashed) models, all at 35 kK.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f9.eps} \end{figure}$	Figure 9: Comparison between excitation diagnostic [Ar III/II] obtained with Supergiants (light curves) and Dwarfs stars (bold curves), for WM-Basic (dotted) and CMFGEN (dashed) atmosphere models. Models at 35 and 40 kK are shown by filled diamonds and empty squares respectively.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f10.eps} \end{figure}$	Figure 10: Variation of the excitation diagnostics [Ne III/II] versus [Ar III/II] for the same atmosphere models (here WM-Basic and CMFGEN, dotted and dashed thin lines respectively), multiplying the effective recombination coefficient for only Ar⁺⁺ by 10 (bold lines).
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f11.eps} \end{figure}$	Figure 11: Variation of the excitation diagnostic [Ar III/II] according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f12.eps} \end{figure}$	Figure 12: Variation of the optical excitation diagnostic He I 5876 Å/H $_\beta$ with ${T}_{{\rm eff}}$ for supergiant stars. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f13.eps} \end{figure}$	Figure 13: Variation of the optical excitation diagnostic [O III] 5007 Å/ [O II] 3727,29 Å with ${T}_{{\rm eff}}$ for supergiant stars. Codes are the same as in Figs. 2, 3. The set of arrows have the same meaning as in Fig. 6, for WM-Basic models only.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f14.eps} \end{figure}$	Figure 14: Variation of the radiation softness parameter $\eta _{\rm Ar-Ne}$ versus [Ar III/II]. The codes are the same than in Figs. 2, 3. Note that the model predictions for both axis depend also on the assumed ionization parameter $\bar{U}$ . Arrows as in Fig. 6 for WM-Basic models.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f15.eps} \end{figure}$	Figure 15: Variation of the radiation softness parameter $\eta _{\rm Ar-Ne}$ according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. Arrows have the same meaning as in Fig. 6, for WM-Basic models.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f16.eps} \end{figure}$	Figure 16: Variation of the radiation softness parameter $\eta _{\rm S-O}$ (defined by Vilchez & Pagel 1988, see note 8), according to the ${T}_{{\rm eff}}$ of the ionizing star. Codes are the same as in Figs. 2, 3. Arrows have the same meaning as in Fig. 6, for WM-Basic models, for WM-Basic models only.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f17.eps} \end{figure}$	Figure 17: Variation of the radiation softness parameter $\eta _{\rm S-Ne}$ versus $\eta _{\rm Ar-Ne}$ , when changing $\bar{U}$ , ${T}_{{\rm eff}}$ or Z. Arrows codes are the same as in Fig. 6.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f19.eps} \end{figure}$	Figure 19: [Ar III/II] versus metallicity, measured here by the Ne abundance (see text). The line is a linear fit to the observations in log-log space. Arrows as in Fig. 6, for WM-Basic models.
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$\begin{figure} \par\includegraphics[width=8.8cm,clip]{MS4059f20.eps} \end{figure}$	Figure 20: [Ne III/II] versus metallicity, as in Fig. 19. A very similar plot is obtained for [S IV/III].
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