All Figures

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig1.ps}\end{figure}$ Figure 1: Spectral energy distribution (SED) for I Zw 18 NW models. Flux in erg s^-1 eV^-1 multiplied by an arbitrary constant versus photon energy $h\nu$ in eV. The SED (solid line) is the sum of two black bodies (dotted lines), with a discontinuity at 4 ryd (Sect. 4.1). Panel a) is common to N0, N1 and M1. Panels b), c) and d) correspond to M2, M3 and M4 respectively. Vertical arrows appear at 1 and 4 ryd. Straight dashed lines of the form exp ( $-\alpha$ $h\nu$ /ryd) are exponential approximations of the SEDs over the range 1-4 ryd. The exponential expressions are written in the panels corresponding to the SEDs ( $\alpha$ = 0.70, 1.00, 1.05 and 1.00 in panels a) to d), with $\alpha$ a measure of the EUV continuum hardness). Some of the SED data provided in Table 2 ( $Q_{\rm He}$ /Q, discontinuity factor $\delta _4$ and $Q_{\rm abs}$ /Q) are recalled in the panels.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8344fig2.ps}\end{figure}$ Figure 2: Gas pressure versus optical depth $\tau$ for models M1 (dashed line), M2 (solid line) and M3 (dotted line), governed by Eq. (1) with parameters in Table 2. M4, close to M3, is not shown. For comparison, the dash-dotted lines correspond to the constant density runs N0 and N1.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig3.ps}\end{figure}$ Figure 3: Local electron density $N_{\rm e}$ versus shell radius R in I Zw 18 NW models: a) N0, b) N1, c) M2; and versus optical depth $\tau$ at 1 ryd: d) M2. In panels c) and d), the vertical arrows indicate the boundary of the matter-bounded sector 2 of Model M2. Also provided are the optical thickness "max( $\tau$ )'' (">150'' means "radiation bounded''), the fraction of primary photons absorbed $Q_{\rm abs}$ /Q, and, where relevant (N0 and N1), the covering factor $f^{\rm cov}$ , the filling factor $\epsilon$ and the departure of the computed [S II] doublet ratio from observation in %.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig4.ps}\end{figure}$ Figure 4: As in Fig. 3 for local electron temperature $T_{\rm e}$ /10⁴ K. The maximum $T_{\rm e}$ and average $T_{\rm e}$ 's weighted by $N_{\rm e}$ $\times$ N(H⁺) and by $N_{\rm e}$ $\times$ N(O²⁺) are noted along each curve. In panel c), $\langle$ $T_{\rm e}$ $\rangle$ is given for both the radiation bounded sector ( $\langle$ sect.1 $\rangle$ ) and the full model M2 ( $\langle$ mod. $\rangle$ ).
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig5.ps}\end{figure}$ Figure 5: Local fractional concentrations of O⁰ (dotted line), O⁺ (dot-dashed line), O²⁺ (solid line), O³⁺ (dashed line) and O⁴⁺ (dotted line again) versus $\tau$ in I Zw 18 NW models: a) N0, b) N1, d) M2; and versus R: c) M2. Vertical arrows as in Fig. 3.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{8344fig6.ps}\end{figure}$ Figure 6: As in Fig. 1b for variants of M2 comprising a soft X-ray emitting black body: Model $M2_{\rm X}$ (solid line) and $M2_{\rm X2}$ (dash-dotted line). Ordinate unit flux is now 10⁴¹ erg s^-1 eV^-1. Dash-triple-dotted line: one possible fit to the CHANDRA data for I Zw 18 NW.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig7.ps}\end{figure}$ Figure 7: Model $M2_{\rm X}$ for I Zw 18 NW. a) Electron density $N_{\rm e}$ (solid line) and total hydrogen density N(H) (dotted line) versus optical depth $\tau$ at 1 ryd: N(H) levels out at 300 cm^-3. b) Electron temperature $T_{\rm e}$ /10⁴ K: the maximum value and average values weighted by $N_{\rm e}$ $\times$ N(H⁺) and by $N_{\rm e}$ $\times$ N(O²⁺) are noted for the full 2-sector model ( $\langle$ mod. $\rangle$ ). c) Ratio of $N_{\rm e}$ to atomic hydrogen density N(H⁰). In panels a)- c), the dashed lines correspond to $M2_{\rm X2}$ . d) Local fractional concentrations of O⁰ (dotted line), O⁺ (dot-dashed line), O²⁺ (solid line), O³⁺ (dashed line), O⁴⁺ (dotted line again) and O⁵⁺ (solid line again): the thinner dash-dotted line ( $\tau$ > 200) corresponds to O⁺ in $M2_{\rm X2}$ and the thinner solid line (split into $M2_{\rm X}$ and $M2_{\rm X2}$ for $\tau$ > 200) to Ne⁺.
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In the text

$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig8.ps}\end{figure}$ Figure 8: Cumulative mean squared relative temperature fluctuations t² (Peimbert 1967) weighted by $N_{\rm e}$ $\times$ N(H⁺) (solid line), $N_{\rm e}$ $\times$ N(O⁺) (dotted line) and $N_{\rm e}$ $\times$ N(O²⁺) (dashed line) versus $\tau$ in I Zw 18 NW models: a) N0, b) N1, d) M2; and versus R: c) M2. Note the expanded vertical scale in panel a). The short horizontal bars to the right of panel d) correspond to the means over the full model. Vertical arrows as in Fig. 3.
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{8344fig2.ps}\end{figure}$	Figure 2: Gas pressure versus optical depth $\tau$ for models M1 (dashed line), M2 (solid line) and M3 (dotted line), governed by Eq. (1) with parameters in Table 2. M4, close to M3, is not shown. For comparison, the dash-dotted lines correspond to the constant density runs N0 and N1.
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$\begin{figure} \par\includegraphics[width=9cm,clip]{8344fig5.ps}\end{figure}$	Figure 5: Local fractional concentrations of O⁰ (dotted line), O⁺ (dot-dashed line), O²⁺ (solid line), O³⁺ (dashed line) and O⁴⁺ (dotted line again) versus $\tau$ in I Zw 18 NW models: a) N0, b) N1, d) M2; and versus R: c) M2. Vertical arrows as in Fig. 3.
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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{8344fig6.ps}\end{figure}$	Figure 6: As in Fig. 1b for variants of M2 comprising a soft X-ray emitting black body: Model $M2_{\rm X}$ (solid line) and $M2_{\rm X2}$ (dash-dotted line). Ordinate unit flux is now 10⁴¹ erg s^-1 eV^-1. Dash-triple-dotted line: one possible fit to the CHANDRA data for I Zw 18 NW.
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