All Figures

$\begin{figure} \par\includegraphics[width=8cm,clip]{0534fig01.eps} \end{figure}$ Figure 1: Composite colour image of the Horsehead nebula PDR obtained by the VLT in the B, V and R bands. The arrow indicates the direction of the illuminating star $\sigma$ Ori (Image courtesy of ESO).
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8cm,clip]{0534fig02.eps} \end{figure}$ Figure 2: Visible image from the Digital Sky Survey in the area of IC 63. The cometary shape originates from the strong radiation field emitted by the close-by HD5394 stars 20 $^\prime$ South-West.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0534fig03.eps} \end{figure}$ Figure 3: Close-up of the western border of L1688 mapped by ISOCAM around 7 $\mu$ m (Abergel et al. 1996). The illuminating star HD 147889 direction is indicated by an arrow. The black squares indicate the positions of the cut where molecular data have been collected. The tilted rectangle delimitates the area mapped in ¹³CO and C¹⁸O. Offsets are given with respect to $\alpha _{2000}$ = 16 $^{\rm h}$ 25$^m$58 $^{\rm s}$ , $\delta _{2000}= -24$ $^{\circ }$ 21 $^\prime$ 00 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=15cm,clip]{0534fig04.eps} \end{figure}$ Figure 4: Integrated emission maps ( $T_{\rm A}^*$ , K km s^-1) of the molecular emission in the Horsehead nebula PDR for several species. Contours are: c-C₃H₂(2_1,2-1_0,1) 0.1 to 0.7 by 0.15, C₂H(1-0) 0.3 to 2.1 by 0.3, C₄H(9-8) 0.025 to 0.2 by 0.03, C¹⁸O(2-1) 1 to 6 by 1. Also displayed is the continuum emission between 5 and 8.5 $\mu$ m (aromatic emission) as mapped by ISO (from Abergel et al. 2002, contours are 10 to 25 by 5 MJy/sr), and at 1.2 mm (contours are 3 to 53 by 10 mJy/11 $^{\prime \prime }$ beam) mapped by the MAMBO 117-channel bolometer. The white squares indicated the position labelled as CO-peak ( top) and IR-peak ( bottom).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{0534fig05.eps} \end{figure}$ Figure 5: Maps of IC 63. Top: integrated emission of the ISOCAM CVF spectra between 5.98 and 6.54 $\mu$ m after subtraction of a 1-order baseline. Contours are 70 to 310 by 40 MJy/sr. The dashed box indicates the area covered by the molecular millimetre data displayed below. Bottom: integrated emission maps ( $T_{\rm A}^*$ , K km s^-1) of ¹²CO(2-1) (contours 5 to 40 by 5) and C₂H(1-0) (contours 0.10 to 0.35 by 0.05). The white squares indicate the position of the IR peaks.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{0534fig06.eps} \end{figure}$ Figure 6: Channel maps of the C¹⁸O(1-0) emission (K km s^-1, $T_{\rm A}^*$ ) in the L1688-W PDR, as well as integrated intensity maps of C¹⁸O(1-0) and ¹³CO(2-1) ( (a) and (b)) in the velocity interval 3.4-4.2 km s^-1, corresponding to the emission associated to the PDR as seen by ISO. The velocities of each channel map is indicated in the upper left corner. Offsets are given with respect to $\alpha _{2000}=16$ $^{\rm h}$ 25$^m$58 $^{\rm s}$ , $\delta _{2000}= -24$ $^{\circ }$ 21 $^\prime$ 00 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8cm,clip]{0534fig07.eps} \end{figure}$ Figure 7: Scatter diagrams in the Horsehead nebula. The scatter around the zero emission points is indicative of the noise in the data.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=14.5cm,clip]{0534fig08.ps} \end{figure}$ Figure 8: Sample spectra of heavier hydrocarbons and other species obtained at the so-called CO peak (indicated by dashed ¹²CO contours, 30% and 80 to 95% of the peak, from Abergel et al. 2003) and IR peak (indicated by full line ISO contours 15 to 30 MJy/sr, from Abergel et al. 2002). All temperature are in K, $T_{\rm A}^*$ scale. The same scales are used for both positions to ease the comparison between the various molecules. The missing channels in the C₆H spectrum were removed due to spurious features in the backend response. For this line, we also display the Gaussian fit to the line. Offsets are given with respect to $\alpha _{2000}=05$ $^{\rm h}$ 40$^m$59.0 $^{\rm s}$ , $\delta _{2000}= -02$ $^{\circ }$ 25 $^\prime$ 31.4 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=14.5cm,clip]{0534fig09.ps} \end{figure}$ Figure 9: Sample spectra of weaker hydrocarbons and other species obtained at some dedicated positions in IC 63 displayed over contours of the 6.2 $\mu$ m ISO integrated emission map. It illustrates in particular the non-detection of the cyclic and linear forms of C₃H around 92 and 98 GHz respectively. The probed positions are marked with filled and open circles. The open circles indicate the cuts displayed in Fig. 10. All temperature are in K, $T_{\rm A}^*$ scale. Offsets are given with respect to $\alpha _{2000}=00$ $^{\rm h}$ 59$^m$00.7 $^{\rm s}$ , $\delta _{2000}= 60$ $^{\circ }$ 53 $^\prime$ 19 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=8.8cm,clip]{0534fig10.eps} \end{figure}$ Figure 10: Normalized line intensities across the PDR fronts of the Horsehead nebula and IC 63. Horsehead: iso-declination cut performed through the IR-peak (see Fig. 4) at $\delta _{2000}= -2$ $^{\circ }$ 28 $^\prime$ 00 $^{\prime \prime }$ . All intensities are normalized to the peak (respectively 0.24, 0.9, 2.3 and 8.7 K from top to bottom, and 24 MJy/sr for the ISO LW2 profile). The illuminating source lies towards the negative offsets. IC 63: cut performed along the direction indicated by open circles in Fig. 9. All intensities are normalized to the peak (respectively 0.03, 0.27, 0.5 and 0.2 K from top to bottom, and 310 MJy/sr for the ISOCAM CVF profile). The illuminating source lies towards the positive offsets. All error bars are $\pm$ 1 $\sigma$ . Upper limits indicate non-detections at a 2 $\sigma$ level.
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In the text

$\begin{figure} \par\includegraphics[angle=270,width=15cm,clip]{0534fig11.eps} \end{figure}$ Figure 11: Normalized line intensities over the cut displayed in Fig. 3 across the L1688-W PDR. Error bars are $\pm$ 1 $\sigma$ . The star lies towards the negative offsets. The 0-offset refers to the most North-East point of the cut (Fig. 3). All intensities are normalized to the peak (respectively 0.08, 0.2, 0.43 and 1.6 K from top to bottom, and 117 MJy/sr for the ISO LW2 profile). An upper limit is indicated for the rightmost point of C₄H. Sample spectra at two offsets are also shown. The dashed line indicates the centroid velocity of the optically thin lines, illustrating the self-absorption in ¹³CO. positive offsets). All temperature are in K, $T_{\rm A}^*$ scale.
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In the text

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{0534fig12.eps} \end{figure}$ Figure 12: Prediction for the abundances of various species derived from an updated Le Bourlot et al.'s PDR model, using two chemistry networks, namely Herbst et al.'s New Standard Model ( left) and UMIST95 of Millar and collaborators ( right). We assumed a gas density of $n_{\rm H}=2\times 10^4$ cm^-3 and a radiation field G₀=100. Both models predict too low abundances for c-C₃H₂ and C₄H, but also that C₂H is present in the UV illuminated gas as observed.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{0534fig01.eps} \end{figure}$	Figure 1: Composite colour image of the Horsehead nebula PDR obtained by the VLT in the B, V and R bands. The arrow indicates the direction of the illuminating star $\sigma$ Ori (Image courtesy of ESO).
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$\begin{figure} \par\includegraphics[angle=270,width=8cm,clip]{0534fig02.eps} \end{figure}$	Figure 2: Visible image from the Digital Sky Survey in the area of IC 63. The cometary shape originates from the strong radiation field emitted by the close-by HD5394 stars 20 $^\prime$ South-West.
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$\begin{figure} \par\includegraphics[angle=270,width=8cm,clip]{0534fig07.eps} \end{figure}$	Figure 7: Scatter diagrams in the Horsehead nebula. The scatter around the zero emission points is indicative of the noise in the data.
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