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Figure 1: The field of view covered when mapping small hydrocarbons at 3.4 with the Plateau de Bure Interferometer (PdBI) is shown as a square over this ESO-VLT composite image (B, V and R bands) of the Horsehead nebula. Each circle indicates the 3.4 primary beam of the PdBI at one of the 7 observed positions. Those positions are largely oversampled at the hydrocarbon wavelength (3.4 ) to ensure simultaneous Nyquist-sampling at 1.4 used to observe C18O. A linear combination of the 7 pointed observation is done to obtain the final dirty image.

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In the text

Figure 2: Integrated emission maps obtained with the Plateau de Bure Interferometer. Maps of i) the H2 v=1-0 S(1) emission (Habart et al. 2004,2005); ii) the mid-IR emission (Abergel et al. 2003, labeled ISO-LW2); and iii) the 1.2 dust continuum (Teyssier et al. 2004, labeled 1.2mm) are also shown for comparison. The center of all maps has been set to the mosaic 1 phase center: RA(2000 , Dec(2000 . The map size is , with ticks drawn every 10''. Either the synthesized beam or the single dish beam is plotted in the bottom left corner. The emission of all the lines observed at PdBI is integrated between 10.1 and 11.1  . Values of contour level are shown on each image wedge (contours of the H2 image have been computed on an image smoothed to 5'' resolution). The sharp edge of the H2 emission ( upper right panel) defines a boundary, which is used as a numerical support (in the language of signal processing) for deconvolution of the other images. This deconvolution support is overplotted in red on each panel.

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In the text

Figure 3: Same as Fig. 2 except that maps have been rotated by 14 counter-clockwise around the image center to bring the exciting star direction in the horizontal direction as this eases the comparison of the PDR tracer stratifications. Maps have also been horizontally shifted by 20'' to set the horizontal zero at the PDR edge delineated as the vertical red line. Horizontal red lines delimit the two lanes that have been vertically averaged to produce the two series of cuts shown in Fig. 5.

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In the text

Figure 4: Joint histogram of the integrated emission of i) the second brightest CCH line ( top); ii) c-C3H2 ( middle); and iii) one C4H line ( bottom) vs. the main CCH line. The value at a given position of this joint histogram is the percentage of pixels of the input images whose intensities lies in the respective vertical and horizontal bins. Only image pixels lying inside the deconvolution support (shown in Fig. 2) have been used in the histogram computation. Contour levels are set to 0.125, 0.25, 0.5, 1, 2, 4 and 8% of points per pixel.

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In the text

Figure 5: Emission profiles along the exciting star direction ( in the equatorial coordinate system). To improve the signal-to-noise ratio, those emission profiles have been integrated along the perpendicular direction between ( left column) and ( right column). The comparison of those two series of mean cuts gives an idea of the influence of the local conditions (either excitation effects or chemical differentiation). We show from top to bottom, H2 v=1-0 S(1) (full resolution in black, smoothed at a 5''-resolution in red), 12CO J=2-1, 12CO J=1-0, C18O J=2-1, CCH lines (1 black, 2 green, 3 red and 4 blue, respectively following the ratios 1:0.5:0.5:0.25), c-C3H2, C4H lines (1 black, 2 red), ISO-LW2 and the 1.2 dust continuum. For the PdBI data, the 3- noise level is indicated by the dashed lines. It rises at the cut edges due to the primary beam correction. Note that the fields of view of the 12CO and C18O data are smaller than the field of view of the hydrocarbon data because of the smaller mosaic size and/or the higher frequency. The blue vertical lines are guidelines to localize the hydrocarbon peaks. They have been drawn at ( left column) and ( right column). Note that the bumps at in the ISO-LW2 cut ( left column) and at in the H2 cut ( right column) are due to field stars.

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In the text

Figure 6: Spatial variation along the direction of the exciting star of a) the 12CO J=2-1 brightness temperature (convolved at the same angular resolution as the J=1-0 transition); b) 1-0/2-1 ratio and c), d), e) the kinetic temperature. offsets refer to the coordinate system defined in Fig. 3. The cuts at are drawn in blue, those at in green and those in red. The data cuts have been taken from the 10.6 velocity channel corresponding to the 12CO line peak. The kinetic temperature is derived from an LVG model assuming i) unity beam filling factor; and ii) uniform total hydrogen density of (dashed lines), (full lines), (dotted lines) and (dotted-dashed lines). Arrows indicate lower limits.

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In the text

Figure 7: CO spectra (convolved at the same angular resolution) along the direction of the exciting star at . In the cuts, the label indicates the IR peak position (cf. Table 3). Note that 12CO J=2-1 peak intensity decreases at positions and 15'' while 12CO J=1-0 increases even reaching its maximum at . In addition, both 12CO lines show a small but clear dip (i.e. the center channel intensity is lower than its first neighbours) at . Finally, while C18O spectra are very close to Gaussian, 12CO spectra show asymmetric profiles. Spectra cuts at other close values show the same trends.

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In the text

Figure 8: Predictions of the spatial variation of the abundance relative to H2, using a unidimensional PDR code. For each model, the abundance of the population of the the upper level of the 2.12  H2 line (i.e. v=1, J=3), written [H, is shown on a linear scale ( top). The C, CO and hydrocarbon abundances are shown on a logarithmic scale ( bottom). The modeled cloud is illuminated from the right-hand side. The -axis origin has been set so that [H peaks at the position of the observed H2 peak (i.e. ). The vertical dotted blue line indicates the peak of the c-C3H2 and C4H abundances. Each model is described in 4 lines: i) the density structure; ii) the UV-field properties; iii) the chemical network used; and iv) the gaseous sulfur abundance. 6 different models are compared here. Our reference model is labeled A. The total hydrogen density is kept uniform at a value of . The far-UV intensity of the radiation field is G0=100 (in Draine units) and the extinction curve is the mean Galactic one. The chemical network rate file is the New Standard Model one with minor modifications described in the text. Charge exchange reactions between C+ and PAHs are not taken into account. The gaseous sulfur abundance is low compared to solar (i.e. ). The parameters varied in other models are emphasized in red. Model B uses a different extinction curve. Model C adds reactions of charge exchange between C+ and PAHs. Model D decreases the uniform total hydrogen density. Models E and F use the density profile derived from the model of the H2 observations (Habart et al. 2004,2005). Model F uses a solar gaseous sulfur abundance (i.e. ).

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In the text

	Figure 9: Spatial variation of the total hydrogen density in models A to F. In model E and F, the density increases as a power law of scaling exponent 4 in the first 10'' and then is kept constant at a value of . As in Fig. 8, the x-axis origin has been set so that [H peaks at the position of the observed 2.12  , H2 line peak (i.e. ).
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In the text

Figure 10: Comparison between our two best models (curves) and observed (points with error bars) abundances of the small hydrocarbons. CCH is shown as a green solid line, c-C3H2 as a dashed red line and C4H as a blue dotted line. The top and bottom panels respectively show abundances relative to the total hydrogen density and CCH. The dashed vertical line shows the position where the total hydrogen density profile changes from a steep gradient to a constant.

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In the text