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Appendix A: PDR calculation details

In this appendix, the general outputs of the PDR model used in the article are presented. The model is run, as detailed in the article, with a constant pressure P = 1.6 × 10⁶ K cm^-3 (corresponding to a dense cloud hydrogen density of n_H ≈ 2 × 10⁵ cm^-3), an incident UV flux χ set to 60 times the interstellar radiation field in Draine units (see Draine 1978; Habart et al. 2005), and a cosmic ray ionization rate set to ζ = 5 × 10^-17 s^-1 (Goicoechea et al. 2009). The initial gas phase abundances are those used for the Horsehead Nebula (see Goicoechea et al. 2006, Table 6) with the sulfur abundance set to S / H = 3.5 × 10^-6.

Figure A.1 presents the temperature, density, and ionization degree as a function of the visual extinction (A_v) at the output of the PDR code.

	Fig. A.1 Temperature (purple dotted line), total hydrogen density (blue dashed line), and ionization degree (green solid line) as a function of visual extinction.
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	Fig. A.2 Photodissociation rates of CO (blue dashed dotted lines) and H2 (red dashed lines) and photoionization rates of C (green dotted lines) and S (purple solid lines) as a function of the visual extinction in the Meudon code (no symbols) and in Nahoon (diamond lines).
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	Fig. A.3 Abundance of H (brown long dashed line), electrons (red solid line), CO (yellow dotted lines), C (green dash-dotted line), C+ (blue short dotted line), and H+ (purple short and long dashed line) for times of 300 yr (circle), 1000 yr (triangle), 104 yr (pentagon), and 108 yr (diamond) as a function of the visual extinction.
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	Fig. A.4 Coefficient Fi as a function of time for Av = 0.5 (blue solid line), Av = 1 (purple dashed line), Av = 3 (green dotted lines), Av = 5 (orange dashed dotted line), and Av = 7 (brown dashed line).
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	Fig. A.5 Ratio of the abundance of CCH with or without the inclusion of photo-desorption reactions as a function of the percent of carbon in a–C:H for Av = 3 at 1000 yr (blue dash-dotted line) and for Av = 4 at 10 000 yr (purple solid line).
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Figure A.2 shows the photodissociation rates of H₂, CO, and the photoionization rates of C and S in the Meudon and in the Nahoon codes before our modifications. We have modified the photodissociation rates in the Nahoon code in order to make them equal to those in the Meudon code. In particular, the photo-dissociation rates of H₂ and CO were very different (by several orders of magnitude) in the Meudon code and in Nahoon.

As shown in Fig. A.3, the abundance of key species at different times is not affected by the addition of the a–C:H photolysis reactions. This implies that the added photolytic reactions do not perturb the initial opacity of the cloud model.

The rate coefficient factor F_i, discussed in the article, was checked for all visual extinctions in the model. As shown in Fig. A.4, the higher the visual extinction, the longer it takes for F_i to decrease. It takes approximately 200 yr, 5000 yr, and 2 × 10⁷ yr to reduce F_i by about a factor of two at visual extinctions equal to 1, 3, and 7 mag, respectively.

In Fig. A.5 the influence of the adopted fraction of interstellar carbon abundance locked into a–C:H grains is shown. The abundance of gas phase carbonaceous molecules in the model increases linearly with the percent fraction of carbon contained in the a–C:H. The case of CCH is presented in Fig. A.5. With 5% of carbon locked in a–C:Hs, the abundance of CCH increases by a factor of 40 at A_v = 4 and 10 000 yr, and by a factor 60 at A_v = 3 and 1000 yr.

Appendix B: Influence of the sulfur abundance

Fig. B.1 Output of the Horsehead Nebula photon dominated region model III with a low sulfur abundance in the initial conditions (S / H = 5.8 × 10-8) for the CCH hydrocarbon abundance as a function of visual extinction in the cloud illuminated by the σOri star (see Appendix A for details). The abundance of CCH is shown in the top panel at different times after injection of the a–C:H grain photolysis in the model. The corresponding abundance ratio of CCH with the addition of photolytic reactions divided by the abundance without the addition of the photolytic reactions is shown in the bottom panel for the same times.

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In the modeling we initally used the sulfur abundance as derived by Goicoechea et al. (2006) for the Horsehead Nebula. Because the sulfur abundance to be adopted is generally highly debated, we show the influence of setting the sulfur abundance to a lower abundance value S / H = 5.8 × 10^-8 on our results.

	Fig. B.2 Same as Fig. B.1 with a low sulfur abundance in the initial conditions for C3H2: abundance of C3H2 (top) and abundance ratio of C3H2 with the addition of photolytic reactions divided by the abundances without the addition of photolytic reactions (bottom) for different times.
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	Fig. B.3 Same as Fig. B.1 with a low sulfur abundance in the initial conditions for C4H: abundance of C4H (top) and abundance ratio of C4H with the addition of photolytic reactions divided by the abundances without the addition of photolytic reactions (bottom) for different times.
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© ESO, 2015