Table 1: PDR models of the Horsehead nebula.
Constant density models                    
Model $n_{\rm H}$ G0 $A_{\rm V}$ CO   CO   CO   13CO C18O C I   $\chi^2$
        (2-1) (4-3) (6-5) (2-1) (2-1) (1-0)  
Pos A       40.5 24.8 12.0 15.2 7.3 11.1  
A1 $\ast$ $5 \times 10^4$ 25 36 43.2 29.7 8.9 14.9 7.8 11.6 0.27
A2 $3 \times 10^4$ 25 36 38.9 23.8 5.1 14.7 8.1 11.7 1.11
A3 $7 \times 10^4$ 25 36 46.2 33.8 12.5 14.9 7.5 11.4 0.88
A4 $5 \times 10^4$ 55 36 45.2 31.8 10.4 16.2 8.4 12.9 0.61
A5 $5 \times 10^4$ 10 36 41.5 27.9 7.7 13.5 7.0 10.1 0.39
A6 $5 \times 10^4$ 25 26 42.7 29.3 8.7 14.0 6.3 11.6 0.33
Pos B       32.5 18.2 - 9.5 0.78 5.1  
B1 $\ast$ $3 \times 10^4$ 1 6 30.9 17.2 2.0 8.0 0.92 5.4 0.32
B2 $3 \times 10^4$ 2 6 32.7 19.0 2.6 8.1 0.56 7.1 1.29
Pos C       40.2 25.0 - 9.20 0.89 8.5  
C1 $\ast$ $5 \times 10^4$ 7 8 38.8 26.5 7.1 9.5 0.93 9.2 0.12
Pos D       51.7 37.8 22.7 13.8 6.0 7.4  
D1 $\ast$ $7 \times 10^4$ Star 22 54.7 42.9 19.6 16.1 5.9 8.0 0.32
Isobaric models                    
Model p G0 $A_{\rm V}$ CO   CO   CO   13CO C18O C I   $\chi^2$
        (2-1) (4-3) (6-5) (2-1) (2-1) (1-0)  
D2 $4 \times 10^6$ Star 22 60.0 51.1 29.2 15.7 5.7 6.23 1.16
D3 $2\times 10^6$     54.0 42.5 20.7 15.6 5.7 6.6 0.23
D4 $1\times 10^6$     48.4 34.7 13.7 15.4 5.8 7.2 0.77
N OTES: Entries in the table are: Model number, computed intensities of the CO (2-1), (4-3), (6-5), 13CO (2-1), C18O (2-1), and C I (1-0) emission (K km s-1), as well as the $\chi^2$ value of the fit. The observed line intensities for positions A-C that are used in the fit are also given (we estimate the calibration uncertainty to be $\sim $20%). Best-fit models are marked with an asterisk. The radiation field G0 (Pos A - C) in Draine units. The neck is illuminated double-sided with G0 from the front $\underline {and}$ back side.

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