3. THE INPUT PARAMETERS

                    3.1 Input parameters depending on the shock

Shock velocity V_s: although the FWHM of the line profiles indicate the velocity of the emitting gas rather than that of the shock, as a first trial FWHM roughly suggest the V_s range.

Preshock density n₀ : for a first trial, it is indicated from the characteristic line ratios (e.g. [SII]6548/[SII]6584), considering, however, that n₀ is by a factor 5-100 less than the density of the downstream gas emitting the lines, depending on V_s.

Preshock magnetic field B₀:  it is generally assumed from the observations related to polarization. Its value is then refined phenomenologically. B₀ is a crucial parameter because a high component transversal to the shock direction can substantially reduce compression.

                   3.2 Input parameters depending on photoionization

   Different shapes of the primary radiation can be used:

a) Power-law radiation: it is  characterized by the spectral indices   α_UV and α_X, in the UV and in the X-ray domains, respectively., and by   F_ν that is the flux intensity at 1 Rydberg, in units of photons cm^-2 s^-1 eV_-1 and is constrained mainly by the [OIII] 5007/[OII] 3727 line ratios, while the spectral index by the UV line ratios.

b) Black-body radiation:  it is characterized by the stellar temperature T_S, intended as a color temperature, and the ionization parameter U. Both values are strongly constrained by the HeII/HeI line ratios.

c) Black-body + power-law

The composite spectrum consisting of black-body in the UV and power-law in the X-ray domain is also considered by the code.

                  3.3 Input parameters depending on  the dust

Dust-to-gas ratio d/g: it is directly related to the intensity of the infrared peak of the reprocessed radiation  by dust., and depends on the ratio of  the reprocessed radiation to bremsstrahlung in the IR, throughout the SED of the continuum. Notice that a high d/g, speeding up the cooling rate downstream, may change a non-radiative shock into a radiative one.

Grain radius a_gr:  the smaller the grains, the higher the temperature reached by radiation heating, and  the faster they are eroded by sputtering in grain-gas collisions.

                 3.4 Relative abundance of the elements

The following elements are considered : H, He, C, N, O, Ne, Mg, Si, S, Cl, A,and Fe. They are constrained by the ratios of single element lines. The high abundance of a strong coolant element (e.g. oxygen), affecting the cooling rate, may substantially change both line and continuum spectra.

                 3.5 Geometrical thickness of the nebula

The parameter D (together with the flux intensity and the density) determines whether the model is matter-bound or radiation-bound. In case str=1, a low D reduces the region of relatively low temperature in the internal region of the cloud, and leads to less intense, even negligible, neutral lines (Fig. 1).