The D/H transition in photon dominated regions

E. Roueff  & T. Nodé-Langlois 

DAEC, Observatoire de Paris, Place Janssen, 92190 Meudon, France

We have introduced the deuterium chemistry in our equilibrium model of Photon Dominated Regions. The geometry is supposed to be plane parallel with a UV field impinging on one side of the cloud (Abgrall et al. 1992, Le Bourlot et al. 1993). The HD molecule has the same electronic structure as H₂, but has a small permanent dipole moment due to the non coincidence between the centre of mass and the centre of charges. Then, electric rotational transitions may occur inside the ground level.

The new developments introduced in our numerical code are:

• the Lyman and Werner transitions of HD (whose transition probabilities have been calculated including the rotational centrifugal terms) to calculate the UV pumping and the photodissociation probabilities.

• the radiative dipolar cascades taking place inside the ground level.

• the chemical formation/destruction processes on the grains and in the gas phase.

• the collisional excitation and desexcitation of rotational levels of HD due to H, He, H₂ and electrons.

The photodissociation probabilities of the molecules for which an explicit UV transfer is performed will be shown. The hierarchy of photodissociation probabilities is consistent: the self-shielding of the H₂ molecule is the most efficient. The same process takes place for HD at larger visual magnitudes. Finally, predissociation occurs in CO.

We have calculated the rotational equilibrium of HD and the rotational transitions emissivities for different values of the density and interstellar radiation fields which will be presented.