6 Conclusions

As mentioned in the introduction, the problem is to determine accurate relaxation rates by collision with H atoms. In the case of sodium, hyperfine rates should be useful for modelling the linear polarization observed in the center of the D₂ line (Kerkeni & Bommier 2002). Collisional depolarization in the upper level occurs if the rate is larger than the spontaneous emission coefficient A. Assuming $A \simeq 10^{8} ~$s^-1, collisional depolarization in the upper state is complete if $G_{2} \geq 10^{8} ~$s^-1. As the G₂ coefficients are of the same order of magnitude as $g^{\kappa}$ coefficients, and assuming a temperature of 5000 K, we can conclude that the upper level is depolarized for hydrogen densities larger than (10¹⁷  cm^-3).

As previously mentioned (see Paper I), depolarization in the ground level occurs for much lower densities (10¹⁴  cm^-3). Of course, the correct diagnostic requires the resolution of the rate equations including radiative and collisional processes between hyperfine levels. This work is in progress (Kerkeni & Bommier 2002).

Acknowledgements

The advice and assistance of N. Feautrier and A. Spielfiedel is gratefully acknowledged. I wish to thank E. Landi Degl'Innocenti for helpful discussions. I acknowledge the referees for helpful comments that improved and clarified the understanding of the paper. The computations were performed on the work stations of the computer center of Observatoire de Paris.