Collisional broadening of Mg, Sr, Ca and Na resonance lines by atomic hydrogen

International audienceThis paper compares different approaches used in the calculation of the broadening of spectral lines by H-atom collisions. Firstly, the validity of the semi-classical approach for the collision versus the quantum one is discussed. It is shown that, at the temperatures typical of stellar atmospheres (from 3000 to 10 000 K), a classical approach (with the advantage of reduced computation times) is sufficient. The dependence of the broadening constants on interatomic potentials is also studied. Two different approaches were used to derive these potentials: in the first approach, the interaction energy is determined by the usual methods of quantum chemistry. The second approach, developed by Anstee, Barklem and O'Mara (ABO potentials), is based on second-order perturbation theory. In the case of Mg H, a hybrid potential obtained from ab initio values for the short distances and from the perturbation method in the asymptotic region was also tested. The results for the Na resonance line show that even significant differences in the potentials lead to relatively small changes in the calculated widths. From the comparison of the results for the Mg, Sr and Ca resonance lines, it appears that ABO potentials give results of the order of 8 20% smaller than results from ab initio and hybrid potentials. This difference is attributed to the presence of an avoided ionic crossing in the upper singlet Sgr states that coincides roughly with the Weisskopf radius