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Astron. Astrophys. 329, 792-798 (1998)

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5. Asymmetry

In electron broadening, asymmetry is due: firstly to the difference between the lower cut-offs ([FORMULA]) in the integration over the perturber energies for both wings; and secondly to the introduction of the short range potentials contribution. Therefore, in the dipole approximation, the asymmetry is expected to decrease as the temperature increases. The asymmetry of the profile is represented by the quantity:

[EQUATION]

The variations of [FORMULA] due to the lower cut-off are presented in Fig. 2 at [FORMULA] and 40000K. For comparison, short range effects due to quadrupole and polarization potentials (Tran Minh et al. 1980) are shown in Fig. 2 for [FORMULA] and 10000K.

[FIGURE] Fig. 2. Asymmetry of the total electronic profile; a corresponds to T=5000K, b to T=5000K(*), c to T=10000K(*) and d to T=40000K. The asterisks point out the cases where the short range effects (quadrupole + polarization) are taken into account.

Fig. 2 leads to two main observations. At [FORMULA] K the asymmetry is very small (close to 0). This was to be expected since in this case the lower cut-off tends to [FORMULA] tends to 0. More generally, the asymmetry decreases with increasing temperature.

It is to be mentioned (Allard et al. 1994) that quasi-molecular effects due to [FORMULA] and [FORMULA] collisions are a source of asymmetry in the far wings. Further, when ions are included, they contribute significantly to asymmetry. These last two points have not been taken into account in our work since we are interested in the electronic contribution to the lineshapes.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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