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Astron. Astrophys. 363, 1091-1105 (2000)

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6. Comparison of broadening mechanisms

We now compare the relative strengths of broadening mechanisms in the wings of lower Balmer lines through a model solar atmosphere. As the profiles are not necessarily of the same shape, the best way to do this is to compare the depth of the normalised profile at some suitable detuning from line centre.

We plot the profile depth of each broadening mechanism profile at 5 Å detuning through the Holweger & Müller (1974) model solar atmosphere in Fig. 5. When one considers that the wings of the lines are formed in the region around [FORMULA], we clearly see that the new theory makes a significant difference when compared with Ali & Griem's theory. In this region, rather than being weaker than the Stark broadening contribution, the contribution of self-broadening is now often comparable.

[FIGURE] Fig. 5. The broadening mechanisms through the model solar atmosphere of Holweger & Müller (1974) (left) and a MARCS (Asplund et al. 1997) model atmosphere of a metal poor [Fe/H][FORMULA] star of solar temperature and gravity (right) for H[FORMULA] (top), H[FORMULA] (middle) and H[FORMULA] (bottom). The lines plot the depth of the line profile for our self-broadening theory (full), the Ali & Griem (1966) resonance broadening theory (dot-dash) and for the Stehlé (1994) Stark broadening theory (dash), at 5Å detuning from the line centre. Note that the Stark profiles are folded with Doppler profiles, however, Doppler profiles make negligible contribution at this detuning for these temperatures.

In cool metal poor stars, the electrons and ions are out-numbered by hydrogen atoms by an even greater number than in stars around solar metallicity. Thus self-broadening in hydrogen lines becomes even more important. Fig. 5 also shows similar plots to those shown for the solar model, for a MARCS model (Asplund et al. 1997) with solar temperature ([FORMULA] K) and surface gravity ([FORMULA]) but [Fe/H][FORMULA]. Here we see clearly that the new theory has a significant effect on the contribution to broadening in the line forming region. At great depth the Stark broadening always dominates due to the higher ion/electron density while self-broadening (using our theory) dominates above an optical depth of 0.1 for solar composition and above an optical depth of 1 in the metal deficient case.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000