Astron. Astrophys. 319, 669-672 (1997)

4. Discussion and conclusion

When interpreting the observed prominence emission lines, one has to consider that theoretical models generally assume they are optically thin. Thicker prominences are modelled by superpositions of several intrinsically thin slabs, the number of threads along the line-of-sight then determining the final brightness. However, the physical slab width influences e.g. the He singlet-to-triplet ratio. The calculations by Heasley et al. (1974) and by Heasley & Milkey (1976) show that this ratio is sensitive to the optical thickness in the He 584 Å resonance line. In the optically thin case, photons escape in the He 584 line leading to a depopulation of the lower level of the He 6678 transition. In the optically thick case, this depopulation is reduced and the He 6687-to-He 4471 emission ratio increases. Higher non-thermal line broadening, , also increases the escape probability in the He 584 line, thus lowering the singlet-to-triplet ratio.

Comparing our quasi-simultaneous, high resolution obervations with the model calculations by Heasley & Milkey (1976), we find that our largest singlet-to-triplet ratios of 0.25 (filled squares in Fig. 1) require thicker slabs than assumed in the models, explaining the increase of the singlet-to-triplet ratios. Indeed, pictures from the preceeding days indicate that the corresponding filament is rather 'clumpy'. The largest ratios are found in low-lying (h  arcsec) regions of prominences W-15 and W-25 (cf. Fig. 1), for W-15 these are regions of highest emission.

The high slab widths are also required to explain the significantly low He-D₃ -to- ratios down to 0.43 (cf. Fig. 2) occurring in the brightest parts of prominence W-15 since the calculations by Heasley & Milkey (1976) show a systematic decrease of this relation for increased slab width.

The faint isolated knots at the top of the prominences (ejecta) which exhibit high He-D₃ / ratios (up to 0.9), require small physical slab widths, in agreement with their small spatial widths of 1 arcsec. The observed mean singlet-to-triplet ratio E(He 6687)/E(He 4471) implies that the He-to-H number ratio cannot be much lower than the value of y=0.1 assumed in the models.

Besides the line ratios, the absolute line emissions, E, of our prominences are much higher than the values calculated by Heasley & Milkey (1976; Tables 1, 2). The theoretical models could achieve higher emissions via larger optical thickness by linear superposition of many elements along the line-of-sight keeping the individual slab width constant and the density low. Their model C ( =9500 K, =0.065 dyn/cm²) with and and =2.5  cm^-3 could then also match our mean singlet-to-triplet ratios of found for large E(He 4471) values. The low densities in that model are also conform with the pressure values P  dyn/cm² deduced from our observed Ca -to- ratio.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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