Astronomy and Astrophysics, volume 483, 557-566 (2008/5-4)
The Paschen-Back effect in the Li I 6708 Å line and the presence of lithium in cool magnetic Ap stars.
Abstract (from CDS):
A number of cool magnetic Ap stars show a prominent feature at λ 6708Å. Its identification with LiI remains controversial due to the lack of knowledge of the spectra of rare-earth elements that are strongly enhanced in peculiar stars so they can potentially provide an alternative identification. We suggest investigating the 6708Å line in Ap stars with strong magnetic fields. In these objects, the magnetic broadening and splitting provides an additional, powerful criterium for line identification, allowing the whole line profile to be used instead of depending on a mere coincidence in the observed and predicted wavelengths. The small separation of the LiI doublet components means that their magnetic splitting pattern deviates from the one expected for the Zeeman effect, even in relatively weak fields. We carried out detailed calculations of the transition between the Zeeman and Paschen-Back regimes in the magnetic splitting of the LiI line and computed polarised synthetic spectra for the range of field strength expected in Ap stars. Theoretical spectral synthesis is compared with the high-resolution observations of cool Ap stars HD116114, HD166473, and HD154708, which have a mean field strength of 6.4, 8.6, and 24.5kG, respectively, and show a strong 6708Å line. High-resolution spectra for the 6708Å region were analysed for 17 magnetic Ap stars. The presence of the 6708Å line is confirmed for 9 stars and reported for the first time for 6 stars. The strength of the LiI doublet does not correlate with the absorption features of any other element. The stars HD75445 and HD201601 provide an extreme example of the two objects, which are dissimilar with respect to the 6708Å line, but very close in the atmospheric parameters and abundances of other elements. We demonstrate that the observed profiles of the 6708Å line in the strong field stars HD116114, HD166473, and HD154708 correspond fairly well to the theoretical calculations when assuming the LiI identification. Including the Paschen-Back effect improves the agreement with observations, especially for HD154708. Result. of our study confirm the LiI identification proposed for the 6708Å line in cool Ap stars.