The Paschen-Back effect in the Li I 6708 Å line and the presence of lithium in cool magnetic Ap stars ^*

Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden e-mail: oleg@astro.uu.se

Received: 16 October 2007
Accepted: 17 February 2008

Abstract

Context. A number of cool magnetic Ap stars show a prominent feature at λ 6708 Å. Its identification with Li I 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.

Aims. We suggest investigating the 6708 Å 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.

Methods. The small separation of the Li I 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 Li I 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 HD 116114, HD 166473, and HD 154708, which have a mean field strength of 6.4, 8.6, and 24.5 kG, respectively, and show a strong 6708 Å line.

Results. High-resolution spectra for the 6708 Å region were analysed for 17 magnetic Ap stars. The presence of the 6708 Å line is confirmed for 9 stars and reported for the first time for 6 stars. The strength of the Li I doublet does not correlate with the absorption features of any other element. The stars HD 75445 and HD 201601 provide an extreme example of the two objects, which are dissimilar with respect to the 6708 Å line, but very close in the atmospheric parameters and abundances of other elements. We demonstrate that the observed profiles of the 6708 Å line in the strong field stars HD 116114, HD 166473, and HD 154708 correspond fairly well to the theoretical calculations when assuming the Li I identification. Including the Paschen-Back effect improves the agreement with observations, especially for HD 154708.

Conclusions. Results of our study confirm the Li I identification proposed for the 6708 Å line in cool Ap stars.