A number of cool magnetic Ap stars show a prominent feature at lambda 6708 A. Its identification with Li I remains controversial due to a poor knowledge of the spectra of rare-earth elements that are strongly enhanced in peculiar stars and can potentially provide an alternative identification. We suggest to investigate the 6708 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 to use the whole line profile instead of a mere coincidence of the observed and predicted wavelength. Due to a small separation of the Li I doublet components, their magnetic splitting pattern deviates from the one expected for the Zeeman effect even in relatively weak fields. We carry out detailed calculations of the transition between the Zeeman and Paschen-Back regimes in the magnetic splitting of the Li I line and compute polarized 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 A line. High-resolution spectra for the 6708 A region are analysed for 17 magnetic Ap stars. The presence of the 6708 A line is confirmed in 9 stars and reported for the first time in 6 stars. We demonstrate that the observed profiles of the 6708 A line in the strong field stars HD 116114, HD 166473 and HD 154708 correspond rather well to the theoretical calculations assuming the Li I identification. Inclusion of the Paschen-Back effect improves the agreement with observations, especially for HD 154708. Results of our study confirm the Li I identification proposed for the 6708 A line in cool Ap stars.
Formation of the optical spectra of the coolest M- and L-dwarfs and lithium abundances in their atmospheres
理论方面的建模的光谱晚M-和L-矮星进行了讨论。我们指出，M-矮星和L-矮星光谱的形成过程是基本不同的。与M-矮星的情况不同，Ti和V的原子在L-矮星的大气中应该被耗尽成颗粒。L-矮星光谱的整体形状受K I + Na I共振线翼的巨大强度控制。为了拟合在最冷的矮星光谱中观察到的锂线，我们使用了两个额外的建议：a）在L-矮星的光学光谱中吸收了一些额外的分子种类; b）可能有（一些？）额外的（"dusty“？）大气层中的不透明源。锂线的形成和“自然”的限制，他们使用的“锂测试”的情况下，L-矮星的问题被认为是。
The dispersion in lithium abundance at fixed effective temperature in young cool stars like the Pleiades has proved a difficult challenge for stellar evolution theory. We propose that Li abundances relative to a mean temperature trend, rather than the absolute abundances, should be used to analyze the spread in abundance. We present evidence that the dispersion in Li equivalent widths at fixed color in cool single Pleiades stars is at least partially caused by stellar atmosphere effects (most likely departures from ionization predictions of model photospheres) rather than being completely expl…
We have determined Li abundances in 55 metal-poor (3.6 < [Fe/H] < -0.7) stars with extreme orbital kinematics. We find the Li abundance in the Li-plateau stars and examine its decrease in low-temperature, low-mass stars. The Li observations are primarily from the Keck I telescope with HIRES (spectral resolution of ~48,000 and median signal-to-noise per pixel of 140). Abundances or upper limits were determined for Li for all the stars with typical errors of 0.06 dex. Our 14 stars on the Li plateau give A(Li) = log N(Li)/N(H) + 12.00 of 2.215 +-0.110, consistent with earlier results. We find a d…
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
