Non-LTE line formation for heavy elements in four very metal-poor stars ^*

¹ Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany e-mail: lyuda@usm.lmu.de
² Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia e-mail: lima@inasan.ru
³ National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, PR China
⁴ National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan
⁵ Liberal Arts Education Center, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
⁶ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China

Received: 11 June 2007
Accepted: 16 November 2007

Abstract

Aims.Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor (VMP) stars (Fe/H). For two of them, HD 84937 and HD 122563, the fraction of the odd isotopes of Ba derived for the first time.

Methods.Determination of an effective temperature, surface gravity, and element abundances was based on non-local thermodynamic equilibrium (non-LTE) line formation and analysis of high-resolution ( and 90 000) high signal-to-noise () observed spectra. A model atom for is presented. An effective temperature was obtained from the Balmer H_α and H_β line wing fits. The surface gravity was calculated from the Hipparcos parallax if available and the non-LTE ionization balance between and . Based on the hyperfine structure affecting the resonance line , the fractional abundance of the odd isotopes of Ba was derived from a requirement that Ba abundances from the resonance line and subordinate lines of must be equal.

Results.For each star, non-LTE leads to a consistency of T_eff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from the ionization balance between and . For each star with a known trigonometric surface gravity, non-LTE abundances from the lines of two ionization stages, and , agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for all investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd, equals 0.. The latter can serve as an observational constraint on r-process models. The lower Eu/Ba ratio and found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star.