3. Method of analysis and physical data
The spectra were analysed using a differential model atmosphere technique. The Eqwidth and Synthetic Spectrum program packages, developed at the Uppsala Astronomical Observatory, were used to carry out the calculations of theoretical equivalent widths of lines and synthetic spectra. A set of plane parallel, line-blanketed, flux constant LTE model atmospheres was computed with an updated version of the MARCS code of Gustafsson et al. (1975) using continuous opacities from Asplund et al. (1997) and including UV line blanketing as described by Edvardsson et al. (1993). Convection was treated in the mixing-length approximation ().
The Vienna Atomic Line Data Base (VALD, Piskunov et al. 1995) was extensively used while preparing the input data for the calculations. Atomic oscillator strengths for this study were taken mainly from two sources: the first being an inverse solar spectrum analysis done in Kiev (Gurtovenko & Kostik 1989, Gurtovenko et al. 1983, 1985a, 1986), the second being high-precision laboratory measurements done in Oxford (Blackwell et al. 1982, 1983, 1986). The coincidence of these two sets of gf values is very good, and the errors in the least-squares fit do not exceed dex (Gurtovenko et al. 1985b). For Ca I the gf values have been also taken from Smith & Raggett (1981) and for Zr I from Bogdanovich et al. (1996).
Using the gf values and solar equivalent widths of analysed lines from the cited sources we have obtained the solar abundances, later used for the differential determination of abundances in the programme stars. We used the solar model atmosphere from the set calculated in Uppsala (Edvardsson et al. 1993) with a microturbulent velocity of 0.8 , as derived from Fe I lines.
Abundances of carbon, nitrogen and europium were determined using the spectrum synthesis technique. The interval of 5632-5636 Å was synthesized and compared with observations in the vicinity of the Swan 0-1 band head at 5635.5 Å. The same atomic data of as used by Gonzalez et al. (1998) were adopted for the analysis. The interval of 7980-8130 Å, containing strong CN features, was analysed in order to determine the nitrogen abundance and ratios. The molecular data for and were taken from ab initio calculations of CN isotopic line strengths, energy levels and wavelengths by Plez (1999), with all gf values increased by +0.03 dex in order to fit our model spectrum to the solar atlas of Kurucz et al. (1984). Parameters of other lines in the intervals of spectral synthesis were compiled from the VALD database. In order to check the correctness of the input data, synthetic spectra of the Sun were compared to the solar atlas of Kurucz et al. (1984) and necessary adjustments were made to the line data.
An interval of 6643-6648 Å, containing the Eu II line at 6645 Å, was analysed in order to determine the europium abundance. The oscillator strength of the Eu II line, log gf=0.17, was adopted from Gurtovenko & Kostik (1989). The solar abundance of europium, later used for the differential analysis, log=0.49, was determined using the same procedure as for other heavy chemical elements. Parameters of other lines in the interval were compiled from the VALD database. CN lines were also included, but none of them seems to affect the europium line significantly.
In addition to thermal and microturbulent Doppler broadening of lines, atomic line broadening by radiation damping and van der Waals damping were considered in the calculation of abundances. Radiation damping parameters for the most of lines were taken from the VALD database. When they were not available at the VALD database, published oscillator strengths of strong lines were used for determination of life times and thus radiation damping for relevant energy levels. Correction factors to the classical van der Waals damping widths were taken from the literature: Na I: Holweger (1971), Ca I: O'Neill & Smith (1980), Ba II: Holweger & Müller (1974), Fe I: Simmons & Blackwell (1982). For all other species a correction factor of 2.5 was applied to the classical =+1.0), following Mckle et al. (1975). For lines stronger than W=100 mÅ, the correction factors were selected individually by the inspection of the solar spectrum.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000