3. General method of analysis
3.1. Numerical code
The WIDTH9 code and new grid of atmosphere models (Kurucz 1992) were used to derive the abundances. Although these models do not take into account the sphericity of the supergiant atmospheres, as well as possible increased helium content (that could be expected for Cepheids), they are commonly in use and this allows us to perform a correct comparison of the independently obtained results on elemental abundances.
3.2. Oscillator strengths
Special attention was given to oscillator strengths selected for abundance analysis. In this work we used so-called "solar" values. They were derived by us using unblended solar lines (based on solar spectrum by Kurucz et al. 1984). The equivalent widths of selected solar lines were accurately measured using the software of Galazutdinov (1992). Then, for each line of the considered element we corrected the initially adopted value to produce the solar elemental abundances (Grevesse et al. 1996) in the calculations. This was performed using the WIDTH9 code, solar atmosphere model from Kurucz's grid with the microturbulent velocity of 1 km s-1. Oscillator strengths adopted for the further analysis are collected in the Appendix.
A comparison of our oscillator strengths for 415 common lines with of Gurtovenko & Kostyk (1989) shows a small systematical difference (about 0.1 dex). This difference is easily explained taking into account the difference in the adopted solar iron abundance (Gurtovenko & Kostyk used , while we employed 7.50). The result of the comparison is given in Fig. 1.
In their fundamental spectroscopic study of 23 galactic Cepheids, Fry & Carney (1997) found a similar small difference comparing their "solar" values with those of Gurtovenko & Kostyk (1989). Finally, we have compared for 101 common lines with Fry & Carney (1997) and found an excellent agreement (see Fig. 2). It should be mentioned that our oscillator strengths were also compared with the compilative data used by Luck (1996) in his studies and no significant differences were found.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999