6. The model atmosphere analysis
The high-resolution spectra were analyzed with the model atmosphere technique. Stellar parameters were estimated from Strömgren photometry, from spectrophotometry, from H profiles, from IUE ultraviolet colours and (for nine stars) from () colours. The estimates obtained by these different methods and the values of the parameters that we adopted are given in Table 7.
Table 7. Comparison of the stellar parameters obtained by different methods.
Table 7. (continued)
Table 7. (continued)
Table 7. (continued).
Having fixed the model atmosphere, we computed abundances from both the equivalent widths and line profiles for each star observed at KPNO, except for BD +00 0145 and HD 14829. For these two stars and for the BHB stars that were observed at ESO (whose spectra only extended over 50 Å) we estimated the abundances from the equivalent widths alone.
We used model atmospheres and fluxes that were computed by Castelli with an updated version of the ATLAS9 code (Kurucz 1993a). We adopted models for stars with an -element enhancement =+0.4 (which is generally appropriate for halo stars). The symbol "a" near the metallicity in Column 6 of Table 7 indicates when these models were used. The convective models ( 8750 K) were calculated with the no-overshooting approximation. More details about these models can be found in Castelli et al. (1997) and in Castelli (1999). The synthetic grids of Strömgren indices, Johnson () indices and H profiles were also computed by Castelli from the above models. Grids of models, fluxes and colours are available either at the Kurucz website (http://kurucz.harvard.edu ) or upon request.
We derived abundances from the equivalent widths using the WIDTH code (Kurucz 1993a), modified so that we could derive the Mg abundance from the measured equivalent width of the doublet Mg II 4481 Å. The SYNTHE code (Kurucz 1993b), together with the atomic line lists from Kurucz & Bell (1995), were used to compute the synthetic spectra.
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000