2.1. Spectroscopic material
The stars selected for observation are described in Table 1. Classification as a blue straggler is given in accordance with the catalogue of Ahumada & Lapasset (1995). Because some of the program stars belong to spectral class A, we decided also to observe the A-type star CMa (Sirius) as a control on our data reduction method and abundance analysis.
Table 1. Program stars.
The observations were performed with the ESO 3.6-m telescope equipped with the CASPEC echelle spectrograph during May 1985. Resolving power and S/N ratio were 20000 and about 100 at the continuum level, respectively (for the faintest main sequence stars the S/N is slightly smaller).
2.2. Reduction of the spectra
All of the echelle orders were extracted from the images, flat-fielded and wavelength calibrated using the MIDAS software. Preliminary visual inspection showed that the blue stragglers usually possess sharp, well-defined spectral lines, while the observed main sequence stars appeared to be rather fast rotators. For sharp-lined spectra the continuum placement is rather reliable, but for the rotationally broadened spectra, continuum within some orders is not well defined. We excluded those orders from the further analysis.
Where it was possible (i.e. only for the sharp unblended lines), equivalent widths were measured directly under the assumption that the shapes of the lines can be approximated by a gaussian profile. After visual inspection of the rotationally broadened spectra of the main sequence stars, it became clear that the Am star CMa could hardly be considered an appropriate reference for our abundance analysis of those stars. Nevertheless, we kept CMa as a check on the abundance determinations.
We compared our equivalent widths from the CMa spectrogram with those given by Sadakane & Ueta (1989). It appeared that on the average our values are systematically about 10% greater than those of Sadakane & Ueta (1989). This small difference is likely due to the higher quality of the spectroscopic material used by these authors (the reciprocal dispersion was 1 Å mm-1, while we estimate our value as about 7Å mm-1).
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000