2 Observations

2.1 The sample

We have undertaken a high-resolution optical spectroscopic survey of the upper main sequence stars brighter than V = 10 for which only low- or medium-dispersion spectra (120-40 Å mm^-1), or only one radial-velocity value, are available in the literature. We also selected several spectroscopic binaries (SB1 and SB2) to improve the orbital elements. The final programme contains a total of 37 stars, including 9 candidate members in the cluster outer part proposed by Raboud et al. (1997).

2.2 High dispersion spectra

The programme stars were observed at CASLEO in the optical region, between $\lambda$ 3750 Å, and $\lambda$ 4861 Å, at the 2.1 m telescope, equipped with a REOSC echelle spectrograph with a CCD TEK $1024 \times 1024$ detector. We have used a grating of 400 grooves mm^-1 as a cross disperser. This instrumental configuration gives a 2 pixel resolution of 0.30 Å. In this configuration, the echelle spectrograms cover a useful wavelength range of about 1500 Å, with a resolving power of $\sim$14000. A total of 113 spectrograms for the 37 programme stars, were obtained between 1995 and 1999. Most spectra present a S/N ratio between 50 and 80 for typical exposure time of 30 min.

We have obtained the usual series of bias and flat-field frames to carry out the reduction of the images. A Th-Ar lamp was used for wavelength calibration. The pixel-to-pixel variations were removed by flat-field division and the spectra were extracted, wavelength calibrated, normalized and measured using IRAF routines from version 2.10.

We also obtained spectra for standard stars from Fekel (1985, 1991) to establish the zero point correction for our observations, which we find to be negligible.

2.3 Heliocentric radial velocities in NGC 6231

We have measured the radial velocities for the program stars by fitting Gaussians, using splot routine in IRAF.

Mean values are presented in Table 1, described below (see Sect. 2.6) and individual observations are listed in Table 2. The latter table gives successively the identification in the system of Col. 2 in Table 1 the heliocentric Julian Day, the heliocentric radial velocities, the probable error, number of lines measured. On several occasions, line doubling was observed and radial velocities for both components are tabulated. The radial velocity of the primary component is given first.

2.4 Spectral types

We have estimated spectral types for 11 program stars lacking MK classification in the literature, using the cluster members with known spectral types as comparison, to complete the description of this sample.

2.5 Axial rotation

In order to have approximate values for the projected axial rotational velocities, we have measured on the spectra of the stars with rotational velocities published by LM83, the FWHM of the spectral lines usualy taken into account for this type of study, namely HeI $\lambda$ 4026 and HeI $\lambda$ 4471, and we have used the average values to obtain the coefficients of a linear regression ($V\sin i$ vs. FWHM). From these data, we have obtained new $V\sin i$ values for the program stars of luminosity classes III, IV, and V and not earlier than O9. For supergiant stars or stars of earlier spectral types, the broadening of the spectral lines is modified by turbulence. In the cases of the stars S290 and 501, we have assigned a probable value of projected rotational velocity based on the width of the HeI lines, to give an estimate of their behaviour; the first star shows broadened lines, while star 501 presents very narrow profiles. We have transformed all data to the new Sletteback system (Slettebak et al. 1975) using García & Levato (1984) calibration.

2.6 Results

Table 1 lists fundamental photometric and spectroscopic data for the stars brighter than V = 10.40 ($V_{0}\; <$ 9.00) in NGC 6231. The identifications are given in the first three columns: (1) the HD/CoD numbers, (2) numbers <295 are from Seggewiss (1968) and those >501 from Raboud (1996), and (3) numbers from Sung et al. (1998). Numbers larger than 853 are an extension of their numbering for the brightest stars they did not observe, as displayed in the WEBDA database. The next two columns reproduce the V₀ magnitudes (4) and X colour indices (5) in the Geneva photometry from Raboud et al. (1997). Then we tabulate the $V\sin i$ data (6) and their sources (7). Reference 1 corresponds to this paper, Ref. 3, to LM83, Ref. 8, to Buscombe (1969), Ref. 9 to Conti & Ebbets (1977), and Ref. 10 corresponds to Killian et al. (1994). The mean radial velocities from our observations, the uncertainties and the number of spectra obtained are given in the next three Cols. (8-10). Published and new spectral types are given as well as the sources (11-12). Reference 1 corresponds to our estimates, while Ref. 2 corresponds to Levato & Malaroda (1980), Ref. 4 to Sahade (1958), Ref. 5 to Schild et al. (1971), Ref. 6 to PHYB, and Ref. 7 corresponds to Garrison & Schild (1979). Column (13) gives the magnitude differences between the star V magnitude and the magnitude on the ZAMS at the star colour. This quantity measures the vertical displacment above the ZAMS. Finally, notes describe the binary characteristics (14).

No radial velocities have been given for S28, S110, S266, S286, S253, and S312 because they are double-lined systems or binaries with large amplitudes which makes mean values meaningless.

Details about the cluster or the star members and bibliographic information can be obtained from the WEBDA open cluster database, including a map of the cluster.