6 Conclusion

By combining new high-resolution spectroscopic observations of 37 stars with those available in the literature for the members of NGC 6231, we got the following results:

Figure 8: Normalized CCD observations for S292 and S309, in H$_{\gamma }$ region

Figure 9: Same as Fig. 8 for S34

Figure 10: Same as Fig. 8 for S34, in HeI $\lambda$4026 region

Figure 11: Same as Fig. 8 for 769

Figure 12: Radial velocity components for S28, in H$_{\beta }$ region

• We have confirmed the binary nature for 28 members of the cluster;
• The orbital elements of 7 SB systems were calculated, three of them for the first time;
• We found a global binary frequency of 82% for the stars earlier than B1.5V;
• 79% of the O-type stars, are binary systems, and among the binaries, 86% are double-lined objects;
• The average baricentric velocity we obtain, -30.7 $\pm$ 0.9 km s^-1, is in agreement with previous studies;
• The average ratio between the rotation of the cluster members and field stars is 0.67 $\pm$ 0.04;
• The percentage the binaries with periods lower than 100 days, fits fairly well on the Abt & Sanders (1973) correlation. This result contributes to demonstrate that the overall axial rotation is lower in clusters rich in binary systems (see Morrell & Levato 1991, Fig. 4);
• It is also remarkable that all except one periods in NGC 6231 O-type binaries are less than 10 days, with non-zero eccentricities.

From the comparison with other very young open clusters containg O-type stars, we distinguish two different scenarios: either the clusters have many (more than 6) O-type stars which are distributed over the cluster field and are mostly binaries or "single'' or they have one or two O-type stars, which form a double-lined spectroscopic binary, belonging to herarchical triple or quadruple systems, or to trapezia, with a period close to 3 days and are often located at the cluster center. In the latter case, most of the mass is strongly concentrated in the central multiple system.

The similarity of the value of the periods of the inner spectroscopic binary, around $P \sim$ 3 days, is quite striking. We can wonder what are the reasons for such a similarity and if the short 3 days periods result from the formation of the O-type binaries or from the early evolution of the multiple systems or inner part of the clusters. Due to the very small ages of these clusters, we are inclined to conclude that the observed difference must result, for a large part, from the formation processes, rather than from early dynamical evolution inside the molecular cloud, but appropriate simulations will be necessary to settle the question.

Figure 13: Same as Fig. 8 for S110

Figure 14: Radial velocity components for S110, in H$_{\beta }$ region

Figure 15: Same as Fig. 8 for S161, in He I $\lambda$4471 region

Figure 16: Same as Fig. 8 for S253, in HeI $\lambda$4026 region

Figure 17: Same as Fig. 8 for S253, in H$_{\beta }$ region

Figure 18: Same as Fig. 8 for S286, in HeI $\lambda$4471 region

Raboud & Mermilliod (1998) found mass segregation among the cluster members and between binaries and single stars in NGC 6231, although this cluster is likely still not dynamically relaxed. The massive stars occupy a much smaller volume than the less massive stars. The present study contributes to prove, by spectroscopic methods, the binary nature of several stars, which was based so far mostly on photometric criteria. Radial-velocity observations are especially important in the upper part of the main sequence where the seqeunce is nearly vertical. For most clusters listed in Table 5 the massive multiple systems are located close to the cluster center.

In this study, we tentatively conclude that the cluster density depends on the duplicity rate for rich clusters with many O-type stars. But in less rich clusters, O-type stars belongs to multiple systems which may be massive enough to ensure the stability of the surrounding clusters. The overall structure and the evolutionary history of these clusters are probably different.

NGC 6231 is an exceptional cluster because of its young age ($\log t$ = 6.75) and the rich stellar content. Multi-fiber spectrographs mounted on large telescopes will permit to study the duplicity and rotation of stars all along the main sequence to describe the characteristics of a very young open cluster just emerged from its parent cloud.

Further spectroscopic observations, as well as a search for "wider'' components with adaptive optics or speckle interferometry, of O-type stars in more very young open clusters would help confirming and understanding the differences found in this paper.

Acknowledgements

We thank the anonymous referee for his comments which helped to improve the paper. The authors acknowledge use of the CCD and data acquisition system supported under US National Science Foundation grant AST-90-15827 to R. M. Rich, and the staff of the CASLEO for their support during the observing time. We deeply thank Prof. B. Hauck for the hospitality of the Institute d'Astronomie de l'Université de Lausanne, which permitted to work on this project during a two-month stay of one of the authors (BG). We are grateful to A.W. Fullerton, D. Gies and C. T. Bolton for their observing data on HD 152233 (S306). The present work was partially supported by the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina.