Over the last twenty years, the interest in wind interaction has grown, and numerous studies both from theoretical (e.g. Stevens et al. 1992; Walder 1998; Pittard 2000) and observational (e.g. Corcoran 1996; Rauw et al. 1999; Hill et al. 2000) points of view have been undertaken. These studies demonstrate that considerable insight into the mass loss process can be gained from a detailed investigation of the wind interaction phenomenon. In addition, binary systems provide precious astrophysical laboratories allowing a direct determination of the fundamental parameters of the stars that are crucial in order to test evolutionary models and to probe the impact of binarity on the evolution of the components of the system.
HD152248 is an O+O binary system that belongs to the young open cluster NGC6231 which is considered to be the nucleus of the rich SCOOB1 association. Situated at a distance of 1.99 kpc (Baume et al. 1999), this cluster contains a number of interesting single O stars as well as O binaries and a WR+O system (WR79). Not only is the HD152248 system one of the brightest objects of the cluster (V=6.06, Baume et al. 1999) but it also appears from ROSAT X-ray observations to be the brightest X-ray source in NGC6231. Corcoran (1996) has further shown that the ROSAT X-ray flux from HD152248 displays phase-locked variations, though the poor phase coverage of the ROSAT data did not allow elucidation of their origin. On these grounds, NGC6231 has been selected as a target for the XMM X-ray observatory as part of the Liège project for guaranteed time. The pointings will be centered on HD152248 for a total duration of 180 ksec, consisting of six 30 ksec exposures in order to adequately sample the orbital variations.
In this context, we have undertaken a spectroscopic campaign to monitor the optical counterpart of most of the ROSAT X-ray sources of the NGC6231 cluster. Some preliminary results have already been presented by Sana et al. (2001). The present paper is the first of a series devoted to the O stars in NGC6231. We present here a complete and detailed spectroscopic study of the HD152248 system (the main target of our campaign) in the optical domain. One aim is to derive accurate, contemporaneous ephemerides required to correctly interpret the forthcoming XMM data and to clarify the evolutionary status of both components of the system. Another objective of this study is the search for the optical signature of a wind interaction which is suspected to exist within the system because of its high and variable X-ray luminosity.
This paper is organised as follows. The next section is devoted to a review of the main results already published about this object in the literature. In the third section, we describe our spectroscopic observing campaign, the data we collected and the data reduction techniques we applied. The determination of the orbital parameters is presented in the fourth section. The fifth one is devoted to the study of the variations of the equivalent widths of the absorption lines and their link to the Struve-Sahade effect (S-S effect). We next turn to the emission lines displayed by the spectrum of this system. Particular attention is paid to the He II
4686 and H
lines and to their phase-locked variability. Section 7 is devoted to the discussion of the evolutionary status of the system, and, among other things, we re-investigate its spectral and luminosity classification.
Finally, we present a wind-wind interaction model that is consistent with most of the observational features of the emission lines. The last section is devoted to a brief summary of our main results.
Copyright ESO 2001