6 Conclusions

The high-resolution spectroscopic observations and the near IR photometry have allowed the confirmation of four low-mass PMS stars among the original sample proposed by PF96 and investigated by FL97 using low-resolution optical spectroscopy. These four stars have radial velocities consistent with those of the Lower Centaurus-Crux subgroup of the Sco-Cen association and hence are very likely part of the loose group of low-mass PMS stars of the Sco-Cen association. Many more widely spread X-ray emitting PMS stars are expected to be identified in the Lower Centaurus-Crux region. Given the high velocity dispersion and low spatial density of the Crux stars, the possibility that they form a small aggregate is unlikely. However, more X-ray observations with higher spatial resolution, extended to lower flux limits than those so far available will help to establish if there are more X-ray emitting PMS stars around $\beta$-Crux, and in that case, if they form a small cluster similar to the one around $\eta$ Cha.

Two of the stars (Cru-1 and Cru-3) are found to be a visual binary and a triple system, respectively. The number statistics is too low however for any conclusion regarding the binary fraction.

The PMS star Cru-3 has been found to be a hierarchical triple system in which one of the components is a SB2 itself. The separation of the hierarchical triple of about 4 AU, which means about 36 mas at the distance of 110 pc, as well as its orbital period of about 4.6 years, makes it a good candidate to be observed in the coming years with the Very Large Telescope Interferometer (VLTI), in order to fully solve the orbital elements, allowing the determination of the true mass of the components. The latter is crucial to put observational constraints on theoretical PMS evolutionary tracks, which are specially uncertain at the low-mass end.

Acknowledgements

We thank the referee Prof. W. A. Lawson for his comments. We thank the technical support by K. Brooks, A. Gonzalez, E. Matamoros, A. Sánchez and E. Wenderoth at the ESO 3.6 m telescope and by R. Vega, A. Torrejón, E. Pompei and P. Francois at the ESO 1.5 m telescope. This work has been financed by the Italian Ministero per la Ricerca Scientifica e Tecnologica.