A near IR adaptive optics search for faint companions to early-type multiple stars

We report on a high dynamical range (> 10(m)) and high angular resolution ( down to 0.2 ") search for low-mass components of early-type primaries in 7 intermediate-age (5 Myr) multiple systems with the ESO 3.6 m telescope and the adaptive optics system ADONIS. The images were obtained in the J and SK bands with and without a coronographic mask of 2 " diameter. The census is nearly complete in the angular separation range 1 " - 6 ", corresponding t o linear separations of 200-1200 AU, which have remained unstudied so far d ue to the intrinsic brightness of the massive primaries. The best detection limits are around K = 15(m) at 1 " and K = 19(m) at 5 " separation from th e primary. In 13 fields of 12.5 " x 12.5 " 6 new faint stars were detected. Their magnitudes and colours indicate that 2 of them can be physical low-m ass components. The bright (K = 11.2, J - K = 1.2)companion to HD 108250 C at 2.17 " has an IR excess and deserves further study. Several fields were explored without a mask; somewhat surprisingly, no new components at sub-ar csecond separations down to 0.2 " were detected. The first order stability analysis indicates that the studied multiple systems are hierarchical at sp atial scales of 40-1000 AU. The data are briefly discussed in the light of the formation and evolution of multiple systems with massive primaries.