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 0.2arcsec ) 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 2arcsec diameter. The census is nearly complete in the angular separation range 1arcsec - 6arcsec , corresponding to linear separations of 200-1200 AU, which have remained unstudied so far due to the intrinsic brightness of the massive primaries. The best detection limits are around K=15(m) at 1arcsec and K=19(m) at 5arcsec separation from the primary. In 13 fields of 12.5arcsec x 12.5arcsec 6 new faint stars were detected. Their magnitudes and colours indicate that 2 of them can be physical low-mass components. The bright (K=11.2, J-K=1.2) companion to HD 108250 C at 2.17arcsec has an IR excess and deserves further study. Several fields were explored without a mask; somewhat surprisingly, no new components at sub-arcsecond separations down to 0.2arcsec were detected. The first order stability analysis indicates that the studied multiple systems are hierarchical at spatial scales of 40-1000 AU. The data are briefly discussed in the light of the formation and evolution of multiple systems with massive primaries. Based on observations collected at the ESO La Silla Observatory (program 58.D-0455).