ADONIS observations of hard X-ray emitting late B-type stars in Lindroos systems

We present adaptive optics JHK(S) imaging observations of three main-sequen ce late B-type stars listed in the Lindroos Catalogue: HD123445, HD127971 a nd HD129791. Given their spectral types, these stars should not be X-ray em itters. However, they have been detected by ROSAT and their X-ray emission has been attributed to possible unresolved late-type companions. We have ca rried out near-IR observations with ADONIS at the ESO 3.6 m but have not de tected any late-type companions close to HD127971 and HD129791. This result leads us to conclude that either (i) they are spectroscopic binaries with unresolved low-mass companions, or (ii) they are intrinsic X-ray emitters. While the former case would be consistent with the reported high multiplici ty of early-type (A and B) stars, the latter would yield a revision of stel lar activity theories which do not predict X-ray emission from these stars. On the other hand, HD123445 does indeed show visual companions, namely an apparent subarcsecond faint (K-s similar to 10) binary system at a projecte d separation of 5 " from the late-B type star. The JHKS magnitudes and colo rs of the components are consistent with (i) a pair of Pre Main Sequence (P MS) K-type stars at 140 pc (i.e. possible members of the Upper Centaurus Lu pus association), (ii) a pair of Main Sequence M-type stars at 60 pc and (i ii) a pair of K-type giants at 2.6 kpc. While in the first case the reporte d X-ray emission can be ascribed to the new objects, in the second and thir d case it cannot, and we have to assume the late B-type star to be either a spectroscopic binary itself or a single star with intrinsic X-ray emission . Spectroscopy is required to confirm the possible PMS nature of the new bi nary and Chandra X-ray high spatial resolution (astrometric) imaging observ ations are required to definitely determine the source of the X-ray emissio n. If the B9 star results to be the X-ray emitter, near-IR spectroscopy can be used to investigate the presence of a T Tauri like spectroscopic compan ions.