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

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85741 Garching, Germany
² University of Hawaii, Institut for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
³ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching, Germany
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Corresponding author: N. Huélamo, huelamo@mpe.mpg.de

Received: 16 March 2001
Accepted: 3 May 2001

Abstract

We present adaptive optics JHK_S imaging observations of three main-sequence late B-type stars listed in the Lindroos Catalogue: HD 123445, HD 127971 and HD 129791. Given their spectral types, these stars should not be X-ray emitters. However, they have been detected by ROSAT and their X-ray emission has been attributed to possible unresolved late-type companions. We have carried out near-IR observations with ADONIS at the ESO 3.6 m but have not detected any late-type companions close to HD 127971 and HD 129791. 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 multiplicity of early-type (A and B) stars, the latter would yield a revision of stellar activity theories which do not predict X-ray emission from these stars. On the other hand, HD 123445 does indeed show visual companions, namely an apparent subarcsecond faint () binary system at a projected separation of 5´´from the late-B type star. The JHK_S magnitudes and colors of the components are consistent with (i) a pair of Pre Main Sequence (PMS) K-type stars at 140 pc (i.e. possible members of the Upper Centaurus Lupus association), (ii) a pair of Main Sequence M-type stars at 60 pc and (iii) a pair of K-type giants at 2.6 kpc. While in the first case the reported X-ray emission can be ascribed to the new objects, in the second and third 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 binary and Chandra X-ray high spatial resolution (astrometric) imaging observations are required to definitely determine the source of the X-ray emission. 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 companions.