EDP Sciences
Free access
Volume 373, Number 2, July II 2001
Page(s) 657 - 664
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010690

A&A 373, 657-664 (2001)
DOI: 10.1051/0004-6361:20010690

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

N. Huélamo1, W. Brandner2, 3, A. G. A. Brown3, R. Neuhäuser1, 2 and H. Zinnecker4

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

(Received 16 March 2001 / Accepted 3 May 2001 )

We present adaptive optics $JHK_{\rm S}$ 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 ( $K_{\rm s}\sim10$) binary system at a projected separation of 5´´from the late-B type star. The $JHK_{\rm S}$ 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.

Key words: stars: early-type -- stars: binaries -- infrared: stars -- X-rays: stars

Offprint request: N. Huélamo, huelamo@mpe.mpg.de

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© ESO 2001