EDP Sciences
Free access
Volume 433, Number 2, April II 2005
Page(s) 629 - 634
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20042291

A&A 433, 629-634 (2005)
DOI: 10.1051/0004-6361:20042291

BS Indi: An enigmatic object in the Tucana association

E. W. Guenther1, E. Covino2, J. M Alcalá2, M. Esposito1, 3 and R. Mundt4

1  Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
    e-mail: guenther@tls-tautenburg.de
2  INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
3  Università di Salerno, via S. Allende, 84081 Baronissi (SA), Italy
4  MPI für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

(Received 1 November 2004 / Accepted 30 November 2004 )

BS Ind (=HD 202947) is a young K0V star in the Tucana association. Photometric observations with the Hipparcos satellite show an eclipse-like light-curve with primary and secondary eclipse. The eclipsing binary has a period of 0.435338 days and a circular orbit. Our spectroscopic observations however show that the K0V primary is a single-line spectroscopic binary with a period of 3.3 years. The minimum mass of the invisible component is about 0.9 $M_\odot$ which means that the mass of the companion is about the same as that of the primary. The first inspection of our FEROS spectra with a resolution of 48 000, as well as a CES spectrum with a resolution of 220 000 shows no obvious companion. However, when the FEROS spectra are cross-correlated with an M-star, a secondary becomes visible as a broad peak in the cross-correlation function. The width and the position of this broad peak is variable on a short time. When phased to a period of 0.435338 days, the radial velocity variations of the broad peak show the characteristic sine-wave of a spectroscopic binary in a circular orbit. The best interpretation of this data is that the broad peak in the cross-correlation function is caused by an eclipsing binary consisting of two late-K, or early-M stars with an orbital period of 0.435338 days. This is the eclipsing system. These two stars then orbit the K0V-primary with a period of 3.3 years. The assumption that BS Ind is a triple system consisting of a K0V star and two late-K, or early-M stars also explains the unusual brightness of the object and the near infrared excess. Thus, BS Ind is unique, as it contains by far the shortest-period young binary star, and these stars are eclipsing.

Key words: stars: individual: BS Ind -- binaries: eclipsing -- binaries: spectroscopic -- stars: formation -- stars: evolution

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© ESO 2005