BS Indi: An enigmatic object in the Tucana association^*

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

Abstract

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  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.