Properties and nature of Be stars

25. A new orbital solution and the nature of a peculiar emission-line binary  Sagittarii

¹ Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic e-mail: koubsky@sunstel.asu.cas.cz
² Astronomical Institute of the Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
³ Department of Physics and Astronomy, University of Victoria, PO Box 3055 STN CSC, Victoria, B.C. V8W 3P6, Canada
⁴ Institue of Theretical Physics and Astrophysics, Faculty of Science, Masaryk University Brno, Kotlářská 2, 611 37 Brno, Czech Republic

Received: 25 March 2006
Accepted: 28 April 2006

Abstract

Context.Binaries observed in the initial rapid phase of mass exchange between the components are very rare since the statistical probability of finding them is low. At the same time, thorough studies of them are extremely important for better understanding the process of large-scale mass exchange and possible mass loss from the system. One of these objects is probably υ Sgr.

Aims.By analyzing 35 new electronic spectra and numerous published spectral and photometric observations, we derived the new orbital elements, an upper limit to a secular period change, and also the peculiar RV curve of the blue-shifted Hα absorption. Possible models of the binary and its evolutionary stage are then discussed critically.

Methods.Reduction of new spectra was carried out with the IRAF and SPEFO programs. All orbital elements were derived with the FOTEL program and period searches were carried out using the phase-dispersion minimalization technique.

Results.The peculiar RV curve of the blue-shifted Hα absorption rules out the model of a coronal flow of matter from the brighter component. The presence of bipolar jets that are perpendicular to the orbital plane and similar to those found for β Lyr seems probable. An upper limit to the secular period decrease of 24 s per year was estimated and should be tested by future RV observations. The peculiar character of the line spectrum of the brighter component could also be understood as originating from a pseudo-photosphere of an optically thick disk rather than from a stellar spectrum.

Conclusions.Better understanding of the nature of υ Sgr will not be possible without interferometric resolution of the binary and especially without determining its orbital inclination.