9 Sagittarii: uncovering an O-type spectroscopic binary with an 8.6 year period
Gosset, E.; Rauw, G.; De Becker, M.; Spano, M.; Sana, H.; Mahy, L.; Eenens, P.
Belgium, Netherlands, Switzerland, Mexico
Abstract
Context. The O-type object 9 Sgr is a well-known synchrotron radio emitter. This feature is usually attributed to colliding-wind binary systems, but 9 Sgr was long considered a single star.
Aims: We have conducted a long-term spectroscopic monitoring of this star to investigate its multiplicity and search for evidence for wind-wind interactions.
Methods: Radial velocities are determined and analysed using various period search methods. Spectral disentangling is applied to separate the spectra of the components of the binary system.
Results: We derive the first ever orbital solution of 9 Sgr. The system is found to consist of an O3.5 V((f+)) primary and an O5-5.5 V((f)) secondary moving around each other on a highly eccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emission lines that could be formed in the wind interaction zone in agreement with the expected properties of the interaction in such a wide system.
Conclusions: Our results provide further support to the paradigm of synchrotron radio emission from early-type stars being a manifestation of interacting winds in a binary system.