Non-thermal radio emission from O-type stars
III. Is Cyg OB2 No. 9 a wind-colliding binary?

S. Van Loo (1), R. Blomme (2), S. M. Dougherty (3)and M. C. Runacres (4)

(1) School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
(2) Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel, Belgium
(3) National Research Council of Canada, Herzberg Institute for Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia V2A 6J9, Canada
(4) Erasmus University College, Nijverheidskaai 170, 1070 Brussel, Belgium

The star Cyg OB2 No. 9 is a well-known non-thermal radio emitter. Recent theoretical work suggests that all such O-stars should be in a binary or a multiple system. However, there is no spectroscopic evidence of a binary component. Re-analysis of radio observations from the VLA of this system over 25 years has revealed that the non-thermal emission varies with a period of 2.35+-0.02 yr. This is interpreted as a strong suggestion of a binary system, with the non-thermal emission arising in a wind-collision region. We derived some preliminary orbital parameters for this putative binary and revised the mass-loss rate of the primary star downward from previous estimates.

Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted