Non-thermal radio emission from O-type stars^*

III. Is Cygnus OB2 No. 9 a wind-colliding binary?

¹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK e-mail: svenvl@ast.leeds.ac.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

Received: 8 January 2008
Accepted: 6 March 2008

Abstract

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.