First orbital solution for the non-thermal emitter Cyg OB2 #9

Abstract

After the first detection of its binary nature, the spectroscopic monitoring of the non-thermal radio emitter Cyg OB2 #9 (P=2.4yrs) has continued, doubling the number of available spectra of the star. Since the discovery paper of 2008, a second periastron passage has occurred in February 2009. Using a variety of techniques, the radial velocities could be estimated and a first, preliminary orbital solution was derived from the HeI5876 line. The mass ratio appears close to unity and the eccentricity is large, 0.7--0.75. X-ray data from 2004 and 2007 are also analyzed in quest of peculiarities linked to binarity. The observations reveal no large overluminosity nor strong hardness, but it must be noted that the high-energy data were taken after the periastron passage, at a time where colliding wind emission may be low. Some unusual X-ray variability is however detected, with a 10% flux decrease between 2004 and 2007. To clarify their origin and find a more obvious signature of the wind-wind collision, additional data, taken at periastron and close to it, are needed.

15 pages, 4 figures, accepted by ApJ

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• Additional Massive Binaries in the Cygnus OB2 Association
• A multi-epoch XMM-Newton campaign on the core of the massive Cyg OB2 association
• The 2.35 year itch of Cyg OB2 #9. II. Radio monitoring
• A test for the theory of colliding winds: the periastron passage of 9 Sagittarii I. X-ray and optical spectroscopy
• Multiplicity of northern bright O-type stars with optical long baseline interferometry
• The kinematic characteristics of magnetic O-type stars
• Non-thermal radio emission from colliding-wind binaries: modelling Cyg OB2 No. 8A and No. 9