Research Note

A binary signature in the non-thermal radio-emitter Cyg OB2 #9^*

¹ Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5C, 4000 Liège, Belgium e-mail: naze@astro.ulg.ac.be
² Dipartimento di Astronomia, Universitá degli studi di Padova, vicolo Osservatorio 2, 35122 Padova, Italy

Received: 31 January 2008
Accepted: 17 March 2008

Abstract

Aims. Non-thermal radio emission associated with massive stars is believed to arise from a wind-wind collision in a binary system. However, the evidence of binarity is still lacking in some cases, notably Cyg OB2 #9.

Methods. For several years, we have been monitoring this heavily-reddened star from various observatories. This campaign allowed us to probe variations both on short and long timescales and constitutes the first in-depth study of the visible spectrum of this object.

Results. Our observations provide the very first direct evidence of a companion in Cyg OB2 #9, confirming the theoretical wind-wind collision scenario. These data suggest a highly eccentric orbit with a period of a few years, compatible with the 2 yr-timescale measured in the radio range. In addition, the signature of the wind-wind collision is very likely reflected in the behaviour of some emission lines.