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

TL;DR

This study provides the first direct evidence of a binary companion in the non-thermal radio-emitter Cyg OB2 #9, supporting the wind-wind collision model and revealing a highly eccentric orbit with a period of a few years.

Contribution

It offers the first in-depth optical spectroscopic evidence confirming binarity in Cyg OB2 #9, aligning with radio observations and advancing understanding of non-thermal emission mechanisms.

Findings

Detected a binary companion in Cyg OB2 #9

Confirmed a highly eccentric orbit with a period of a few years

Linked emission line behaviour to wind-wind collision signatures

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 2yr-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.