Cyg OB2 #5: When three stars are just not enough

TL;DR

This study re-examines 20 years of radio observations of Cyg OB2 #5, revealing a third star and suggesting the system is a quadruple, with variable emission caused by a non-thermal source orbiting within the stellar wind envelope.

Contribution

It provides the first detailed analysis of the primary radio emission, identifies a third stellar companion, and models the system as a quadruple with a non-thermal source orbiting the binary.

Findings

The variable radio emission has a 6.7-year period.

The non-thermal source is likely orbiting within the stellar wind envelope.

Cyg OB2 #5 is a quadruple star system.

Abstract

Archival observations from the Very Large Array (VLA) at frequencies between 1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 over 20 years are re-examined. The aim is to determine the location and character of its known variable radio emission. The radio emission consists of a primary component associated with the binary, and a non-thermal source (NE), $0.8" to the NE. This work reveals that NE shows no evidence of variation demonstrating that the variable emission arises in the primary component. With NE constant, the radio flux from the primary can now be well determined for the first time, especially in observations that do not resolve both the primary and NE components. The variable radio emission from the primary has a period of 6.7+/-0.3 years which is described by a simple model of a non-thermal source orbiting within the stellar wind envelope of the binary. Such a model implies the presence of a third, unresolved stellar companion (Star C) orbiting the 6.6-day binary with a period of 6.7 years. The non-thermal emission arises from either a WCR between Star C and the binary system, or possibly from Star C directly. Examination of radial velocity observations suggests reflex motion of the binary due to Star C, for which a mass of 23^+22_-14 solar mass is deduced. Together with the star associated with NE, this implies that Cyg OB2 #5 is a quadruple system.