MODELING THE RADIO EMISSION FROM Cyg OB2 NO. 5: A QUADRUPLE SYSTEM?

Abstract

Fifty observations at frequencies between 1.4 GHz and 43 GHz of the 6.6-day O6.5-7+O5.5-6 binary Cyg OB2 #5 using the Very Large Array over 20 years are re-examined. The aim is to determine the location and character of the previously detected variable radio emission. The radio emission from the system consists of a primary component that is associated with the binary, and a non-thermal source (NE), 0.8 ′′ to the NE of the binary that has been ascribed to a wind-collision region (WCR) between the stellar winds of the binary and that of a B-type star (Star D) to the NE . Previous studies have not accounted for the potential contribution of NE to the total radio emission, most especially in observations where the primary and NE sources are not resolved as separate sources. NE shows no evidence of variation in 23 epochs where it is resolved separately from the primary radio component, demonstrating that the variable emission arises in the primary component. Since NE is non-variable, the radio flux from the primary can now be well determined for the first time, most especially in observations that do not resolve both the primary and NE components. The variable radio emission from the primary component 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 and independent of Star D to the NE. The variable non-thermal emission arises from either a WCR between Star C and the binary system, or possibly from Star C directly. The model gives a mass-loss rate of 3.4 × 10 −5 M ⊙ yr −1 for Cyg OB2 #5, unusually high for an Of supergiant and comparable to that of WR stars, and consistent with an unusually strong He i 1.083-µm emission line, also redolent of WR stars. An examination of radial velocity observations available from the literature suggests reflex motion of the binary due to Star C, for which a mass of 23 +22 −14 M ⊙ is deduced. The natures of NE and Star D are also examined. If NE is a WCR, as suggested by other authors, then the required mass-loss rate is an order of magnitude higher than expected for an early B-type dwarf, and only just consistent with a supergiant. This raises the question of NE as a WCR, but its nonthermal luminosity is consistent with a WCR and a comparison of reddening between Cyg OB2 #5 and Star D do not rule out an association, implying Cyg OB2 #5 is a quadruple system. Pursuing alternative models for NE, such as an unassociated background source, would require very challenging observations.

Catalogue of particle-accelerating colliding-wind binaries

“…Until a few years ago, this system was known to be a triple system, made of a close early-type binary with a much more distant B-type third component. The presence of a fourth star in this multiple system is strongly suggested by a well-defined radio variability (of non-thermal origin) on a time scale of about 6.7 years (Kennedy et al 2010). Some additional convincing information comes from X-ray observations revealing a hard spectrum in the soft X-ray domain (Linder et al 2009).…”

Wind collisions in three massive stars of Cygnus OB2

“…The system is a nonthermal radio emitter, which is an indication of the presence of a WWC. Strangely, the radio emission of the binary is not modulated with the 6.6 d period, but with a period of about 6.7 yr. Kennedy et al (2010), therefore, deduced that a third star exists in the system, orbiting the binary in a 6.7 yr orbit. This third component can be associated with a late O/early B-type star (Kennedy et al 2010).…”

“…Strangely, the radio emission of the binary is not modulated with the 6.6 d period, but with a period of about 6.7 yr. Kennedy et al (2010), therefore, deduced that a third star exists in the system, orbiting the binary in a 6.7 yr orbit. This third component can be associated with a late O/early B-type star (Kennedy et al 2010). These authors proposed several possible orbital solutions for the tertiary orbit that could explain the radio emission.…”

Non-thermal radio emission from O-type stars

“…The Cyg OB2 #5 system is a 6.6 d spectroscopic binary, but the 6.7 yr period in the radio fluxes indicates the presence of a third companion. Furthermore, a CWR is visible at radio wavelengths between Cyg OB2 #5 and a nearby B-type star, implying that Cyg OB2 #5 could be a quadruple system (Kennedy et al 2010;Dzib et al 2013). From spectroscopy we know that HD 167971 is a 3.3 d binary, with a third component which may or may not be gravitationally bound (Leitherer et al 1987).…”