Multi-Epoch VLBA Observations of the Compact Wind-Collision Region in the Quadruple System Cyg OB2 #5

TL;DR

This study uses multi-epoch VLBA observations to analyze the wind-collision region in Cyg OB2 #5, confirming its morphology, measuring its distance, and providing insights into its orbital and stellar properties.

Contribution

It provides the first direct parallax measurement of Cyg OB2 #5 and constrains the properties of the unseen companion star.

Findings

Confirmed arc-shaped morphology of the emission.

Measured a parallax of 0.61 ± 0.22 mas, indicating a distance of ~1.65 kpc.

Estimated the companion star's spectral type between B0.5 and O8.

Abstract

We present multi--epoch VLBA observations of the compact wind collision region in the Cyg OB2 #5 system. These observation confirm the arc-shaped morphology of the emission reported earlier. The total flux as a function of time is roughly constant when the source is "on", but falls below the detection limit as the wind collision region approaches periastron in its orbit around the contact binary at the center of the system. In addition, at one of the "on" epochs, the flux drops to about a fifth of its average value. We suggest that this apparent variation could result from the inhomogeneity of the wind that hides part of the flux rather than from an intrinsic variation. We measured a trigonometrical parallax, for the most compact radio emission of 0.61 $\pm$ 0.22 mas, corresponding to a distance of 1.65 $^{+0.96}_{-0.44}$ kpc, in agreement with recent trigonometrical parallaxes measured for objects in the Cygnus X complex. Using constraints on the total mass of the system and orbital parameters previously reported in the literature, we obtain two independent indirect measurements of the distance to the Cyg OB2 #5 system, both consistent with 1.3--1.4 kpc. Finally, we suggest that the companion star responsible for the wind interaction, yet undetected, is of spectral type between B0.5 to O8.