The long period eccentric orbit of the particle accelerator HD167971 revealed by long baseline interferometry

TL;DR

This study used long baseline interferometry to resolve the components of HD167971, revealing a highly eccentric wide orbit and providing insights into its particle acceleration and radio emission behavior.

Contribution

First direct imaging of HD167971's wide components, establishing a highly eccentric orbit and constraining the binary's spectral types and evolutionary status.

Findings

Components are on a very eccentric orbit (e>0.5).

The wide orbit is not coplanar with the inner binary.

Maximum radio emission occurs near periastron.

Abstract

Using optical long baseline interferometry, we resolved for the first time the two wide components of HD167971, a candidate hierarchical triple system known to efficiently accelerate particles. Our multi-epoch VLTI observations provide direct evidence for a gravitational link between the O8 supergiant and the close eclipsing O + O binary. The separation varies from 8 to 15 mas over the three-year baseline of our observations, suggesting that the components evolve on a wide and very eccentric orbit (most probably e>0.5). These results provide evidence that the wide orbit revealed by our study is not coplanar with the orbit of the inner eclipsing binary. From our measurements of the near-infrared luminosity ratio, we constrain the spectral classification of the components in the close binary to be O6-O7, and confirm that these stars are likely main-sequence objects. Our results are discussed in the context of the bright non-thermal radio emission already reported for this system, and we provide arguments in favour of a maximum radio emission coincident with periastron passage. HD167971 turns out to be an efficient O-type particle accelerator that constitutes a valuable target for future high angular resolution radio imaging using VLBI facilities.