9 Sgr: uncovering an O-type spectroscopic binary with an 8.6 year period

TL;DR

This study reveals that 9 Sgr is an O-type spectroscopic binary with an 8.6-year orbit, supporting the idea that its synchrotron radio emission results from wind-wind interactions in a binary system.

Contribution

First orbital solution of 9 Sgr showing it is a wide, highly eccentric binary system with detailed spectral analysis.

Findings

9 Sgr is a binary with an 8.6-year period.

The system has a highly eccentric orbit (e=0.7).

No variable emission lines indicative of wind interaction were observed.

Abstract

The O-type object 9 Sgr is a well-known synchrotron radio emitter. This feature is usually attributed to colliding-wind binary systems, but 9 Sgr was long considered a single star. We have conducted a long-term spectroscopic monitoring of this star to investigate its multiplicity and search for evidence for wind-wind interactions. Radial velocities are determined and analysed using various period search methods. Spectral disentangling is applied to separate the spectra of the components of the binary system. We derive the first ever orbital solution of 9 Sgr. The system is found to consist of an O3.5 V((f+)) primary and an O5- 5.5 V((f)) secondary moving around each other on a highly eccentric (e = 0.7), 8.6 year orbit. The spectra reveal no variable emission lines that could be formed in the wind interaction zone in agreement with the expected properties of the interaction in such a wide system. Our results provide further support to the paradigm of synchrotron radio emission from early-type stars being a manifestation of interacting winds in a binary system.