A close encounter of the massive kind

We have used (i) Hubble Space Telescope Advanced Camera for Surveys imaging and Space Telescope Imaging Spectrograph spectroscopy, (ii) ground-based Precision Integrated-Optics Near-infrared Imaging ExpeRiment/Very Large Telescope long-baseline interferometry, and (iii) ground-based spectroscopy from different instruments to study the orbit of the extreme multiple system HD 93 129 Aa,Ab, which is composed of (at least) two very massive stars in a long-period orbit with e > 0.92, which will pass through periastron in 2017/2018. In several ways, the system is an ¿ Car precursor. Around the time of periastron passage, the two very strong winds will collide and generate an outburst of non-thermal hard X-ray emission without precedent in an O+O binary since astronomers have been able to observe above Earth's atmosphere. A coordinated multiwavelength monitoring in the next two years will enable a breakthrough understanding of the wind interactions in such extreme close encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a binary system itself. In that case, we could witness a three-body interaction which may yield a runaway star or a stellar collision close to or shortly after the periastron passage. Either of those outcomes would be unprecedented, as they are predicted to be low-frequency events in the Milky Way.
JMA acknowledges support from the Spanish Government Ministerio de Economía y Competitividad (MINECO) through grant AYA2013-40 611-P and from HST GO programs 10205, 10602, and 10898. RHB acknowledges support from FONDECYT Projects1 140 076 and11 121 550. The HST data were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.