Volume 620, December 2018
|Number of page(s)||26|
|Section||Stellar structure and evolution|
|Published online||07 December 2018|
Multiple star systems in the Orion nebula
MPE – Max Planck Institute for Extraterrestrial Physics, Gießenbachstraß, 85741
2 Department of Physics, Le Conte Hall, University of California, Berkeley, CA, 94720 USA
3 Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
5 CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal; Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
6 CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal; Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
7 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
8 Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
9 European Southern Observatory, Casilla, 19001 Santiago 19, Chile
10 MPIA – Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
11 LESIA – Observatoire de ParisUniversitè PSL, CNRS, Sorbonne Universitè, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
12 Sterrewacht Leiden, Leiden University, Postbus 513, 2300 RA Leiden, The Netherlands
13 Max-Planck-Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
14 ESA – European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218 USA
15 Dublin Institute for Advanced Studies, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
16 University of Vienna, Universitätsring 1, 1010 Wien, Austria
17 School of Physics, Astrophysics Group, University of Exeter, Stocker Road, Exeter EX4 4QL UK
Accepted: 26 September 2018
This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium cluster with the recently comissioned GRAVITY instrument. We observed a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for θ1 Ori B, θ2 Ori B, and θ2 Ori C. We determined a separation for the previously suspected companion of NU Ori. We confirm four companions for θ1 Ori A, θ1 Ori C, θ1 Ori D, and θ2 Ori A, all with substantially improved astrometry and photometric mass estimates. We refined the orbit of the eccentric high-mass binary θ1 Ori C and we are able to derive a new orbit for θ1 Ori D. We find a system mass of 21.7 M⊙ and a period of 53 days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about two, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints toward a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We excluded collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion.
Key words: techniques: interferometric / astrometry / celestial mechanics / binaries: close / binaries: general / stars: massive
GRAVITY is developed in a collaboration by the Max Planck Institute for extraterrestrial Physics, LESIA of Paris Observatory/CNRS/UPMC/Univ. Paris Diderot and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Centro de Astrofísica e Gravitação, and the European Southern Observatory.
© ESO 2018
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