Multiple star systems in the Orion nebula

¹ MPE – Max Planck Institute for Extraterrestrial Physics, Gießenbachstraß, 85741 Garching, Germany
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

Received: 30 May 2018
Accepted: 26 September 2018

Abstract

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.