Multiple star systems in the Orion nebula
Authors:   GRAVITY collaboration, Karl Martina , Pfuhl Oliver , Eisenhauer Frank , Genzel Reinhard , Grellmann Rebekka , Habibi Maryam , Abuter Roberto , Accardo Matteo , Amorim António , Anugu Narsireddy , Ávila Gerardo , Benisty Myriam , Berger Jean-Philippe , Blind Nicolas , Bonnet Henri , Bourget Pierre , Brandner Wolfgang , Brast Roland , Buron Alexander , o Garatti Alessio Caratti , Chapron Frédéric , Clénet Yann , Collin Claude , du Foresto Vincent Coudé , de Wit Willem-Jan , de Zeeuw Tim , Deen Casey , Delplancke-Ströbele Françoise , Dembet Roderick , Derie Frédéric , Dexter Jason , Duvert Gilles , Ebert Monica , Eckart Andreas , Esselborn Michael , Fédou Pierre , Finger Gert , Garcia Paulo , Garcia Dabo Cesar Enrique , Garcia Lopez Rebeca , Gao Feng , Gendron Éric , Gillessen Stefan , Gonté Frédéric , Gordo Paulo , Grözinger Ulrich , Guajardo Patricia , Guieu Sylvain , Haguenauer Pierre , Hans Oliver , Haubois Xavier , Haug Marcus , Haußmann Frank , Henning Thomas , Hippler Stefan , Horrobin Matthew , Huber Armin , Hubert Zoltan , Hubin Norbert , Jakob Gerd , Jochum Lieselotte , Jocou Laurent , Kaufer Andreas , Kellner Stefan , Kendrew Sarah , Kern Lothar , Kervella Pierre , Kiekebusch Mario , Klein Ralf , Köhler Rainer , Kolb Johan , Kulas Martin , Lacour Sylvestre , Lapeyrère Vincent , Lazareff Bernard , Le Bouquin Jean-Baptiste , Léna Pierre , Lenzen Rainer , Lévêque Samuel , Lin Chien-Cheng , Lippa Magdalena , Magnard Yves , Mehrgan Leander , Mérand Antoine , Moulin Thibaut , Müller Eric , Müller Friedrich , Neumann Udo , Oberti Sylvain , Ott Thomas , Pallanca Laurent , Panduro Johana , Pasquini Luca , Paumard Thibaut , Percheron Isabelle , Perraut Karine , Perrin Guy , Pflüger Andreas , Duc Thanh Phan , Plewa Philipp M. , Popovic Dan , Rabien Sebastian , Ramírez Andrés , Ramos Jose , Rau Christian , Riquelme Miguel , Rodríguez-Coira Gustavo , Rohloff Ralf-Rainer , Rosales Alejandra , Rousset Gérard , Sanchez-Bermudez Joel , Scheithauer Silvia , Schöller Markus , Schuhler Nicolas , Spyromilio Jason , Straub Odele , Straubmeier Christian , Sturm Eckhard , Suarez Marcos , Tristram Konrad R. W. , Ventura Noel , Vincent Frédéric , Waisberg Idel , Wank Imke , Widmann Felix , Wieprecht Ekkehard , Wiest Michael , Wiezorrek Erich , Wittkowski Markus , Woillez Julien , Wolff Burkhard , Yazici Senol , Ziegler Denis , Zins Gérard 
Published: October 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.