Dynamics of a massive binary at birth

Abstract

Almost all massive stars have bound stellar companions, existing in binaries or higher-order multiples1–5. While binarity is theorized to be an essential feature of how massive stars form6, essentially all information about such properties is derived from observations of already formed stars, whose orbital properties may have evolved since birth. Little is known about binarity during formation stages. Here we report high-angular-resolution observations of 1.3\u2009mm continuum and H30α recombination line emission, which reveal a massive protobinary with apparent separation of 180\u2009au at the centre of the massive star-forming region IRAS 07299-1651. From the line-of-sight velocity difference of 9.5\u2009km\u2009s−1 of the two protostars, the binary is estimated to have a minimum total mass of 18 solar masses, consistent with several other metrics, and maximum period of 570\u2009yr, assuming a circular orbit. The H30α line from the primary protostar shows kinematics consistent with rotation along a ring of radius of 12\u2009au. The observations indicate that disk fragmentation at several hundred astronomical units may have formed the binary, and much smaller disks are feeding the individual protostars.High-angular-resolution observations of 1.3\u2009mm continuum and H30α recombination line emission identify a binary star system in formation, with protostars apparently separated by 180\u2009au. The velocity difference between the two protostars constrains the orbital period and total mass of the system.