# Dynamics of a Massive Binary at Birth

## TL;DR

This study presents high-resolution observations of a massive protobinary system, revealing its formation process, orbital properties, and disk structures, providing new insights into the early stages of massive star binary formation.

## Contribution

It is the first to directly observe a massive protobinary during formation, showing disk fragmentation as the likely formation mechanism.

## Key findings

- Binary separation of 180 au with a 9.5 km/s velocity difference.
- Estimated total mass of at least 18 solar masses.
- Maximum orbital period of approximately 570 years.

## Abstract

Almost all massive stars have bound stellar companions, existing in binaries or higher-order multiples. While binarity is theorized to be an essential feature of how massive stars form, 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 mm continuum and H30alpha recombination line emission, which reveal a massive protobinary with apparent separation of 180 au at the center of the massive star-forming region IRAS07299-1651. From the line-of-sight velocity difference of 9.5 km/s 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 years, assuming a circular orbit. The H30alpha line from the primary protostar shows kinematics consistent with rotation along a ring of radius of 12 au. The observations indicate that disk fragmentation at several hundred au may have formed the binary, and much smaller disks are feeding the individual protostars.