Forming spectroscopic massive proto-binaries by disk fragmentation

TL;DR

This study uses advanced simulations to demonstrate that massive protostellar discs can fragment to form close binary systems, which may also produce observable accretion bursts like FU Orionis events.

Contribution

It provides the first detailed 3D simulations showing how disc fragmentation around massive protostars can lead to close binary formation and associated accretion phenomena.

Findings

Disc fragmentation occurs violently in massive protostars.

Some fragments migrate to form close massive proto-binaries.

Disc fragmentation may be traced by FU Orionis-type bursts.

Abstract

The surroundings of massive protostars constitute an accretion disc which has numerically been shown to be subject to fragmentation and responsible for luminous accretion-driven outbursts. Moreover, it is suspected to produce close binary companions which will later strongly influence the star's future evolution in the Hertzsprung-Russel diagram. We present three-dimensional gravitation-radiation-hydrodynamic numerical simulations of 100 Mo pre-stellar cores. We find that accretion discs of young massive stars violently fragment without preventing the (highly variable) accretion of gaseous clumps onto the protostars. While acquiring the characteristics of a nascent low-mass companion, some disc fragments migrate onto the central massive protostar with dynamical properties showing that its final Keplerian orbit is close enough to constitute a close massive proto-binary system, having a young high-mass and a low-mass component. We conclude on the viability of the disc fragmentation channel for the formation of such short-period binaries, and that both processes -close massive binary formation and accretion bursts- may happen at the same time. FU-Orionis-type bursts, such as observed in the young high-mass star S255IR-NIRS3, may not only indicate ongoing disc fragmentation, but also be considered as a tracer for the formation of close massive binaries - progenitors of the subsequent massive spectroscopic binaries - once the high-mass component of the system will enter the main-sequence phase of its evolution. Finally, we investigate the ALMA-observability of the disc fragments.