The Orbital Decay of Embedded Binary Stars

TL;DR

This paper derives an analytic model for how embedded binary stars within dense molecular clouds experience orbital decay due to dynamical friction from acoustic waves, potentially leading to stellar mergers and high-mass star formation.

Contribution

It introduces a simple analytic expression for the braking torque on binaries caused by acoustic wave emission, linking orbital decay to star formation processes.

Findings

Binary orbits decay within 10^5 years in dense clouds.

Acoustic waves carry away angular momentum, causing orbital shrinkage.

Mergers may explain the formation of high-mass stars.

Abstract

Young binaries within dense molecular clouds are subject to dynamical friction from ambient gas. Consequently, their orbits decay, with both the separation and period decreasing in time. A simple analytic expression is derived for this braking torque. The derivation utilizes the fact that each binary acts as a quadrupolar source of acoustic waves. The acoustic disturbance has the morphology of a two-armed spiral and carries off angular momentum. From the expression for the braking torque, the binary orbital evolution is also determined analytically. This type of merger may help explain the origin of high-mass stars. If infrared dark clouds, with peak densities up to 10^7 cm^{-3}, contain low-mass binaries, those with separations less than 100 AU merge within about 10^5 yr. During the last few thousand years of the process, the rate of mechanical energy deposition in the gas exceeds the stars' radiative luminosity. Successive mergers may lead to the massive star formation believed to occur in these clouds.