The orbital decay of embedded binary 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 in explaining the origin of high-mass stars. If infrared dark clouds, with peak densities up to 107 cm-3, contain low-mass binaries, those with separations less than 100 au merge within about 105 years. 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. © 2009 The Author. Journal compilation © 2009 RAS.