A Dynamic Formation Channel for Binaries in Embedded Clusters

We present hybrid MHD+N-body simulations of star cluster formation and evolution including self consistent feedback from the stars in the form of radiation, winds, and supernovae from massive stars. The MHD is modeled with the adaptive mesh refinement code FLASH, while the N-body computations are done with the 4th order Hermite code ph4 and stars are evolved using SeBa. We use a gravity bridge within the Astrophysical MUltipurpose Software Environment (AMUSE) framework to couple the N-body dynamics and stellar evolution of the stars to the gas dynamics in FLASH. Our results include the observation of the formation of binaries in all simulations, with and without feedback, with roughly half of all stars with M > 8 M⊙ found in binaries, without any primordial binary formation needed. We find our binary fraction increases in a power law fashion as a function of primary mass, while our eccentricity distribution appears thermal. For a binary binding energy x, the binaries themselves are well separated into hard (|x| /<kT> > 1) and soft (|x| /<kT> < 1) binaries, with a clear division where the binary energy equals the mean cluster thermal energy, |x| = <kT>. Generally, the hard binaries are also massive, with semi-major axes 1 ≤ a ≤ 20 AU. Finally our mass ratio distribution is well fit by a power law f(q) ∝ q-Γ for q < 0.5 with Γ = 0.5, but is multi-modal for q > 0.5.