Stellar orbit evolution in close circumstellar disc encounters

Abstract

The formation and early evolution of circumstellar discs often occurs within dense, newborn stellar clusters. For the first time, we apply the moving-mesh code arepo, to circumstellar discs in 3D, focusing on disc–disc interactions that result from stellar flybys. Although a small fraction of stars are expected to undergo close approaches, the outcomes of the most violent encounters might leave an imprint on the discs and host stars that will influence both their orbits and their ability to form planets. We first construct well-behaved 3D models of self-gravitating discs, and then create a suite of numerical experiments of parabolic encounters, exploring the effects of pericentre separation rp, disc orientation and disc–star mass ratio (Md/M*) on the orbital evolution of the host stars. Close encounters (2rp ≲ disc radius) can truncate discs on very short time-scales. If discs are massive, close encounters facilitate enough orbital angular momentum extraction to induce stellar capture. We find that for realistic primordial disc masses Md ≲ 0.1M*, non-colliding encounters induce minor orbital changes, which is consistent with analytic calculations of encounters in the linear regime. The same disc masses produce entirely different results for grazing/colliding encounters. In the latter case, rapidly cooling discs lose orbital energy by radiating away the energy excess of the shock-heated gas, thus causing capture of the host stars into a bound orbit. In rare cases, a tight binary with a circumbinary disc forms as a result of this encounter.