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 3-D, focusing on disc-disc interactions that result from stellar fly-bys. 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 3-D models of self-gravitating discs, and then create a suite of numerical experiments of parabolic encounters, exploring the effects of pericenter separation r_p, disc orientation and disc-star mass ratio (M_d/M_*) on the orbital evolution of the host stars. Close encounters (2r_p<~ 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 M_d<~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.
Highlights
The variety of processes that influence circumstellar disc evolution in multistar systems are not yet well understood
We focus on the direct simulation of circumstellar disc flybys, solving the equations of three-dimensional, selfgravitating hydrodynamics discretized over a moving Voronoi mesh as implemented by the AREPO code (Springel 2010b)
We have shown that realistic circumstellar discs can affect the orbital evolution of their host stars provided the encounters have pericentre separations close to or smaller than the disc sizes
Summary
The variety of processes that influence circumstellar disc evolution in multistar systems are not yet well understood. The existence of evolved discs in binary systems implies that earlier, massive rotating structures must have interacted in some way with stars other than their host. Exploring these more violent interactions, where pericentre separation becomes comparable to disc size, requires high resolution, three-dimensional numerical simulations. Andrews & Williams 2007; Andrews et al 2009), very early protostellar objects are expected to have more massive discs or envelopes The role of these massive young discs/envelopes on stellar dynamics has not been explored much theoretically, but it is possible that the gas component plays a short-lived, but important role that cannot be captured by simple collisionless dynamics.
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