Title: Towards a population synthesis model of self-gravitating disc fragmentation and tidal downsizing II: The effect of fragment-fragment interactions,
Abstract: It is likely that most protostellar systems undergo a brief phase where the
protostellar disc is self-gravitating. If these discs are prone to
fragmentation, then they are able to rapidly form objects that are initially of
several Jupiter masses and larger. The fate of these disc fragments (and the
fate of planetary bodies formed afterwards via core accretion) depends
sensitively not only on the fragment's interaction with the disc, but with its
neighbouring fragments.
We return to and revise our population synthesis model of self-gravitating
disc fragmentation and tidal downsizing. Amongst other improvements, the model
now directly incorporates fragment-fragment interactions while the disc is
still present. We find that fragment-fragment scattering dominates the orbital
evolution, even when we enforce rapid migration and inefficient gap formation.
Compared to our previous model, we see a small increase in the number of
terrestrial-type objects being formed, although their survival under tidal
evolution is at best unclear. We also see evidence for disrupted fragments with
evolved grain populations - this is circumstantial evidence for the formation
of planetesimal belts, a phenomenon not seen in runs where fragment-fragment
interactions are ignored.
In spite of intense dynamical evolution, our population is dominated by
massive giant planets and brown dwarfs at large semimajor axis, which direct
imaging surveys should, but only rarely, detect. Finally, disc fragmentation is
shown to be an efficient manufacturer of free floating planetary mass objects,
and the typical multiplicity of systems formed via gravitational instability
will be low.