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A theoretical perspective on the formation and fragmentation of protostellar discs

We discuss the factors influencing the formation and gravitational fragmentation of protostellar discs. We start with a review of how observations of prestellar cores can be analysed statistically to yield plausible initial conditions for simulations of their subsequent collapse. Simulations based on these initial conditions show that, despite the low levels of turbulence in prestellar cores, they deliver primary protostars and associated discs which are routinely subject to stochastic impulsive perturbations; consequently misalignment of the spins and orbits of protostars are common. Also, the simulations produce protostars that collectively have a mass function and binary statistics matching those observed in nearby star formation regions, but only if a significant fraction of the turbulent energy in the core is solenoidal, and accretion onto the primary protostar is episodic with a duty cycle > 3000 yr. Under this circumstance a core typically spawns between 4 and 5 protostars, with high efficiency, and the lower-mass protostars are mainly formed by disc fragmentation. The requirement that a proto-fragment in a disc lose thermal energy on a dynamical timescale dictates that {\bf there is a sweet spot for} disc fragmentation at radii 70 AU < R < 100 AU and temperatures 10 K < T < 20 K, and this might explain the Brown Dwarf Desert.