Marriages made in Heaven (astronomy and astrophysics): How brown dwarfs form and why they tend to end up single

Roughly 50% of stars are in stable multiple systems, usually binary systems where the two stars pursue elliptical orbits round each other, but often higher-order multiples involving three or more stars. These systems must be created as part of the star formation process - it is very hard to get two stars together, once they have formed - and indeed, all the evidence suggests that almost all stars form in multiple systems, but then some of the more fragile systems are destroyed by external tidal forces or intrinsic instability. Forming multiple systems with the properties of observed systems is therefore one of the main challenges of star formation theory.
This project will use numerical simulations to explore the formation dynamics, stability and orbital parameters (mass ratios, separations and eccentricities) of multiple systems, starting from initial conditions that are informed by the latest high-resolution observations [in particular Herschel observations processed with the new PPMAP algorithm (Marsh, Whitworth & Lomax, 2015, mnras 454 4282) which delivers both greatly increased resolution and the true distribution of dust temperature], and tracking the evolution with synthetic observations of their spectral energy distributions and their short-term variability (since this reflects the stochastic accretion onto forming protostars). The majority of stars that do not end up in multiple systems are Brown Dwarfs, sometimes called failed stars, but actually just stars that become dense enough to be supported by degeneracy pressure before they become hot enough to start fusing hydrogen to form helium. This project will also shed light on how brown dwarfs form and why they tend to end up single. One strong possibility is that they form in circumstellar discs, but then, if they can’t acquire enough mass, they are prone to being ejected from the disc by interaction with more massive stars. An alternative possibility is that they form in the filaments that funnel matter into forming star clusters. A third possibility is that they form in isolation from especially dense low-mass cores. The student will become expert in numerical hydrodynamics, thermal, chemical and radiative processes in star forming gas, and statistical techniques for converting observations into initial conditions for simulations.