Marriages Made in Heaven: the Origin of Multiple Star Systems

Context. Most high- and intermediate-mass stars are in stable multiple systems, usually binary
systems where the two stars pursue elliptical orbits round each other, but quite 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, after they have formed). Indeed, the observational
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 statistical properties of observed systems is therefore one of the main challenges of star formation
theory.
Project. 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; we will be particularly
concerned with multiple systems forming in cores that have condensed out of filaments. The majority
of stars that do not end up in multiple systems have low-mass and the project will also explore
how these objects form and why they tend to end up single. One possibility is that they form in
circumstellar discs, but then fail to acquire enough mass and are ejected from the disc by interaction
with more massive siblings. 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 exceptionally
dense low-mass cores. We will determine which of these scenarios is most viable and why.
Skills. The student will become expert in numerical hydrodynamics, magnetic, thermal, chemical
and radiative processes in star forming gas, and statistical techniques for converting observations
into initial conditions for simulations