Dynamically triggered star formation in giant molecular clouds

A Lagrangian, particle-based numerical method (tree code gravity plus smoothed particle hydrodynamics) was used to simulate clump-clump collisions occurring within GMCs. The collisions formed shock-compressed layers, out of which condensed approximately co-planar protostellar discs of 7-60 solar masses and 500-1000AU radius. Binary and multiple systems were the usual final state. Lower mass objects were also produced, but commonly underwent disruption or merger. Such objects occasionally survived by being ejected via a three-body slingshot event resulting from an encounter with a binary system. Varying the impact parameter, b, altered the processes by which the protostellar systems formed. At low b a single central disc formed initially, and was then spun-up by an accretion flow, causing it to produce secondaries via rotational instabilities. At mid b the shocked layer w hich formed initially broke up into fragments, and discs were then formed via fragment merger. At large b single objects formed within the compressed leading edge of each clump. These became unbound from each other as b was increased further. The effect of changing numerical factors was examined by : (i) colliding clumps which had been re-oriented before the collision (thus altering the initial particle noise), and (ii) by quadrupling the number of particles in each clump (thus increasing the resolution of the simulation). Both changes were found to affect the small-scale details of a collision, but leave the large scale morphology largely unaltered. It was concluded that clump-clump collisions provide a natural mechanism by which multiple protostellar systems may form.Comment: 15 pages, 12 low resolution figures in 50 files, accepted by MNRA

Numerical simulations of protostellar encounters III. Non-coplanar disc-disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. If protostars are formed individually within a cluster before falling together and interacting, there should be no preferred orientation for such interactions. As star formation within clusters is believed to be coeval, it is probable that during interactions, both protostars possess massive, extended discs. We have used an SPH code to carry out a series of simulations of non-colpanar disc-disc interactions. We find that non-coplanar interactions trigger gravitational instabilities in the discs, which may then fragment to form new companions to the existing stars. (This is different from coplanar interactions, in which most of the new companion stars form after material in the discs has been swept up into a shock layer, and this then fragments.) The original stars may also capture each other, leading to the formation of a small-N cluster. If every star undergoes a randomly oriented disc-disc interaction, then the outcome will be the birth of many new stars. Approximately two-thirds of the stars will end up in multiple systems.Comment: 12 pages, submitted to MNRAS; low resolution figures onl

Numerical simulations of protostellar encounters I. Star-disc encounters

It appears that most stars are born in clusters, and that at birth most stars have circumstellar discs which are comparable in size to the separations between the stars. Interactions between neighbouring stars and discs are therefore likely to play a key role in determining disc lifetimes, stellar masses, and the separations and eccentricities of binary orbits. Such interactions may also cause fragmentation of the discs, thereby triggering the formation of additional stars. We have carried out a series of simulations of disc-star interactions using an SPH code which treats self-gravity, hydrodynamic and viscous forces. We find that interactions between discs and stars provide a mechanism for removing energy from, or adding energy to, the orbits of the stars, and for truncating the discs. However, capture during such encounters is unlikely to be an important binary formation mechanism. A more significant consequence of such encounters is that they can trigger fragmentation of the disc, via tidally and compressionally induced gravitational instabilities, leading to the formation of additional stars. When the disc-spins and stellar orbits are randomly oriented, encounters lead to the formation of new companions to the original star in 20% of encounters. If most encounters are prograde and coplanar, as suggested by simulations of dynamically-triggered star formation, then new companions are formed in approximately 50% of encounters.Comment: 17 pages, submitted to MNRAS; low resolution figures onl

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

Meeting abstrac

Star formation and the singular isothermal sphere

We caution against approximating the initial conditions for protostellar collapse with a singular isothermal sphere. First, it is very unlikely that nature can assemble anything like a singular isothermal sphere. Secondly, collapse of a singular isothermal sphere would seem to be severely prejudiced against binary formation - either by fragmentation during collapse, or by disc instability following collapse. It is therefore appropriate to explore star formation paradigms which (a) involve initial conditions which are far less focused than the singular isothermal sphere, and (b) take into account impulsive dynamical and thermodynamic processes - reflecting the rapidly varying environments in which stars are formed. Even in relatively quiescent regions like Taurus, such processes must be reckoned with; they cannot realistically be relegated to the status of small perturbations on an essentially quasistatic theme

Estimating density in smoothed particle hydrodynamics

Flebbe et al. have suggested that, when estimating the density at the position of an SPH particle, it is appropriate to discount the contribution from the particle itself. We explain why this suggestion is incorrect, and present numerical results in support of our explanation

Star formation and the singular isothermal sphere

We caution against approximating the initial conditions for protostellar collapse with a singular isothermal sphere. First, it is very unlikely that nature can assemble anything like a singular isothermal sphere. Secondly, collapse of a singular isothermal sphere would seem to be severely prejudiced against binary formation - either by fragmentation during collapse, or by disc instability following collapse. It is therefore appropriate to explore star formation paradigms which (a) involve initial conditions which are far less focused than the singular isothermal sphere, and (b) take into account impulsive dynamical and thermodynamic processes - reflecting the rapidly varying environments in which stars are formed. Even in relatively quiescent regions like Taurus, such processes must be reckoned with; they cannot realistically be relegated to the status of small perturbations on an essentially quasistatic theme