We analyse a hydrodynamical simulation of star formation. Sink particles in the simulations which represent stars show episodic growth, which is presumably accretion from a core that can be regularly replenished in response to the fluctuating conditi
ons in the local environment. The accretion rates follow $dot{m} propto m^{2/3}$, as expected from accretion in a gas-dominated potential, but with substantial variations over-laid on this. The growth times follow an exponential distribution which is tapered at long times due to the finite length of the simulation. The initial collapse masses have an approximately lognormal distribution with already an onset of a power-law at large masses. The sink particle mass function can be reproduced with a non-linear stochastic process, with fluctuating accretion rates $propto m^{2/3}$, a distribution of seed masses and a distribution of growth times. All three factors contribute equally to the form of the final sink mass function. We find that the upper power law tail of the IMF is unrelated to Bondi-Hoyle accretion.
The presence of young massive stars orbiting on eccentric rings within a few tenths of a parsec of the supermassive black hole in the Galactic centre is challenging for theories of star formation. The high tidal shear from the black hole should tear
apart the molecular clouds that form stars elsewhere in the Galaxy, while transporting the stars to the Galactic centre also appears unlikely during their stellar lifetimes. We present numerical simulations of the infall of a giant molecular cloud that interacts with the black hole. The transfer of energy during closest approach allows part of the cloud to become bound to the black hole, forming an eccentric disc that quickly fragments to form stars. Compressional heating due to the black hole raises the temperature of the gas to 100-1000K, ensuring that the fragmentation produces relatively high stellar masses. These stars retain the eccentricity of the disc and, for a sufficiently massive initial cloud, produce an extremely top-heavy distribution of stellar masses. This potentially repetitive process can therefore explain the presence of multiple eccentric rings of young stars in the presence of a supermassive black hole.
We present SPH simulations of protoclusters including the effects of winds from massive stars. Using a particle-injection method, we investigate the effect of structure close to the wind sources and the short-timescale influence of winds on protoclus
ters. Structures such as disks and gaseous filaments have a strong collimating effect. By a different technique of injecting momentum from point sources, we compare the large-scale, long-term effects of isotropic and intrinsically-collimated winds and find them to be similar. Both types of wind dramatically slow the global star formation process, but the timescale on which they expel significant mass from the cluster is rather long (approaching 10 freefall times). Clusters may then experience rapid star formation early in their lifetimes, before switching to a mode where gas is gradually expelled, while star formation proceeds much more slowly. This complicates conclusions regarding slow star formation derived from measuring the star-formation efficiency per freefall time. Estimates of the efficacy of winds in dispersing clusters derived simply from the total wind momentum flux may not be very reliable. This is due to material being expelled from deep within stellar potential wells, often to velocities well in excess of the cluster escape velocity, and also to the loss of momentum flux through holes in the gas distribution. Winds have little effect on the accretion--driven stellar IMF except at the very high-mass end, where they serve to prevent some of the most massive objects accreting. We also find that the morphology of the gas, the rapid motions of the wind sources and the action of accretion flows prevent the formation of bubble-like structures. This may make it difficult to discern the influence of winds on very young clusters.
