CORDIS - EU research results

Star Formation in the Turbulent Interstellar Medium

Final Report Summary - SFTISM (Star Formation in the Turbulent Interstellar Medium)

The objective of this research project is to develop and test the theory of star formation using high-dynamic-range simulations that cover the whole range of scales necessary to describe the Galactic fountain, while at the same time modeling the formation of each individual star. We achieved this by using a computational method known as “adaptive-mesh refinement”, which consists of focusing the computational resources on the most interesting regions. The large-scale Galactic-fountain flow is described at relatively low resolution, while the regions where dense cores collapse into stars are computed at much higher resolution. This method is difficult to implement efficiently in large supercomputers, so part of the challenge consisted in developing a numerical code that could take advantage of the huge number of processors available in modern supercomputers.
We have carried out ambitious galactic-fountain simulations, thanks to large supercomputing allocations awarded to the PI of this project, under the European PRACE program and the NASA High-End-Computing program. We were able to show that the mass distribution of massive stars is a natural result of turbulent flows driven by supernova explosions, and that the rate of star formation is comparable to what is observed in actual galaxies. Besides the derivation of the star formation rate and the stellar mass function, our simulations provide large samples of star-forming clouds that can be studied and compared with observational surveys.
As part of our study of the origin of stellar masses, we have addressed the process of protostellar growth, and the later mass accretion on pre-main-sequence stars (the youngest stars after the heavily gas-embedded protostellar phase has ended). Regarding protostars, we have proposed a natural solution to the long-standing luminosity problem (protostars appear to be dimmer than they should be). We demonstrated that the accretion rates controlled by interstellar turbulent flows are decreasing over time, and also highly variable. Regarding pre-main-sequence stars, we have shown that their circumstellar disks cannot be considered as isolated systems, as the accretion rate on the disks from the surrounding ambient gas can explain the measured accretion rates of these stars.
Our research has been extended to the study of the origin of planetesimals, the low-mass progenitors of fully-fledged planets. In a series of papers, we have tackled both theoretically and numerically the challenging problem of the transport and collisional growth of dust grains transported by the gas turbulence. In particular, we have stressed the importance of accounting for the full probability distribution of collision velocities (rather than just the mean velocity), which can be highly non-Gaussian, with important consequences for the solution of the bouncing and fragmentation barriers to particle growth.
Finally, we have also studied the problem of star formation at high redshift, in order to understand the origin of globular clusters. We have proposed a new model for the origin of globular clusters, based on the merging of mini-halos.