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High resolution hydrodynamic simulations of star formation and protoplanetary disc evolution

Final Activity Report Summary - SPH-DISC-SIMULATIONS (High resolution hydrodynamic simulations of star formation and protoplanetary disc evolution)

Stars form by the contraction of interstellar matter. A significant concentration of gaseous material is required before this matter starts falling together at a high rate, forced by its own gravity. This stage is called gravitational collapse. However, not all of this matter ends up in the stellar embryo, called protostar, which cannot accommodate material that moves too fast. Such material is first accumulated in a disc surrounding the protostar. The gas in this disc is then slowly channelled onto the protostar.

At the early stages of their formation, such protostellar discs contain significant amount of material. During these early stages, additional low-mass objects are likely to form in such discs, via a process called disc fragmentation. Depending on their final mass, these additional objects can be stellar in nature, thus forming tight binary and higher-order stellar systems together with the original central protostar. In addition, planetary-type objects can also form by this process, hence giving birth to planetary systems orbiting around the original central protostar. There is speculation that this may be indeed the origin of our Solar system.

The above process of star and planet formation is simultaneously repeated in different parts of interstellar space and in several cases the protostars form in great proximity with each other, i.e. they form in small groups or clusters. The members of such young clusters are known to dynamically interact with each other. Despite the fact that both the formation of clusters of protostellar discs and the formation of planets in isolated discs were extensively studied in the past, there was no study combining the two processes and examining e.g. the role of disc-disc interactions within a realistic cluster environment on the evolution of these discs and the probability of planet formation. With this project we aimed to address this problem, i.e. to investigate whether interactions within such a cluster of protostars could induce or inhibit the formation of low-mass objects in protostellar discs.

We employed advanced numerical techniques to simulate the formation of young clusters with members attended by protostellar discs. We found that sometimes disc-disc interactions could be destructive leading to either the merging of discs or the formation of larger discs attending tight binary stellar systems formed during the merging process, or the stripping of one or more of the interacting discs with disc material being lost during the interaction. However, there were also interactions that induced the formation of low-mass objects in the discs and in particular the formation of planets. At this stage, there could not be conclusive evidence on whether the planetary-type objects formed in these simulations would remain in orbit around their star, as the simulations ended at a stage giving still room for dynamical evolution both in the disc and the cluster. It was also established that our models needed to be improved so that we could take into account in more detail the thermal behaviour of the gas in the discs, i.e. include into our calculations the way that gas emitted, in the form of thermal energy, the radiation that it received during its compression. This was a project we would like to work on in the future.

Because of the fact that we had to treat accurately enough the compression of interstellar gas, that moved very fast, in our models, we implemented a direct comparison of the performance of different numerical schemes on the modelling of the behaviour of such gas during the project. This exercise showed that the numerical techniques employed for the above mentioned problem of star and planet formation included a reasonable treatment of the process of fast gas compression, called compressible turbulence.