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The origins of stellar properties

Final Report Summary - STELLARPROP (The origins of stellar properties)

Less than a century ago it was realised that stars are still forming in our Galaxy today. Over the decades since, the questions of what physical processes dominate the star formation process and how the statistical properties of stars are determined have been some of the key questions in astrophysics. This project has advanced theoretical studies of star formation from those that have been constantly searching for a mixture of initial conditions and physical processes that can reproduce the stellar properties that we observe, to a predictive theory that can predict how stellar properties may vary in different environments and with different initial conditions. We have predicted how stellar properties should vary if the gas that forms stars has a substantially lower or higher abundance of heavy elements than our Sun, finding that there is little change in the distribution of stellar masses, but that there is a substantial increase in the proportion of close binary star systems when the gas has a lower abundance of heavy elements. We have predict that in very dense star-forming clouds there should be a slightly higher fraction of brown dwarfs and low-mass stars formed relative to solar-type and more massive stars. For the first time, we have also predicted the distributions of the mass and size of the discs of gas and dust that form around very young stars. These discs are where planets form, and our predictions are now being tested by the latest radio observations of the discs around young stars. Along with making predictions of how stellar properties should vary in different star-forming regions, we have substantially advanced the understanding of the role that magnetic fields play in the star formation process. We have shown that in realistic star-forming clouds, magnetic fields do not substantially inhibit the formation of discs around young stars, contrary to earlier studies that began with more idealised initial conditions and simplified magnetic field equations (which did not treat non-ideal magnetohydrodynamic processes such as the Hall effect, ambipolar diffusion, and/or Ohmic resistivity). We have also studied how magnetic fields may be dragged in with collapsing gas and implanted in a star even as it is just beginning to form. We have also performed the first dusty hydrodynamical calculations of how dust particles evolve during the collapse of a gas cloud to form a star. Finally, in order to conduct these studies, we have developed new numerical methods for modelling the thermodynamics of star-forming clouds, magnetic fields, and dust-gas mixtures.