Magnetic fields play a dominant role in the dynamics of interstellar matter. Their implementation in computer models is however non-trivial in systems where the gas is partially ionised. Only charged atoms feel the presence of magnetic fields, and collisions between ions and neutrals lead to magnetic diffusion. An early prescription for the coupling between gas dynamics and magnetic fields (known as magneto-hydrodynamics; MHD) ignored this detail (the ‘ideal’ approximation). Its use in simulations of star formation is now known to have pathological side-effects, preventing the formation of proto-planetary disks, and hence planets. This project makes use of a state-of-the-art module for diffusive MHD. The strength of the diffusion is quantified by “resistivities”, that depend on the temperature and abundances of the chemical species. The first task was to compute new tables of resistivities using a chemical network. The tables were published (Marchand et al. 2016, A&A, 592, A18) and were made available publicly.
I then carried out two simulations evolving a large molecular cloud to assess the exact importance of magnetic diffusion; the first used ideal MHD, while the second included magnetic diffusion (see image “Large-scale star formation”). Strong magnetic fields inhibit fragmentation, and many more isolated structures form in the second (middle panel) simulation. This affects the number of stars that are formed and their masses; more massive objects are present in the ideal MHD run (right panel), while the protostellar distribution in the diffusive framework is a far better match to recent telescope observations represented by the purple curve.
The aim of the third task was to refine present models used to represent stars (“star particles”) in global simulations. The end goal is to run stellar evolution software inside each star particle, but this requires an initial starting point, corresponding to the birth of the protostar. We needed to determine which are the starting conditions, depending on the mass, size and temperature of each star’s parent body. We created a large set of star formation models, describing the formation of the stellar embryo, and found this seed’s properties to be independent of the initial conditions, implying all stars have the same starting point. The results were published (Vaytet & Haugbølle 2017, A&A, 598, A116) and presented during an international conference. The raw data from the simulations were also made publicly available.
The second half of the research proposal, the local approach, focussed on the describing all the stages involved in the formation of a protostar, with particular emphasis on magnetic diffusion. I performed two 3D simulations of isolated protostellar formation, using ideal and diffusive MHD. In the ideal MHD run, the magnetic field removes all rotation from the protostar, while enough is preserved to form a small disk in the diffusive run. The rotation also affects the topology of the magnetic field around the protostar (see image “Magnetic field topology in protostars”), where wrapped-up field lines are believed to be responsible for launching observed protostellar jets. These results were published (Vaytet et al. 2018, arXiv:1801.08193) and presented at 3 international conferences.
In the fifth task, I coupled the hydrodynamics of a collapsing cloud to a chemical network counting 65 gas and dust species with over 700 reactions. Chemical abundances in the protostellar and its envelope vary by orders of magnitude, which has important consequences for comparison with observations. We are in the process of identifying the most important reactions to help reduce the size of chemical networks before applying them to full 3D models.
The final task involved studying the formation of high-mass stars, which differs significantly from their low-mass counterparts because of extreme luminosities and winds. Computational models to date have ignored magnetic fields, and we carried out new simulations with MHD. We found that magnetic processes dominate the wind launching mechanism, focusing them over great distances. The masses and accretion rates of the star-disk system are strongly affected.
