EXETER Highlight: The formation of a star and its magnetized outflows in radiation magneto-hydrodynamic simulations

EXETER Highlight: The formation of a star and its magnetized outflows in radiation magneto-hydrodynamic simulations

As a gravitationally unstable molecular cloud core undergoes collapse to form a star, it undergoes several distinct phases. Initially it collapses almost isothermally as energy is easily radiated away. However, eventually the density rises to the point where radio and infrared radiation cannot freely escape from the gas, and the temperature begins to increase. This substantially increases the pressure, resulting in the formation of a pressure supported gaseous object with a typical size of ~10 AU (known as the first core). This object increases in mass as it accretes surrounding gas and, as it does so, its central temperature continues to rise. When its central temperature reaches ~2000 K, the molecular hydrogen from which it is primarily composed dissociates into atomic hydrogen. This is an endothermic process which results in a second collapse occurring inside the first core. This second collapse continues until the hydrogen is completely atomic, where upon a second, or stellar, core is formed with a size similar to that of our Sun. This object then accretes to its final mass to become a star. If reasonably strong magnetic fields are present initially, outflows from both the first core and the stellar core can be launched at speeds of several km/s. These outflows can carry some of the mass, angular momentum, and energy away from the forming star.

Figure 1. Visualizations of the magnetic field direction in the outflows from the first cores (top) and stellar cores (bottom) in three calculations with different initial magnetic field strengths (magnetic field strength increases from left to right). Published in Bate, Tricco & Price (2013).

The DiRAC facility, Complexity, has been used to perform the first fluid dynamical simulations that include both magnetic fields and radiation transport to follow this whole process. Such calculations are essential for predicting what may be observed in highresolution observations that will be made using the Attacama Large (Sub-)Millimetre Array (ALMA) over the next few years, and interpreting these observations. In the Figure, we show visualizations of the magnetic field structure in the outflows. Animations of the calculations are at: http://www.astro.ex.ac.uk/people/mbate/Animations/stellarcore.html and the full dataset from the calculations is on the Open Research Exeter archive at http://hdl.handle.net/10871/13883 .