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Disc Formation during the Star Formation process
Magnetic fields are an integral part of star formation, and have been included in most recent numerical models. However, when ideal magnetic fields are included, the numerical results contradict observations. For example, numerical simulations cannot form massive discs around these young pre-star objects (e.g. the first hydrostatic core, or protostar). This has been studied by modelling the collapse of a one solar mass, rotating gas cloud embedded in a vertical magnetic field. When non-ideal magnetohydrodynamics (MHD; Ohimc resistivity + Hall effect + ambipolar diffussion) are included in these collapse simulations, discs are able to form. Bipolar outflows are suppressed.
A collapsing, one solar mass gas cloud, modelled in the presence of a strong magnetic field. The video starts at 0.95 free-fall times. Left: Ideal MHD. Right: Non-ideal MHD.
The Hall effect is sensitive to the direction of the magnetic field. When modelling a collapse with the Hall effect as the only non-ideal MHD term, no disc is formed when the magnetic field is aligned with the rotation vector of the cloud, while a massive disc is formed when the magnetic field is anti-aligned with the rotation vector of the cloud.
A collapsing, one solar mass gas cloud, modelled in the presence of a strong magnetic field; both models include the Hall effect as the only non-ideal MHD term. The video starts at 0.95 free-fall times. Left: aligned magnetic field and rotation vectors. Right: anti-aligned magnetic field and rotation vectors.
In these models, ambipolar diffusion is the dominant non-ideal MHD term, and is not dependant on the direction of the magnetic field. In models that include all three non-ideal MHD terms, the Hall effect can still influence the size of the resulting discs, impliying that the Hall effect does not need to be strong in order to yield an observable result.
A collapsing, one solar mass gas cloud, modelled in the presence of a strong magnetic field; both models include all three non-ideal MHD terms. The video starts at 0.95 free-fall times. Left: aligned magnetic field and rotation vectors. Right: anti-aligned magnetic field and rotation vectors.
The above study has been performed using the smooth particle magnetohydrodynamics (SPMHD) code Phantom. The results are published in Wurster, Price & Bate (2016).
To determine the strengths of the non-ideal MHD terms, ionisation levels are required to determine the number densities of the charged particles. The module to self-consistently calculate the ionisation and subsequently calculate the number densities in Phantom will soon be publically released as a stand-alone module. There will be several options and parameters for the user to modify, thus this module will be relevant for a wide range of problems. Although this module has been fully tested for SPMHD, it has been designed to be platform-independent, thus can also be implemented into grid code. The module is written in Fortran95.
The library and details of how to download, implement and use it, will be released shortly.
When a galaxy's central region is very luminous (more luminous than expected, and often brighter than the rest of the galaxy), it is termed an active galactic nuclus (AGN). It is generally accepted that AGN are fuelled by gas flowing onto the supermassive black hole located at the at the centre of the galaxy. Some of the gas that is accreted onto the black hole is converted to energy and released to the surrounding environment. This feedback energy has the ability to shape both the large and small scale environment around the black hole.
There are many different numerical algorithms to describe AGN feedback. However, it is impossible to numerically compare them when they are modelled using different numerical codes and different initial conditions. Thus, to properly compare the algorithms, I model them under the same initial conditions using the same numerical code. My simulations start from two Milky Way-sized galaxies on parabolic orbits around one another, and after 1 Gyr, the two galaxies merge. Thus, I am able to track the affects the various AGN algorithms have on the evolution of the merging system. As expected, the various algorithms produce large and small scale differences, and I am able to quantify the differences since all simulations start from the same initial conditions and are all run using the numerical code, Hydra. Below are movies showing the evolution of the system, and comparing the evolution of six different models.
The evolution of Model WT.
The face-on evolution of gas density of six different models.
The edge-on evolution of gas density of six different models.
The evolution of gas density of Model WT. The simulation pauses to rotate at apoapsis, core merger and at the end of the simulation.
