Principal Investigator Helen Pongracic Project g35
Research Centre for Theoretical Astrophysics, Machine VP
University of Sydney
Co-Investigator Sue Byleveld
Research Centre for Theoretical Astrophysics, School of Physics, University of Sydney
The Influence of Magnetic Fields on Star Formation
Stars, even our own sun, were born deep within giant molecular clouds, which are large clouds of hydrogen gas and dust inhabiting the space between stars. The dust shrouds the stars as they are born, obscuring the light they emit and making observations difficult. With the recent advancements in speed and memory capabilities of computers, numerical simulations have become a valuable tool with which to probe the physical processes at work as a star is formed.
We are particularly interested in the influence of magnetic fields on star formation. Just as the Earth possesses its own magnetic field, so too do giant molecular clouds. What is the effect of this field the dynamical processes at work within these clouds which have been proposed as resulting in star formation? Specifically, we are investigating the supersonic collisions which occur between ``sub-clouds'', or regions or enhanced density inside giant molecular clouds. As the ``sub-clouds'' collide they compress the gas between them which then may collapse under its own gravitational forces and form a dense, disk-like structure, the precursor to a star.
The giant molecular clouds in which stars form are composed not only of neutral molecules and atoms, but also of ions which may form from neutral particles via interactions with photons from young, newly formed stars, or with cosmic ray particles. As only the ions couple to the magnetic field in the cloud, and slip between the ions and neutral particles is possible, the magnetic field is no longer fixed, but can move relative to the neutral particles. A new aspect of our research is to consider star formation in this more realistic environment.
A large number of particles (~20000) must be used so that our three dimensional simulations can be as realistic as possible. Not only do we require many particles, but our consideration of hydrodynamic, gravitational and magnetic effects using a vectorized code, means that only very powerful computers, such as the VP2200 are capable of running our simulations.
What are the basic questions addressed?
In this project we are investigating what influences magnetic fields have on star formation which occurs via dynamic processes at work within giant molecular clouds. Although many studies have considered the influence of magnetic fields on the quasi-static, slow collapse of single molecular clouds leading to the formation of protostellar disks, less is know about dynamic star formation. In particular, we are interested in the formation of protostellar disks resulting from density enhancements in shock compressed material. The shock compression arises from the supersonic collision of "sub-clouds'' within the giant molecular clouds themselves. As the presence of shocks is an integral part of our model, we are also considering the structure and propagation of shocks themselves in molecular clouds.
Questions we are addressing about the influence of magnetic fields on this system include:
Do magnetic fields (of a magnitude comparable with the galactic field) slow the formation of "protostars''?
What magnetic field strength is required to arrest their formation entirely?
How do these collisions of ``sub-clouds'' affect the morphology of the magnetic field? How do magnetic fields influence the formation of binary or multiple systems?
Can we realistically reproduce propagating continuous (C) and jump (J) shocks in a partially ionized plasma (the ambient medium of a molecular cloud) ?
How does the presence of either a C or J shock influence shock compression in this mechanism of star formation?
What are the results to date and future of the work?
To date we have successfully run simulations with a range of initial conditions including various magnetic field morphologies and strengths. In all cases where the field resembled the non-random component of the galactic magnetic field collapse to a protostellar disk still occurred, but took ~ 10-20% longer than in the corresponding unmagnetized system. However, for a magnetic field strength an order of magnitude greater than the galactic field no collapse occurs. The ``sub-clouds'' collide, compressing material between them, but then re-expand before collapse under self-gravity can occur.
One of the major questions we have investigated during the last year is whether the presence of a magnetic field inhibits the formation of binary or multiple stellar systems via this mechanism of star formation. In results presented at the Astronomical Society of Australia meeting in 1995 we found that only a single protostellar system formed under the same initial conditions as used in a study by Chapman et al. (1992) where Chapman and his colleagues found a binary star was produced. The only notable difference in the two simulations was that our calculation included a constant magnetic field. Further work has shown that the formation of binary systems is still possible via this mechanism in the presence of a magnetic field of typical of the galactic magnetic field. However the formation of such systems is less likely to occur than indicated in previous studies where magnetic fields were not considered.
As mentioned in our description of the project, star forming regions are only partially ionized. This leads to the effect of ambipolar diffusion which has been shown to be of great importance in quasi-static theories of star formation. We have developed our code further to encorporate the physics of partially ionized plasmas. SPH is well suited to such a problem in that, being a particle method, it can keep track of the neutral and ionized particles as they interact with each other. We have commenced tests of this new code and have presented preliminary test results at a recent workshop held by the RCfTA at Sydney University on "Shocks in Molecular Clouds". The code should soon be ready to be applied to the simulation of C and J shocks in molecular clouds.
What computational techniques are used and why is a supercomputer required?
The code is three dimensional and uses a magnetic version of smoothed particle hydrodynamics with tree-code gravity. A super computer is required because:
The code is well vectorized (it was developed to run on a CRAY and has been adapted to run on a VP2200) and runs more efficiently on a vector machine than local Sparc stations and Dec-Alpha machines. The magnetic fields have been included in the code in such a way as to retain the vectorized structure.
With a supercomputer more particles can be used which is necessary to represent ``sub-clouds'' and the surrounding medium in three dimensions.
We require a large amount of memory. (There are 6 arrays for the magnetic variables alone).
We require a realistic turn-around time for large jobs which cannot be obtained using a local workstation. This is especially true of the latest version of the code which simulates a partially ionized plasma. In this code the drag between the ions and neutral particles restricts the allowed timestep size considerably.
The Influence of Magnetic Fields on Star Formation,
S.E. Byleveld, H Pongracic,. PASA, 13, 71. (1996).