Astronomy and Space Science Theory Group (ASSTG)
http://hdl.handle.net/2104/5513
2016-11-17T22:03:52ZMultipole Expansions of Aggregate Charge: How Far to Go?
http://hdl.handle.net/2104/9881
Multipole Expansions of Aggregate Charge: How Far to Go?
Matthews, Lorin; Coleman, Douglas A.; Hyde, Truell W.
Aggregates immersed in a plasma or radiative environment will have charge distributed over their extended surface. Previous studies have modeled the aggregate charge using the monopole and dipole terms of a multipole expansion, with results indicating that the dipole-dipole interactions play an important role in increasing the aggregation rate and altering the morphology of the resultant aggregates. This study examines the effect that including the quadrupole terms has on the dynamics of aggregates interacting with each other and the confining electric fields in laboratory experiments. Results are compared to modeling aggregates as a collection of point charges located at the center of each spherical monomer comprising the aggregate.
2016-04-01T00:00:00ZCoagulation in the Vicinity of a Gap-Opening Jupiter-Mass Planet
http://hdl.handle.net/2104/9880
Coagulation in the Vicinity of a Gap-Opening Jupiter-Mass Planet
Carballido, Augusto
We analyze the coagulation of dust in and around a gap opened by a Jupiter-mass planet. To this end, we carry out a high-resolution magnetohydrodynamic (MHD) simulation of the gap environment, which is turbulent due to the magnetorotational instability. From the MHD simulation, we obtain values of the gas velocities, densities, and turbulent stresses (a) close to the gap edge, (b) in one of the two gas streams that accrete onto the planet, (c) inside the low-density gap, and (d) outside the gap. The MHD values are then input into a Monte Carlo dust-coagulation algorithm which models grain sticking and compaction. We also introduce a simple implementation for bouncing, for comparison purposes. We consider two dust populations for each region: one whose initial size distribution is monodisperse, with monomer radius equal to 1 μm, and another one whose initial size distribution follows the Mathis–Rumpl–Nordsieck distribution for interstellar dust grains, with an initial range of monomer radii between 0.5 and 10 μm. Without bouncing, our Monte Carlo calculations show steady growth of dust aggregates in all regions, and the mass-weighted (m-w) average porosity of the initially monodisperse population reaches xtremely high final values of 98%. The final m-w porosities in all other cases without bouncing range between 30% and 82%. The efficiency of compaction is due to high turbulent relative speeds between dust particles. When bouncing is introduced, growth is slowed down in the planetary wake and inside the gap. Future studies will need to explore the effect of different planet masses and electric charge on grains.
2016-06-01T00:00:00ZPhotophoretic Force on Aggregate Grains, Monthly Notices of the Royal Astronomical Society
http://hdl.handle.net/2104/9597
Photophoretic Force on Aggregate Grains, Monthly Notices of the Royal Astronomical Society
Matthews, Lorin; Kimery, J. B.; Wurm, G.; de Beule, C.; Kuepper, M; Hyde, T. W.
The photophoretic force may impact planetary formation by selectively moving solid particles based on their composition and structure. This generates collision velocities between grains of different sizes and sorts the dust in protoplanetary discs by composition. This numerical simulation studied the photophoretic force acting on fractal dust aggregates of μm-scale radii. Results show that aggregates tend to have greater photophoretic drift velocities than spheres of similar mass or radii, though with a greater spread in the velocity. While the drift velocities of compact aggregates continue to increase as the aggregates grow larger in size, fluffy aggregates have drift velocities which are relatively constant with size. Aggregates formed from an initially polydisperse size distribution of dust grains behave differently from aggregates formed from a monodisperse population, having smaller drift velocities with directions which deviate substantially from the direction of illumination. Results agree with microgravity experiments which show the difference of photophoretic forces with aggregation state.
2016-01-21T00:00:00ZMeasurement of net electric charge and dipole moment of dust aggregates in a complex plasma
http://hdl.handle.net/2104/9203
Measurement of net electric charge and dipole moment of dust aggregates in a complex plasma
Yousefi, Raziyeh; Davis, Allen; Carmona-Reyes, Jorge; Matthews, Lorin; Hyde, Truell
Understanding the agglomeration of dust particles in complex plasmas requires knowledge of basic properties such as the net electrostatic charge and dipole moment of the dust. In this study, dust aggregates are formed from gold-coated mono-disperse spherical melamine-formaldehyde monomers in a radiofrequency (rf) argon discharge plasma. The behavior of observed dust aggregates is analyzed both by studying the particle trajectories
and by employing computer models examining three-dimensional structures of aggregates and their interactions and rotations as induced by torques arising from their dipole moments. These allow the basic characteristics of the dust aggregates, such as the electrostatic charge and dipole moment, as well as the external electric field, to be determined. It is shown that the experimental results support the predicted values from computer models for aggregates in these environments.
2014-09-02T00:00:00Z