Astrophysics
1) Experimental method to generate "astrophysical" dust
To model the size distribution and composition of interstellar and interplanetary dust grains, and their effect on a wide range of phenomena, it is vital to understand the mechanism of dust-shock interaction. We demonstrate a new laser experiment that subjects dust grains to pressure spikes similar to those of colliding astrophysical dust, and that accelerates the grains to astrophysical velocities. This new method generates much larger data sets than earlier methods; we show how large quantities (thousands) of grains are accelerated at once, rather than accelerating individual grains, as is the case of earlier methods using electric fields. We also measure the in-flight velocity (~4.5km/s) of hundreds of grains simultaneously by use of a particle image velocimetry (PIV) technique.
2) Carbon grain collision
In this study we analyze, using molecular dynamics, the collision dynamics of two clusters of diamond of radius 1nm, with different velocities and impact parameters. The research includes the analysis of the structural and thermodynamic changes produced by the collision. When the impact speed is high, the configuration of the atoms changes, i.e. the sp3 hybridization of diamond changes to sp2 (graphitic) hybridization. This transition occurs after a few picoseconds and modifies greatly the properties of nanograins.
This type of collisions occurs in the interestellar dust clouds, for clusters of carbon and others materials.
Collision scheme. X is the impact paramer, d the diameter and v/2 the individual velocity.
Film of the frontal cluster collision. The color scale indicates the coordination number. The coordination number 4 (associated with a sp3 hybridization) changes to coordination number 3 (sp2 hybridization)
3) Silicate grain collisions
The collision of granular clusters can result in a number of complex outcomes, from sticking, to partial or full destruction of the clusters. This outcome will contribute to the size distribution of dust aggregates, changing their optical properties and their capability to contribute to solid-state astrochemistry. We study the collision of two clusters of equal size, formed by approximately 7000 sub-micron grains each, with a mass and velocity range which is difficult to sample in experiments. We obtain the outcome of the collision: compaction, fragmentation and size distribution of ejecta, and type of outcome, as a function of velocity and impact parameter. We compare our results to other models and simulations, at both atomistic and continuum scales, and find some agreement together with some discrepancies.
Final snapshots for different collision events showing the regimes of agglomeration, partial, and total fragmentation. (a) v = 2 m/s (v = 12vfrag ). (b) v = 5 m/s (v = 59vfrag ). (c) v = 10 m/s (v = 176vfrag ). In all cases the impact parameter was selected as b = 0.6R.
4) Sputtering from porous materials
Porous materials are ubiquitous in the universe and weathering of porous surfaces plays an important role in the evolution of planetary and interstellar materials. Sputtering of porous solids in particular can influence atmosphere formation, surface reflectivity and the production of the ambient gas around materials in space. Several previous studies and models have shown a large reduction in the sputtering of a porous solid compared to the sputtering of the non-porous solid. Using molecular dynamics simulations we study the sputtering of a nanoporous solid with 55% of the solid density. We calculate the electronic sputtering induced by a fast, penetrating ion, using a thermal spike representation of the deposited energy. We find that sputtering for this porous solid is, surprisingly, the same as that for a full density solid, even though the sticking coefficient is high.
Publications
1- "A new method to generate dust with astrophysical properties" J.F. Hansen, W. van Breugel, E.M. Bringa, B. Eberly, G.A. Graham, B.A. Remington, E.A. Taylor, and A.G.G.M. Tielens, J. Inst. 6 (2011) 5010.
2- "Collisions of porous clusters: a granular-mechanics study of compaction and fragmentation". Christian Ringl, Eduardo M. Bringa, Dalía S. Bertoldi and Herbert M. Urbassek. Astrophysics Journal (2012)
3-"SPUTTERING FROM A POROUS MATERIAL BY PENETRATING IONS", J.F. Rodriguez-Nieva, E.M. Bringa, T.A. Cassidy, R.E. Johnson, A. Caro, M. Fama, M.J. Loeffler, R.A.Baragiola, and D. Farkas, Astrophys. J. Lett. (2011)
Integrantes:
Colaboradores:
Robert.E. Johnson (UVa, USA)
Raul Baragiola (UVa, USA)
J. Rodriguez-Nieva (MIT, USA)
H.M. Urbassek (TU Kaiserslautern, Germany)