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Project Progress

A. Studies of Multifluid Magnetohydodynamic Shock Waves

The goal of this project is to simulate molecular line emission from shocked interstellar gas including shock waves produced by the bipolar outflows emitted by protostars and the shocks associated with colliding clouds and interacting supernova remnants. We aim to provide tools which can be used to interpret the wealth of molecular line observations of prebiotic molecules to be expected over the next several years from SOFIA, ALMA, and other infrared and radio observatories. For example, the SOFIA Cycle 1 observing program includes observations of molecular line emission from interacting supernova remnants using the GREAT spectrometer (PI J. Hewitt, NASA Goddard). Realistic simulations of shock-excited molecular line emission from weakly-ionized plasmas require time-dependent, magnetohydodynamic calculations, which treat the plasma as three interacting fluids (the neutral gas, ion/electron plasma, and charged dust grains). There is no active research group in the US, to our knowledge, with the capacity to carry out such calculations.

In 2011-12 we made two significant advances:

(i) Ciolek and Roberge (2012a,b) developed an “operator splitting” algorithm for simulating multifluid, MHD shock waves. The algorithm allows for mass, momentum, and energy exchange between different fluids by ion-electron recombination, elastic ion-neutral scattering, or any other process (Fig. 1). The algorithm was implemented on Rensselaer’s 30,000 CPU supercomputer and an extensive suite of benchmark tests was carried out. The tests show that our code can achieve accuracies of at least 10 parts per million. It also has the dynamic range to resolve structures over at least two orders of magnitude in spatial scale (Fig. 2) and many orders of magnitude in time.

(ii) Roberge, Ciolek and Katz (2012) completed a study on the effects of dust on multifluid shock waves. They found that, when charged dust grains are partially coupled to the magnetic field, the speed at which a dusty plasma can communicate compressive disturbances can be modified significantly. When dust decouples from the magnetic field it still exerts a ``passive’’ effect by producing an electric field; this electric field alters the dynamics of the ions and electrons, which modifies the overall shock structure. Roberge et al. carried out numerical simulations of shock formation including dust dynamics and found quasi-self similar behavior at large times.

B. Thermal Environments of Primitive Solar System Bodies

The diverse inventory of amino acids and other organic molecules found in meteorites implies that asteroids once provided environments very conducive to prebiotic chemistry. Menzel and Roberge (2012a,b,c) continued their studies on the thermal environments of asteroids, comets, and other primitive bodies in the solar nebula and protoplanetary disks. Their objective is to understand how the interaction of a primitive body with the ambient disk material may alter its thermal environment. Possible mechanisms include (i) heating of a body’s outer regions by energy dissipation in shear flows near the body surface; and (ii) heating by a new “electrodynamic heating” mechanism. Menzel and Roberge described the physical processes which determine the structure of shear flows in weakly-ionized protoplanetary disks and derived the governing equations which describe the flow for bodies of arbitrary shape. Illustrative solutions were obtained for two very simple body shapes.

C. Collaborative Work with Other NYCA Investigators

Roberge collaborated with the team (Baldwin, Watson, Delano, Swindle et al.) studying argon diffusion in lunar glasses (Gombosi et al. 2012a,b). Roberge devised an algorithm which speeded up numerical simulations of argon diffusion by a factor of ~1 trillion. Roberge and Watson devised numerical and analytical techniques for simulating “multi-event” diffusion, wherein diffusion is driven by discrete heating events. Applications to impact heating of lunar glasses and other extraterrestrial materials are being explored.