2014 Annual Science Report
Rensselaer Polytechnic Institute Reporting | SEP 2013 – DEC 2014
Project 2: Processing of Precometary Ices in the Early Solar System
The discovery of numerous planetary systems still in the process of formation gives us a unique opportunity to glimpse how our own solar system may have formed 4.6 billion years ago. Our goal is to test the hypothesis that the building blocks of life were synthesized in space and delivered to the early Earth by comets and asteroids. We use computers to simulate shock waves and other processes that energize the gas and dust in proto-planetary disks and drive physical and chemical processes that would not otherwise occur. Our work seeks specifically to determine (i) whether asteroids and comets were heated to temperatures that favor prebiotic chemistry; and (ii) whether the requisite heating mechanisms operate in other planetary systems forming today.
A. Molecular Line Emission from Multifluid, Magnetohydodynamic Shock Waves
The goal of this project is to simulate molecular line emission from shock waves, including shocks produced by protostellar outflows as well as 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. 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 2013 we published an algorithm for simulating multifluid shocks (Ciolek & Roberge 2013, ApJ, 768, 78). Work during the period Sept. 2013 – Oct. 2014 has focused on (i) implementing adaptive mesh refinement (AMR) and other techniques to greatly speed up the algorithm; and (ii) adding atomic and molecular physics to make simulations of line emission possible.
To facilitate the AMR project Roberge initiated a collaboration with an applied mathematician, Prof. Pablo Suarez from Delaware State University (DSU). During the reporting period Ciolek, Suarez and Roberge completed a shock code with AMR which dramatically reduces the number of mesh points in a shock model, from typically 20,000 points to a few hundred. The code has been implemented on Rennselaer’s 80,000 CPU Blue Gene/Q supercomputer.
During summer, 2014 Suarez and two DSU undergraduates, Jeremi Frazier and Arthur Newell, worked on implicit numerical methods for integrating certain ordinary differential equations which arise in the shock problem. Suarez, Frazier and Newell successfully demonstrated an implicit method on a “toy” problem. Incorporation of the implicit method into the shock code is in progress.
Grad student Raymond Menzel (PhD, Rensselaer, May 2014) built subroutines to simulate emission from H2, CO, and water molecules. The modules include thousands of spectral lines for comparison with future observations.
Roberge and RPI undergraduate Jacob Leedom worked on a theory of betatron acceleration in multifluid shock waves. Betatron acceleration is a mechanism whereby dust grains are destroyed; the objective is to study how refractory elements locked in dust may be returned to the gas phase. A related paper is in preparation.
Menzel, Ciolek, Suarez and Roberge used the shock code to carry out a critical examination of a “pseudo-time dependent” (PTD) technique proposed by others for modeling multifluid shocks. The PTD method attempts to model time dependent shocks by “stitching together” the results obtained for different steady shocks. Initial results were reported at the January, 2014 meeting of the AAS and a related paper is in preparation.
B. Asteroid Thermal Environments
The diverse inventory of amino acids and other organic molecules found in meteorites implies that asteroids once provided warm environments conducive to prebiotic chemistry. The goal of this project is to understand the physical mechanisms which heated asteroids and their prevalence in other protoplanetary disks. Menzel and Roberge published a paper (Menzel & Roberge 2013, ApJ, 776, 89) which reexamines a heating mechanism (“induction heating”) described in a classic series of papers by C.P. Sonett and collaborators. Classical induction postulates that the asteroids were swept by a powerful, fully-ionized T Tauri wind from the young Sun. Menzel and Roberge revised this scenario to account for modern ideas about protoplanetary disks. Their principal results are (i) that classical induction theory is based on a subtle misunderstanding of the underlying physics; (ii) the correct physics predicts a new “electrodynamic heating” mechanism which is viable in weakly-ionized disks; (iii) for some flow geometries the rate of electrodynamic heating rate is zero; (iv) for other geometries electrodynamic heating can have a rate comparable to classical induction; and (v) that the shear flows around asteroids and other primitive bodies may be interesting environments for prebiotic chemistry.
B.2 Progress Report
Grad student Allycia Gariepy, Suarez and Roberge carried out a stability analysis of the shear flows studied by Menzel and Roberge. They have tentatively identified a new class of instabilities in such flows, though rigorous checks (in progress) are required to verify this conclusion. If the results hold up they will be published.
PROJECT MEMBERS:Wayne Roberge
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Sources of prebiotic materials and catalysts
Origins and evolution of functional biomolecules