Interstellar Silicate Dust: Modeling and Grain Alignment
PhD Dissertation Defense – Indrajit Das
We examine some aspects of the alignment of silicate dust grains with respect to the interstellar magnetic field. First, we consider possible observational constraints on the magnetic properties of the grains. Second, we investigate the role of collisions with gas atoms and the production of H2 molecules on the grain surface in the alignment process when the grain is drifting in the gaseous medium.
Paramagnetism associated with Fe content in the dust is thought to play a critical role in alignment. Min et al (2007) claimed that the Fe content of the silicate dust can be constrained by the shape of the 10 μm extinction feature. They found low Fe abundances, potentially posing problems for grain alignment theories. We revisit this analysis modeling the grains with irregularly shaped Gaussian Random Sphere (GRS). We give a comprehen- sive review of all the relevant constraints researchers apply and discuss their effects on the inferred mineralogy. Also, we extend this analysis to examine whether constraints can be placed on the presence of Fe-rich inclusions which could yield “superparamagnetism”. This possibility has long been speculated, but so far observational constraints are lacking.
Every time a gas atom collides with a grain, the grain’s angular momentum is slightly modified. Likewise when an H2 molecule forms on the surface and is ejected. Here also we model the grain with GRS shape and considered various scenarios about how the colliding gas particles depart the grain. We develop theoretical and computational tools to estimate the torques associated with these aforementioned events for a range of grain drift speeds – from low subsonic to high supersonic speeds. Code results were verified with spherical grain for which analytical results were available. Finally, the above torque results were used to study the grain rotational dynamics. Solving dynamical equations we examine how these torques influence the grain alignment process. Our analysis suggests that these torques, in principle, can align the grain even though the time scale for such an alignment process is too high such that the possible disalignment processes dominate. Drifting grain is found to experience a more complicated drag torque than non-drifting grain in a dynamical situation.
Dr. Joseph Weingartner
Dr. Shobita Satyapal
Dr. Mario Gliozzi
Dr. Padmanabhan Seshaiyer
Notes: The dissertation is on reserve in the Johnson Center Library, Fairfax Campus. All members of the George Mason University community are invited to attend.