First-principles examination of molecule formation in interstellar grains
Abstract
I use a first-principles quantum-mechanical formalism, density-functional theory (DFT), to show that atomic hydrogen can saturate a sub-surface layer in carbonaceous interstellar dust grains under typical conditions, potentially leading to H 2 formation inside grains. I considered a graphene sheet as a model of a carbonaceous grain surface and calculated the potential energy barrier for a hydrogen atom to pass through the surface. I encountered a numerical instability in the algorithms traditionally employed to update the occupancies of the wave-functions in DFT and devised a new algorithm to overcome this problem. The new algorithm divides the occupancies into two subsets based on whether they are partially filled or not. The former are adjusted by an explicit, steepest-descent minimization of the free energy, A. The latter (nearly empty or nearly full) are calculated by imposing thermal equilibrium. I apply the new algorithm to calculate the potential energy of a hydrogen atom in the vicinity of a graphene sheet. The hydrogen-atom flux penetrating a carbonaceous grain in each of the phases of the interstellar medium is estimated. The time for hydrogen to saturate a grain is compared to the timescale for grain destruction. I conclude that a subsurface layer of hydrogen can accumulate inside a grain in that time.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT.........5K
- Keywords:
-
- Interstellar dust;
- Hydrogen