http://hdl.handle.net/2248/5837 2026-04-06T07:34:03Z http://hdl.handle.net/2248/5841 Title: Extraction of Solid State Features of Carbon Dust Grains in the ISM, from X-Ray Absorption Studies Authors: Manpreet Singh Abstract: The subject of dust formation in space is important in diverse environments. The formation of dust proceeds towards the formation of stellar objects and planets. So, it becomes very important for us to know the composition and structure of the dust in order to understand the above mentioned complex systems. Carbon grains are considered for long to be very important candidates for the dust in the interstellar medium (ISM). Existence of carbon in graphite form in the ISM, is an unsolved problem. The present approach will provide a probe, complimentary to the existing ones. It involves extraction of solid state features of carbon dust grains in the ISM, from x-ray absorption studies. A general theory of X-Ray Absorption Fine Structure (XAFS) will be presented. An approximate evaluation of this theory is proposed which allows us to predict the structure of the constituents of the grain. Some case studies are presented for various configurations of graphite and diamond. Description: Thesis Supervisors: Prof. S Chatterjee & Prof. H C Bhatt 2010-06-01T00:00:00Z http://hdl.handle.net/2248/5838 Title: Helicity Constrained Model of a Galactic Dynamo Authors: Avijeet Prasad Abstract: Magnetic fields correlated on kiloparsec scales are seen in disc galaxies. The origin could be due to amplification of small scale seed fields by a turbulent dynamo. Helicity conservation imposes constraints on dynamo action and one can study the minimal field strength of the large scale magnetic field that could arise despite the constraint. The calculation of helicity is technically complicated because of open boundaries and the usual form for the MHD invariant needs to be modified to take this into account. A general formalism has been developed to calculate the field strength using a quasi-linear dynamo model. As a first step, a galactic disk dynamo model has been built analytically while making the usual assumptions of shear and the alpha effect. We find quadrupolar solutions that are matched to a vacuum field with D = 11. The general approach to solving the quasi-linear dynamo using the full set of relative helicity boundary conditions are described Description: Thesis Supervisor Prof. Arun Mangalam; Restricted Access 2010-06-01T00:00:00Z