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I have conducted my PhD dissertation research at the department of Physics and Astronomy at the University of Oklahoma. My thesis adviser, Dr Eddie Baron, and Peter Hauschildt (Hamburger Sternwarte, Germany) have developed an advanced general purpose stellar atmosphere code PHOENIX (Hauschildt et al., 1997) over the past many years. Most of my dissertation research involves working extensively on PHOENIX primarily for SNe related applications. Motivation for the first project was to investigate the importance of time-dependence in the type II SNe atmospheres. It is claimed that the hydrogen recombination time increases due to L&agr; trapping and ionization from the second level and therefore it is necessary to incorporate time dependence to reproduce the Balmer profile of SN 1987A early spectra (Utrob in and Chugai (2005), Dessart and Hillier (2008)). I implemented time dependence in the rate equations for different species and added the 2s-1s non-resonant transition in PHOENIX in order to explicitly estimate the recombination time. I generated the time dependent profiles for the ionization fraction, recombination time and spectra for SN 1987A and SN 1999em. For SN 1987A and SN 1999em I started respectively at 2 and 7 days following the explosion and generated time dependent profiles at intervals of 2-4 days upto 20 and 40 days (since explosion) respectively. I compared the observed spectra for SN 1987A with the synthetic spectra generated from both time dependent (TD) and time independent (TI) rate equations using a 31 level hydrogen atom model. Based on my comparison I concluded that (1) the Balmer profile in SN 1987 A spectra could be produced without TD by tuning the input luminosity at all epochs; and (2) TD is more important at early epochs than later ones. I also compared the recombination times from a 4-level hydrogen atom model to that from the multi-level cases. I concluded that it is very important to include many angular momentum sub-states to get a realistic estimate of the recombination time which is overestimated if the additional angular momentum sub-states are ignored. These results have been described in detail in De et al. (2009). I have also studied the effects of non-resonant transitions (De et al. 2009 in preparation) and concluded that these effects are more important in a multi-level atomic framework in a metal-deficient environment. Recently I have been modifying PHOENIX to apply it to study the cosmic recombination epoch and implementing Compton scattering. The idea is to generate an accurate time-dependent variation of the ionization fraction of the universe during the recombination era which strongly affects the CMB polarization and power spectra. In addition to working on radiative transfer problems I have worked on the cosmological dark matter density field during my graduate school years at Carnegie Mellon University under my then adviser Dr Rupert Croft. I investigated how statistical measures derived from the dark matter mass density field could predict cosmological parameters independent of uncertainties from galaxy formation theories (De and Croft, 2007) and applied this to the 2dF redshift survey to constrain the cosmological parameters (De and Croft, 2009).

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