Abstract
A myriad of information about the physical properties of an astronomical object, such as its composition, temperature, ionization state, velocity, redshift and more, are embedded in its spectral lines. To obtain this information from the observed spectrum, one needs to understand the formation of these spectral lines in the system. In this thesis, I present the spectral line formation study of two systems by comparing the simulated spectral line from theoretical models to the observations. In the first part of the thesis, I investigate Hα and NaD lines in the hot Jupiter transmission spectrum, which are formed due to absorption in the planetary upper atmosphere. In the second part, I study the broadening of narrow emission lines of interacting supernovae due to electron scattering.
When a planet transits in front of its host star, the atmosphere can absorb an extra part of the light from the star besides the planet itself. This frequency dependent absorption of starlight by the planet atmosphere, referred as a transmission spectrum, provides a probe of the atmospheric composition and structure. The Hα and NaD transmission spectra have been observed for the hot Jupiter HD 189733b, which may play an important role in understanding the conditions in the planet's atomic layer. Motivated by the observations, a detailed one-dimensional hydrostatic atmosphere model is constructed over the region dominated by atomic hydrogen, and comparison of model transmission spectra to the data has been made. An atomic hydrogen level population calculation and a Monte-Carlo Lyα radiation transfer simulation are carried out to model the abundance of n=2 state hydrogen. The dependence of the transit profile on Lyman continuum emission and metal abundances is considered. The atmospheric temperature of the model is compared to previous temperature measurements from the Na line profile.
Interacting supernovae, including type IIn and Ia-CSM, are supernovae that show evidence of strong shock interaction between their ejecta and pre-existing circumstellar material (CSM), which may be ejected from the unstable progenitor star before the explosion. After the supernova shock wave has broken out of the progenitor star, the ionizing radiation from the shock region is able to ionize the surroundings. The CSM can have substantial optical depth to electron scattering and the continuum photosphere is in the unshocked CSM due to the electron scattering opacity. The iconic feature of the interacting supernovae are the broad wings (1000's of km/s) on narrow emission lines. To explain the line formation of this feature, we adopt the idea that the narrow emissions are created in the preshock ionized CSM by recombination cascades, and the line profiles are broadened by the electron scattering for the photons that make their way out. A Monte-Carlo simulation has been done to simulate this electron scattering process in a series of possible CSM configurations. The dependence of the line profile on the optical depth, thickness, density distribution, expansion velocity, and continuum absorption has been examined. The scattering model has been applied to a number of supernovae, including Type IIn and Type Ia-CSM events.
