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Abstract:
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Comets are the spectacular objects in the night sky since the dawn of mankind. Due to
their giant apparitions and enigmatic behavior, followed by coincidental calamities, they
were termed as notorious and called as `bad omens'. With a systematic study of these
objects modern scienti c community understood that these objects are part of our solar
system. Comets are believed to be remnant bodies of at the end of evolution of solar
system and possess the material of solar nebula. Hence, these are considered as most
pristine objects which can provide the information about the conditions of solar nebula.
These are small bodies of our solar system, with a typical size of about a kilometer
to a few tens of kilometers orbiting the Sun in highly elliptical orbits. The solid body
of a comet is nucleus which is a conglomerated mixture of water ice, dust and some
other gases. When the cometary nucleus advances towards the Sun in its orbit the ices
sublimates and produces the gaseous envelope around the nucleus which is called coma.
The gravity of cometary nucleus is very small and hence can not in
uence the motion
of gases in the cometary coma. Though the cometary nucleus is a few kilometers in size
they can produce a transient, extensive, and expanding atmosphere with size several
orders of magnitude larger in space. By ejecting gas and dust into space comets became
the most active members of the solar system. The solar radiation and the solar wind
in
uences the motion of dust and ions and produces dust and ion tails, respectively.
Comets have been observed in di erent spectral regions from rocket, ground and
space borne optical instruments. The observed emission intensities are used to quantify
the chemical abundances of di erent species in the comets. The study of various physical
and chemical processes that govern these emissions is essential before estimating chemical
abundances in the coma. Cameron band emission of CO molecule has been used to derive
CO2 abundance in the comets based on the assumption that photodissociation of CO2
mainly produces these emissions. Similarly, the atomic oxygen visible emissions have
been used to probe H2O in the cometary coma. The observed green ([OI] 5577 A) to
red-doublet emission ([OI] 6300 and 6364 A) ratio has been used to con rm H2O as the
parent species of these emissions. In this thesis a model is developed to understand the
photochemistry of these emissions and applied to several comets. The model calculated
emission intensities are compared with the observations done by space borne instruments
like International Ultraviolet Explorer (IUE) and Hubble Space Telescope (HST) and
also by various ground based telescopes. |