Since the 1995 discovery of the first exoplanet orbiting a solar-type star
(Mayor & Queloz 1995), the number of these has grown exponentially.
To date, more than 4,500 planets have been detected (more than 3,200
confirmed). In addition, the size of these planets is very diverse, from giant
Jupiter-type planets (the first to be discovered) to Earth-like planets (the
number of which is booming, thanks to improved detection methods).
To characterize a planetary system, it is necessary to know the chemical
composition of its host star.
In this thesis, we present a uniform study of volatile and refractory
elements, with a total of 14 different elements analysed.
In the case of volatile elements, chemical abundances of nitrogen and
carbon were analysed in a sample of 74 and 1,110 stars, respectively. In
both cases, molecular bands in the near ultraviolet and optical were used.
This is because in the case of nitrogen, there are no atomic lines in the
optical that can be studied; the only available line of atomic nitrogen is at
7,468Å, often outside the spectral range of currently available instrumen-
tation. In the case of carbon, there are lines at 5,380Å and 5,052Å, but
they are difficult to study. Therefore, we opened a door to use molecular
bands as a reliable method to study chemical abundances. To do this,
we developed a continuous tuning method of the stellar photosphere that
allows us to normalize the spectrum reliably. This is especially necessary
in the case of nitrogen, where the points of the continuum are practically
non-existent.
We analysed stars with and without planets, to look for differences in
their chemical composition. In the case of nitrogen and carbon, we found
no significant differences between the samples.
Using the carbon abundances obtained and oxygen, magnesium, and
silicon data from the literature belonging to our group, we calculated the
C/O and Mg/Si ratios. These ratios are especially important in charac-
ixx
terizing the planets orbiting the stars because in the case of Mg/Si, these
will be similar on the planet and in the star. For the C/O, this depends
on the stars ice lines, the distance to the star where the planet formed.
C/O measurements obtained are estimates for those that can be found on
the planets.
In the case of refractory elements, 12 different elements have been
studied: Na, Mg, Al, Si, Ca, Sc (ScI and ScII), Ti (TiI, TiII), V, Cr (CrI,
CrII) Co, Mn, and Ni. We focused our study in metal-poor stars, given the
great interest of these objects. We found overabundances of α elements
(Mg, Si and Ca) in our sample, confirming previous results. In addition,
we obtained results that support the theory that this overabundance is
related to the chemical characteristics of the protoplanetary disk and not
to the membership of a certain stellar population.