Chemical abundances of volatile and refractory elements in stars with and without exoplanets.

Abstract

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.