Detailed analysis of the chemical composition of stars harboring Earth-like planets

Abstract

Context. The planet-metallicity correlation for gaseous giants is widely accepted through spectroscopic
studies. However, whether a similar correlation exists for terrestrial planets is a debated subject. High-precision spectroscopic abundance analysis on Sun-like stars suggested that the Sun is depleted in refractory elements with respect to its solar twins without exoplanets, likely due to the formation of terrestrial planets in the Solar system.
Data. We use high-resolution (R=67000), high signal-to-noise ratio (S/N 200-300) optical spectra of 5 stars hosting terrestrial planets, 12 comparison stars and the Sun. These have been obtained with the High Resolution Echelle Spectrometer HIRES, on the Keck I telescope in Mauna Kea observatory.
Aims. The goal of this work is to obtain the chemical elemental abundances of Sun-like stars with and without exoplanets, with a high precision of 0.01-0.02 dex. This precision will allow us to detect the planet formation signature in planet hosts, and to estimate the bulk chemical composition of their exoplanets.
Methods. We determine the stellar parameters and elemental abundances by performing a strict line-by-line differential analysis. Equivalent widths (EW) are measured individually for all spectral lines with IRAF. Elemental abundances are computed with MOOG, using unidimensional, local thermodynamic equilibrium (1D LTE) model atmospheres. In addition, a
chemical fractionation model that considers Earth’s devolatilisation with respect to the Sun is applied to estimate the bulk composition of exoplanets, giving our differential elemental abundances as input.
Results. Stellar differential parameters and differential elemental abundances are calculated. The latter are obtained with a precision of 0.01-0.02 dex for 19 elements. The differential stellar abundances as a function of condensation temperature show a linear trend, whose slope (Tc slope) shows a dependence on stellar age due to Galactic chemical evolution (GCE) effects. This contribution to the Tc slope is corrected. Corrected differential abundances prove that the
Sun is depleted in refractory elements with respect to the comparison stars. However, only one planet host shows the planet formation signature. On the other hand, most of the estimated planetary bulk compositions in our sample seem to have a similar core mass fraction compared to Earth, although their mantles are more enriched in SiO2.
Conclusion. Our corrected differential abundances confirm that the Sun is depleted in refractories compared to Sun-like stars without exoplanets. The planet formation signature is only present in one out of five planet hosts in our sample. Therefore our results do not favor the planet formation signature hypothesis, although they are limited by small statistics. It is necessary to spectroscopically analyse more planet hosts with high precision, and to constrain
planet densities and compositions in order to understand why the Sun presents these peculiar abundances. In addition, the bulk chemical composition of an exoplanet contributes to estimate its volatile reservoir and formation history.

Popular Abstract

In our Galaxy, the Milky Way, there are billions of stars. Among them, many stars have planets orbiting them, which are named exoplanets since these are found outside the Solar System. A star and its exoplanets form from the same nebula of gas and dust. Each of these planetary systems in our Galaxy was born from a different nebula, which are named proto- planetary nebulae or disks. If we took a look inside them, we would be able to see that dust particles attract each other through gravity and clump to form bigger particles, until they reach the sizes of asteroids or planets. This dust is compound of different elements, including those that first become solid when the protoplanetary nebula cools down and form the cores and mantles of terrestrial planets. Some of these elements are iron (Fe), silicon (Si) and nickel (Ni).
Since the star and its exoplanets are formed from the same protoplanetary nebula, the chem- ical composition of the planets is related to that of the star. We could even ask ourselves, how does the formation of a planet affect the composition of its star? One interesting hypothesis is that stars with terrestrial planets have less atoms of elements like iron, silicon and nickel in their composition, because the dust made of these elements is accreted by the planets instead by the stars when they are forming. This difference in composition is very small, and therefore it requires to be very precise when we calculate the chemical composition of a star. In our work, we compare the composition of stars without any detected planets to the composition of the Sun and other Sun-like stars with terrestrial exoplanets. By comparing these, we can look for a lack of these element atoms in the stars with terrestrial planets, and increase the number of stars in which this difference is looked for. We choose these stars to be as similar as possible in temperature and mass to the Sun so that any differences in their composition are easy to see. The chemical composition of a star is obtained from the light that comes from it to us through space, which we study through the spectrum. Each atom in the atmosphere of the star emits and absorbs light differently, giving to each element a particular fingerprint we can identify in the spectrum of the star. In addition, applications of this work include the detection of terrestrial planets just by looking at the light of the star, and obtaining the exoplanet’s chemical composition.
An important parameter to understand what a planet is made of is its density. However, many different mixtures of chemical elements can explain one single density. To rule out all of these combinations except for one, the composition of a star can help us to figure out how the interior of its planet looks like, since they formed from the same protoplanetary nebula. For example, more calcium (Ca) and aluminum (Al) atoms in the atmosphere of a star means that its planets are more likely to be rich in rubies and sapphires, whereas stars with much more carbon (C) atoms in their composition are likely to host planets whose mantle contains diamonds. The better we understand the relationship between the composition of stars and their planets, the more we will know about these extrasolar worlds.

• author: Acuña Aguirre, Lorena
• supervisor: Fan Liu
• organization: Lund Observatory - Undergoing reorganization
• course: ASTM31 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• publication/series: Lund Observatory Examensarbeten
• report number: 2019-EXA146
• language: English
• id: 8976827
• date added to LUP: 2019-05-24 13:33:28
• date last changed: 2019-05-24 13:33:28