Ministerio de Ciencia, Innovación y Universidades Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

Astrophysics Research Projects

Physic of Stars, Planetary Systems and the Interstellar Medium

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Observational Tests of the Processes of Nucleosynthesis in the Universe (P/300423)

GARIK ISRAELIAN

Jorge Casares Velázquez, Ramón J. García López, Rafael Rebolo López, Jonay González Hernández 

IAC Collaborators: Artemio Herrero Davó, Javier Trujillo Bueno 

M. Mayor, S. Udry, F. Pepe, G. Meynet, A. Maeder (Obs. Ginebra, Suiza); N. Santos, V. Adibekyan, E. Delgado Mena, S. Sousa (Centro de Astrofisica da Universidade do Porto, Portugal); V- Lipunov, V. Fadeev (Univ Moscu), D. Queloz (Univ. de Cambridge)

Introduction 

Several spectroscopic analyses of stars with planets have recently been carried out. One of the most remarkable results is that planet-harbouring stars are on average more metal-rich than solar-type disc stars. Two main explanations have been suggested to link this metallicity excess with the presence of planets. The first of these, the “self-enrichement” hypothesis, attributes the origin of the observed overabundance of metals to the accretion of large amounts of metal-rich H- and He-depleted rocky planetesimal materials on to the star. The opposite view, the “primordial” hypothesis, considers the metallicity enhancement to be caused by the high metal content of the protoplanetary cloud from which the planetary system formed. Light elements may give fundamental information about the mixing, diffusion and angular momentum history of exoplanets hosts, as well as stellar activity caused by interaction with. Studies of Be, Li and the isotopic could give evidences to distinguish between different planet formation theories. Evidences of pollution have been found in HD82943 by Israelian et al. (2001, Nature, 411, 163; 2003, A&A, 405, 753).

The “self-enrichement” scenario should lead to a relative overabundance of refractories, such as Si, Mg, Ca, Ti and the iron-group elements, compared to volatiles, such as CNO, S and Zn. Differents spectroscopic studies of Fe (Santos et al. 2001, A&A, 373, 1019; 2003, A&A, 398, 363; 2004, A&A, 415, 1153) and other elements (Bodaghee et al 2003, A&A, 404, 715; Ecuvillon, Israelian, Santos et al. 2004, A&A, 418, 703; 2004, A&A, 426, 619) have been completed.

The spectroscopic analysis of metal rich stars can also give us a valuable information about yields of chemical elements produced by supernovae during the last 10 Gyr. An alternative method to investigate products of supernova explosions is by studying secondary stars in Low Mass X-ray binary systems (LMXB). The secondary stars in LMXBs have survived the supernova explosions and could have captured a part of the matter ejected during the explosion. This material can be mixed in the convection zone in a way that the final surface abundanced will be altered. Thus, a study of abundance anomalies in the atmospheres of these stars can provide us an information about nucleosynthesis and evolution of massive stars and also about supernova explosions. This new idea was applied for the first time by Israelian et al. (1999, Nature 401, 142) in the spectroscopic study of GRO J1655-40 (Nova Scorpii 1994), a LMXB with a black hole which has the most reliable mass determination. The analysis has shown that the abundances of O, Mg, Si and S are from 6 to 10 times larger compared with the Sun. These results were considered as the evidence that a supernova explosion took place and created the black hole in the system where the low mass secondary star could not produce these elements

The supernova explosions are responsible for the gradual enrichment of the interstellar medium by heavy elements. The observed relative abundance trends as function of metallicity provide some information about nucleosynthesis and formation rate of various types of supernovae. The new generation of 4-10 m telescopes has drastically improved the quality of spectroscopic observations. In the meantime, computing tools allow to study a NLTE spectral line formation in such complex atoms as Fe. Thanks to these advances we have recently discovered new interesting abundance trends for O, S and N (Israelian et al. 1998, ApJ, 507, 805; 2001, ApJ, 551,833; 2004, A&A, 421, 649). Furthermore, It was shown that a standard 1D models of atmospheres of a metal poor giants are unable to resolve conflicts between different abundance indicators of oxygen and magnesium (Israelian et al. 2004, 419, 1095). It is planned to continue consistent abundance studies in a selected sample of metal poor stars with a goal to understand why and when 1D models fail as spectrum synthesis tools.

Milestones 

We have explored the possibility that stars from different galactic populations that have different intrinsic abundance ratios may produce planets with a different overall composition. We compiled abundances for Fe, O, C, Mg, and Si in a large sample of solar neighbourhood stars that belong to different galactic populations. We then used a simple stoichiometric model to predict the expected iron-to-silicate mass fraction and water mass fraction of the planet building blocks, as well as the summed mass percentage of all heavy elements in the disc.

We have presented a detailed study of the Mg/Si and C/O ratios and their importance in determining the mineralogy of planetary companions. Using 499 solar-like stars from the HARPS sample, we determine C/O and Mg/Si elemental abundance ratios to study the nature of the possible planets formed. We separated the planetary population in low-mass planets (< 30 Earth Masses) and high-mass planets (> 30 Earth masses) to test for possible relation with the mass. We found a diversity of mineralogical ratios that reveal the different kinds of planetary systems that can be formed, most of them dissimilar to our solar system. The different values of the Mg/Si and C/O ratios can determine different composition of planets formed.

To understand the formation and evolution of the different stellar populations within our Galaxy it is essential to combine detailed kinematical and chemical information for large samples of stars. The aim of this work is to explore the chemical abundances of neutron capture elements which are a product of different nucleosynthesis processes taking place in diverse objects in the Galaxy, such as massive stars, asymptotic giant branch (AGB) stars and supernovae (SNe) explosions. We derive chemical abundances of Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd, and Eu for a large sample of more than 1000 FGK dwarf stars with high-resolution (R 115 000) and high-quality spectra from the HARPS-GTO program. The abundances are derived by a standard local thermodynamic equilibrium (LTE) analysis using measured equivalent widths (EWs) injected to the code MOOG and a grid of Kurucz ATLAS9 atmospheres.

We have presented a detailed spectroscopic analysis of 1110 solar-type stars, 143 of which are known to have planetary companions. We have determined the carbon abundances of these stars and investigate a possible connection between C and the presence of planetary companions. We used the HARPS spectrograph to obtain high-resolution optical spectra of our targets. Spectral synthesis of the CH band at 4300 Å was performed with the spectral synthesis codes MOOG and FITTING. 

 

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