Abundances in stars

In this contribution we present preliminary estimates of the effect of convection in 3D model stellar atmospheres of red giants on the formation of spectral lines and on the derivation of chemical abundances in stars. 

More stringent constraints on N nucleosynthesis come from the study of the N abundances in stars in the MW since they represent a true evolutionary... 

Elements are formed by three different mechanisms in stars, as elucidated by Burbidge et al. [82]. In brief, these are the s process, involving the capture of slow neutrons by nuclei the r process, involving the capture of rapid neutrons and the p process, which leads to neutron-deficient nuclides that are generally less abundant than those generated by the s and r processes. The relative abundances of the various isotopes of a given element reveal which processes, or combinations thereof, are involved in their creation in the nuclear reactions that power stars. Measurements of isotopic and elemental abundances in stars thus serve to test theories of stellar processes. 

To set an abundance scale for listing the abundances of the elements, astronomers usually set H = 1012 atoms for the size of the sample. Other elemental abundances in stars are then given by their numbers per thousand billion (1012) H atoms. For the heavy elements more reliable information about relative abundances comes from the primitive classes of meteorites, which are dominated by silicon. Thus... 

For the first time a universe composed of H and He has a scientific explanation. Only 1H, 2H, 3He, 4He and much 7Li seem to have inherited their abundances as ashes of that Big Bang. The stars manufactured the remainder of the elements, except for small abundances created by cosmic-ray collisions. Stars in fact slowly destroy 2H by fusing it into He. Cecelia Payne s discovery in stellar spectra that H dominates the abundances in stars became aprime factof the cosmology of the universe. The origin of both isotopes of hydrogen, the first and seventh most abundant nuclei in the universe, as well as ofhelium, in the initial fireball is one of the great achievements of thattheory. 

Magnesium is composed of three stable isotopes, masses 24, 25 and 26, and is animportantelementinnucleosynthesis theory. Itis produced in stars primarily during their carbon-burning thermonuclear phase. Its abundance in stars of widely varying metallicities has been determined by astronomical observations of optical absorption lines in their spectra. 

Chlorine is much the most abundant halogen. It is six times more abundant than fluorine and 450 times more abundantthan bromine. This is understood, because both stable Cl isotopes are produced in the main line ofnuclear reactions during oxygen burning in stars. Cl is the 20th most abundant element in the universe, being almost identical in abundance to potassium and 1.5 times more abundant than titanium. But measurements of Cl abundance in stars are few, because it is rare and its emission lines are unfavorable. 

In contrast, the study of stellar grains permits information to be obtained about individual stars, complementing astronomical observations of elemental and isotopic abundances in stars (e.g., Lambert, 1991), by extending measurements to elements that cannot be measured astronomically. In addition to nucleosynthesis and stellar evolution, presolar grains provide information about galactic chemical evolution, physical properties in stellar atmospheres, mixing of SN ejecta and conditions in the parent bodies of the meteorites in which the grains are found. 

The 6Li/7Li abundance in stars varies widely, and the production mechanisms are far from well understood. Current research examines the relative roles of convection, dMusion, and spallation to gain a greater understanding of stellar evolution. 

A curious circumstance, however, is that calcium appears to be about half as abundant as other alpha-process elements in galaxies. The causes are not clear, but observations indicate that calcium abundance in stars is directly correlated with the mass of the star and the velocity variation within the star at the time of formation of the calcium nuclei. Further studies of supernovae, with their complex velocity distributions, should inform theories of calcium nucleosynthesis. 

Latin carbo, charcoal) Carbon, an element of prehistoric discovery, is very widely distributed in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites. 

Sodium is present in fair abundance in the sun and stars. The D lines of sodium are among the most prominent in the solar spectrum. Sodium is the fourth most abundant element on earth, comprising about 2.6% of the earth s crust it is the most abundant of the alkali group of metals. 

Zirconium is found in abundance in S-type stars, and has been identified in the sun and meteorites. Analysis of lunar rock samples obtained during the various Apollo missions to the moon show a surprisingly high zirconium oxide content, compared with terrestrial rocks. 

Titanium oxide bands are prominent in the spectra of M-type stars. The element is the ninth most abundant in the crust of the earth. Titanium is almost always present in igneous rocks and in the sediments derived from them. 

The hydrogen atom and its spectrum are of enormous importance in astrophysics because of the large abundance of hydrogen atoms both in stars, including the sun, and in the interstellar medium. 

In this paper I discuss overall metallicity, a measure of the overall heavy-element abundance in the star, and direct determination of elemental abundances and abundance ratios of Fe, O, the a-elements Mg, Si, Ca, and Ti, and also the light elements Na and Al. 

Oxygen abundances in clusters show solar [O/Fe] ratios overall, and fall within the envelope of the distribution with [Fe/H] displayed by the disk field stars... 

Abstract. One particular fact that is helping us to understand the mechanisms of planetary formation has to do with the planet host stars themselves. In fact, these were found to have, on average, a metal content higher than the one found in stars without detected planetary companions. In this contribution we will mainly focus on the most recent results on the chemical abundances of planet-host stars, and what kind of constraints they are bringing to the theories of planet formation. 

In this paper we will review the current situation regarding the study of the chemical abundances of stars with giant planets, and discuss the implications these results have on the theories of planetary formation. 

Abstract. High resolution spectral data of red clump stars towards the NGP have been obtained with the spectrograph Elodie at OHP stars. Nearby Hipparcos red clump stars were also observed. We determine the thin and thick properties kinematics and chemical abundances in the solar neighbourhood. We estimate the surface mass density of the galactic disk, we also determine the thin and thick disk chemical properties. 

The discovery of the average metal-rich nature of planet-harbouring stars with regard to disc stars (i.e. [1],[2], [3]) has revealed the key role that metallicity plays in the formation and evolution of planetary systems. If the accretion processes were the main responsible for the iron excess found in planet host stars, volatile abundances should show clear differences in stars with and without planets, since volatiles (with low Tc) are expected to be deficient in accreted materials [4]. Previous studies of the abundance trends of the volatiles N, C, S and Zn [5, 6] have obtained no anomalies for a large sample of planet host stars. 

We have determined oxygen abundances in two large samples, a set of planet-harbouring stars and a volume-limited comparison sample of stars with no known planets, using 3 different indicators [OI] at 6300 A, the OI 7771-5 A triplet, and a set of 5 near-UV OH lines (see [7]). Non-LTE corrections were calculated and applied to the LTE abundance results for the triplet. 

The comparison of coronal and photospheric abundances in cool stars is a very important tool in the interpretation of the physics of the corona. Active stars show a very different pattern to that followed by low activity stars such as the Sun, being the First Ionization Potential (FIP) the main variable used to classify the elements. The overall solar corona shows the so-called FIP effect the elements with low FIP (<10 eV, like Ca, N, Mg, Fe or Si), are enhanced by a factor of 4, while elements with higher FIP (S, C, O, N, Ar, Ne) remain at photospheric levels. The physics that yields to this pattern is still a subject of debate. In the case of the active stars (see [2] for a review), the initial results seemed to point towards an opposite trend, the so called Inverse FIP effect , or the MAD effect (for Metal Abundance Depletion). In this case, the elements with low FIP have a substantial depletion when compared to the solar photosphere, while elements with high FIP have same levels (the ratio of Ne and Fe lines of similar temperature of formation in an X-ray spectrum shows very clearly this effect). However, most of the results reported to date lack from their respective photospheric counterparts, raising doubts on how real is the MAD effect. 

Photospheric abundances of stars within the solar neighborhood may be quite different [1], therefore it is necessary to compare coronal and photospheric abundances of the same star in order to understand this phenomenon. We have conducted a research on a sample of stars of different activity levels, from which we... 

Abstract. A review is presented on abundance determinations in stars of the Galactic bulge, both in the field and in globular clusters. Previous low-resolution spectroscopy results are revised. Recent high resolution and high S/N spectroscopy results based on Keck-Hires, Gemini-Phoenix and VLT-UVES data are presented. Finally, recent analyses of FLAMES data are discussed. 

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