Ministerio de Economía y Competitividad 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)

G ISRAELIAN

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

Colaboradores del IAC: Artemio Herrero Davó, Javier Trujillo Bueno

M. Mayor, S. Udry, F. Pepe, G. Meynet, A. Maeder (Obs. Geneva); N. Santos, S. Sousa (Centro de Astrofisica da Universidade do Porto); P. Bonifacio (Obs. Meudon); P. Molaro (Obs. Trieste); N. Shchukina (Obs. Kiev); C. Melo (ESO, Chile), S. Randich (Arcetri); F. Musaev (SAO); A. Livshits (Univ. Moscú); B. May, M. Rowan Robbinson (IC, RU), S. Dermott (Univ Florida); J. Jenkins (SETI/NASA Ames), M. Showalter (SETI)

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-enrichment” 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-enrichment” 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. Different 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 abundance 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 used a large sample of FGK type dwarf stars for which precise abundances of C have been obtained using high resolution HARPS spectra for a sample of 1110 FGK stars. We have analyzed the possible relationship between the presences or not of planets in these stars. No differences were found between the samples, suggesting that stars with planets do not have a different footprint as far as C is concerned. There is no dependence between the abundance of [C / Fe] with respect to the metallicity of the stars. At low temperatures an abnormal behaviour of [C / Fe] is observed. Due to the amount of blends at these temperatures, the measurements at Teff <5200K should be taken with caution.

Possible correlations between the existence of planetary companions and the abundance of the relative elements of their stars have been studied. We have analyzed the C / O and [C / o] ratio and possible correlations between stars with planets and the abundance of their stars. A first analysis of the data suggests that stars with high mass planet show higher proportions of [℅]. However, we find that the ratio [℅], as well as the ratio [Mg / Si] depends largely on the metallicity through chemical evolution of the galaxy. After the elimination of galactic evolution only the difference in the elemental [C / O] relationship between hosts with high mass planets and stars without planets was present in a significant way. These results suggest that high mass planets are more frequent around stars with high [C / O]. Regarding the ratio ℅, no differences were found between the samples studied. We have also studied the Mg / Si ratio, extending previous studies of [Mg / Si]. We did not find, as with the ratio, differences between the samples.

Combining the results of the ratios, we performed a study of planetary formation, based on the possible formation of compounds. It has been found that from a sample of 502 stars, all have ℅ <0.8, so the presence of graphite, TiC and SiC will be important. In addition, the Mg / Si ratio will determine how we will find these compounds. In both planetary samples, both high and low mass planets, the compounds will divide almost equally between a ratio of less than 1 (where orthopyroxenes and feldspars will form) and a ratio between 1 and 2 (where both olivine and Pyroxenes will be present).

We have been searching for Li-rich giants in a sample of clusters where planets have been searched, thus we can study the planet engulfment scenario to explain Li replenishment using a proper comparison sample of stars without detected giant planets.  We derived Li abundances for a sample of 67 red giant stars in 12 different open clusters using standard spectral synthesis techniques and high-resolution spectra (from HARPS and UVES). We found three stars in different clusters with clearly enhanced Li abundances compared to other stars within the cluster. Interestingly, the only two stars with a detected substellar companion in our sample belong to that group. One of the planet hosts, NGC 2423 No. 3, might lie close to the luminosity bump on the HR diagram, a phase where Li production by the Cameron-Fowler process is supported by extra-mixing to bring fresh Li up to the surface.

We used very high-quality spectra of the two stars retrieved from the HARPS archive to derive very precise stellar parameters and chemical abundances. We derived the stellar parameters with the classical (non-differential) method, while we applied a differential line-by-line analysis to achieve the highest possible precision in abundances, which are fundamental to explore for very tiny differences in the abundances between the stars. We confirm that the abundance difference between ζ2 Ret and ζ1 Ret shows a significant (~2σ) correlation with Tc. However, we also find that the Tc trends depend on the individual spectrum used (even if always of very high quality). In particular, we find significant but varying differences in the abundances of the same star from different individual high-quality spectra.

Our results for the ζReticuli system show, for example, that nonphysical factors, such as the quality of spectra employed and errors that are not accounted for, can be at the root of the Tc trends for the case of individual spectra.

The main goal of this work is to explore the possible dependence of the Tc trend on stellar Galactocentric distances, Rmean.

We used high-quality spectra of about 40 stars observed with the HARPS and UVES spectrographs to derive precise stellar parameters, chemical abundances, and stellar ages. A differential line-by-line analysis was applied to achieve the highest possible precision in the chemical abundances.

We confirm previous results that [X/Fe] abundance ratios depend on stellar age and that for a given age, some elements also show a dependence on Rmean. When using the whole sample of stars, we observe a weak hint that the Tc trend depends on Rmean. The observed dependence is very complex and disappears when only stars with similar ages are considered.  To conclude on the possible dependence of the Tc trend on the formation place of stars, a larger sample of stars with very similar atmospheric parameters and stellar ages observed at different Galactocentric distances is needed.

The goal of this work is to obtain a new spectroscopic calibration for a fast estimate of Teff and [Fe/H] for a wide range of stellar spectral types.

We used spectra from a joint sample of 708 stars, compiled from 451 FGK dwarfs and 257 GK-giant stars. We used homogeneously determined spectroscopic stellar parameters to derive temperature calibrations using a set of selected EW line-ratios, and [Fe/H] calibrations using a set of selected Fe I lines.

We have derived 322 EW line-ratios and 100 Fe I lines that can be used to compute Teff and [Fe/H], respectively. We show that these calibrations are effective for FGK dwarfs and GK-giant stars in the following ranges: 4500 K <Teff< 6500 K, 2.5 < log g< 4.9 dex, and -0.8 < [Fe/H] < 0.5 dex. The new calibration has a standard deviation of 74 K for Teff and 0.07 dex for [Fe/H]. We use four independent samples of stars to test and verify the new calibration, a sample of 56 giant stars, a sample composed of Gaia FGK benchmark stars, a sample of 36 GK-giant stars of the DR1 of the Gaia-ESO survey, and a sample of 582 FGK-dwarf stars. We also provide a new computer code, GeTCal, for automatically producing new calibration files based on any new sample of stars.

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