Measurements of the Isotopic Ratio 6Li/7Li in Stars with Planets

One of the interesting properties of the known stars with planets concerns their metallicity distribution. Several studies (Santos et al. 2000;Gonzalez et al. 2001;Laws et al. 2003;Santos et al. 2005;Fischer & Valenti 2005) have confirmed the result first shown by Gonzalez (1997): stars with giant planets are systematically metal-rich (by ∼ 0.2 dex) relative to field FGK dwarfs not known to harbor planets. Two hypotheses have been proposed to explain this excess: primordial enrichment or pollution. The first indicates that the probability of forming giant planets is a steeply rising function of the intrinsic metallicity of the gas and dust cloud which gave birth to the system. This possibility is in agreement with the core-accretion scenario (e.g., Pollack et al. 1996). The higher metal content would raise the surface density of solid material in the disk, leading to a more efficient agglutination of the cores onto which the gas will be accreted. The pollution scenario, on the other hand, indicates that the presence of planets could alter the metallicity of their hosting stars. During the inward migration process of giant planets, solid material from the protoplanetary disk or even inner planetesimals and planets could be accreted into the convective envelope of the hosting star. As this material is depleted in H and He, the star's metallicity would be enhanced.

Current results (e.g., Fischer & Valenti 2005) show that the frequency of planets increases significantly for higher metallicities, thus giving strong support for the primordial hypothesis. Evidence for the occurence of pollution is still ambiguous. For instance, Ecuvillon et al. (2006) studied the relation between chemical abundances of several elements and their respective condensation temperatures in stars with and without planets, finding no signifcant differences in the two groups. On the other hand, Pasquini et al. (2007) analyzed the metallicity distributions of planet-hosting dwarfs and giants and found that the latter do not favor metal-rich systems. The authors argue that this result could be a strong indication of pollution, as the metal excess could be erased by the dilution process that takes place during the later stages of stellar evolution. Another ambiguous result is the possible detection of 6 Li in the atmospheres of stars with planets, which is a sensitive test of the pollution hypothesis. Both lithium isotopes are destroyed at relatively low temperatures (T = 2 × 10 6 K for 6 Li and T = 2.5 × 10 6 K for 7 Li) in stellar interiors. During the early stages of evolution in solar-type stars (specifically, before entering the main-sequence), these stars are entirely convective and most of the primordial Li is transported to deeper and hotter layers, where it is rapidly burned. The fraction of lithium destruction is, however, a strong function of the stellar mass. For a given metallicity, there is a mass range in which the 6 Li is completely destroyed, while a significant amount of 7 Li is preserved. The lower edge of this range falls just above the solar mass (corresponding to main-sequence late-F spectral types) for near solar metallicities. Thus, one should not expect to find any 6 Li in the atmospheres of solar-type stars. Any positive detection could indicate an external contamination or pollution process. Israelian et al. (2001) measured the isotopic ratio 6 Li/ 7 Li in HD 82943 (with two close-in giant planets) and found 6 Li/ 7 Li = 0.126 ± 0.014. This can be compared to a solar system (meteoritic) ratio of 6 Li/ 7 Li = 0.08. The positive 6 Li detection for this star was interpreted as observational evidence of the pollution process. Reddy et al. (2002) studied 6 Li in 8 planet hosting stars (HD 82943 included) and found no significant amount of this isotope in HD 82943, nor in any of the targets analyzed. The difference from the previously published 6 Li detection for HD 82943 was attributed to the use of a more complete line list, although it noted the presence of an unidentified absorption in the Li region. Israelian et al. (2003) investigated the nature of the unindentified absorption feature at 6708.025 Å, which affects the Li I line, by observing several stars of different effective temperatures. They concluded that a high excitation Si I line first proposed by Müller et al. (1975) is more adequate than the Ti I line used by Reddy et al. (2002). Adopting a revised line list and higher quality spectra, the authors performed a new analysis of HD 82943 and measured 6 Li/ 7 Li = 0.05 ± 0.02. Thus, this most recent result for HD 82943 gave additional support to the pollution scenario.

More recently, Mandell et al. (2004) made a major extension of the previous line lists (especially for the CN contribution) and tested three different possibilities for the unidentified feature at 6708.025 Å (Si I, Ti I and Ti II): for all of the 3 different line lists no 6 Li was detected in a sample of three planet-hosting stars. These resul

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