The situation for other elements

The authors of the majority of the spectroscopic studies on the chemical properties of stars with planets have concentrated their efforts on measuring the abundances of iron as a metallicity proxy. However, the number of studies regarding other elemental abundances is increasing. These concerned the abundances of a variety of elements, from Li and Be (see Section 4), up to alpha and Fe-group elements (e.g. Sadakane et al. 1999, 2002; Santos et al. 2000, 2006b; Gonzalez et al. 2001; Smith etal. 2001; Bodaghee etal. 2003; Beirao etal. 2005; Ecuvillon etal. 2004a, 2004b, 2006a; Gilli et al. 2006; Luck & Heiter 2006) and have unveiled a few interesting trends. The main conclusion, though, is that the stars with planets seem to be the metal-rich component of the Solar-neighborhood population (Figure 2.2).

The study of other elements might be of great importance. If stellar pollution is a common occurrence among planet-host stars, or any field star, e.g. Murray et al. (2001), Quillem (2002), Wilden (2002), Laws & Gonzalez (2003), and Shen et al. (2005), we can expect to find these to be more enriched in refractory elements, since volatiles could evaporate from infalling bodies before being accreted (Smith et al. (2001; Ecuvillon et al. 2006b).

Furthermore, elements like C, O, and N (e.g. Ecuvillon et al. 2004a, 2006a; Robinson et al. 2006) may be essential to form the cores of giant planets. An overabundance of these species could thus increase the efficiency of planet formation. Up to now, however, no clear global evidence of differences between elements of different condensation temperatures has been found regarding any of these matters (Smith et al. 2001; Takeda et al. 2001; Ecuvillon et al. 2006b).

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Figure 2.2. Plots of [X/Fe] versus [Fe/H] for X = Si, Ca, Ti, Cr, Co, and Ni for stars with planets (open triangles) and single field stars (starred symbols). From Gilli et al. (2006).

Figure 2.2. Plots of [X/Fe] versus [Fe/H] for X = Si, Ca, Ti, Cr, Co, and Ni for stars with planets (open triangles) and single field stars (starred symbols). From Gilli et al. (2006).

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