5 Metallicity and orbital parameters

Given that we already have more than 40 extrasolar planets with high-precision metallicity determinations, we can start to think about looking for possible trends in [Fe/H] with planetary mass, semi-major axis or period, and eccentricity.

Gonzalez (1998) and Queloz et al. (2000) have suggested that stars with short-period planets (i.e. small semi-major axes) may be particularly metal-rich, even amongst the planetary hosts. The number of planets that were known by that time was, however, not enough to arrive at a definitive conclusion. Here we revisit that discussion with much better statistics.

In Fig. 6 we present a plot of the distribution of planets for two orbital "radius'' regimes: stars with planets orbiting with semi-major axes lower than 0.1 AU (shaded histogram), and with semi-major axes greater than 0.1 A.U (open histogram). The values were taken from Table 4. Stars with multiple companions are denoted by the vertical lines and were not included in the histograms. These include, among the stars for which we have [Fe/H] values available, the multiple systems around $\upsilon$ And (Butler et al. 1999), HD 83443 (Mayor et al. 2001a), HD 168443 (Udry et al. 2001), and HD 82943 (Mayor et al. 2001b).

Figure 7: Left: distribution of [Fe/H] for stars with planets having masses lower than 1  $M_{\rm Jup}$ (shaded histogram), and greater than 1  $M_{\rm Jup}$ (open histogram). Right: Similar to the left panel, but for eccentricity (limit at 0.1). In both cases, a Kolmogorov-Smirnov test results in a non-significative probability of being two different populations.

Some trend can be seen in the plot (right panel) suggesting that stars with short period planets might be more metal-rich then their long-period counterparts. A Kolmogorov-Smirnov test, however, gives a probability that both distributions make up part of the same sample of $\sim$75%. We have tried to make the plot using different limits of the semi-major axes; no further conclusions can be drawn. It is interesting to note that one of the longest-period planets discovered up to now (around the star 14 Her) has a value of [Fe/H] = +0.50.

Similar conclusions are drawn respecting the eccentricity or the companion mass (Fig. 7). The current results do not suggest any clear difference in [Fe/H] between high- and low-mass/eccentricity planets. If there is some trend, then more data (and probably even more precise abundances) are needed to permit conclusions. In the same way, no conclusions are possible regarding the position of the planetary systems in the plot (Fig. 6).