Results and discussion

The first results concern the general trends of RV variability. As we can see from Table 15.1, which summarizes our results, most of the stars in the N sample are RV variables, at an accuracy of 5 m s-1. The lower percentage ofRV variables in the S sample is clearly due to the lower accuracy obtained with FEROS.

M/Ms

Figure 15.2. The metallicity distribution versus mass for the S stars. The three stars hosting exoplanets are marked with box-and-tick symbols. Our candidates are not preferentially metal-rich.

Another interesting aspect of our statistics is that we have a large number of planet-host candidates (15% in the N sample and, a similar number in the S sample). This percentage is very high, and, if most of these stars are confirmed to host exoplanets, it is much higher than what is usually found in comparable surveys of main-sequence stars. This discrepancy is even larger when considering that among our evolved stars there are no short-period, hot Jupiters (Mayor and Queloz 1995), since they would be engulfed by their extended stellar atmospheres. Our stars have, on average, masses much larger than the typical masses of the main-sequence stars surveyed in RV studies. This might be one possible key for the interpretation of the results; it would indicate a very steeply increasing probability of formation of exoplanets with increasing stellar mass.

Remarkably, of the four stars which have been confirmed to host planets so far, none is metal-rich. Admittedly, our metallicity distribution for giant stars does not peak towards metal-rich stars, but the planet-hosting stars do not belong to the metal-rich tail of our distribution. Conversely, one of them (HD 47536) is the most metal-poor star of both samples. This can be clearly seen from Figure 15.2, which shows the metallicity-mass distribution of the S sample, in which the planet-host candidates are marked.

Figure 15.3. The metallicity-age distribution for our S stars. The sample has been re-sampled to give a similar number of stars for each age bin. In spite of a large metallicity spread for old stars, a clear age-metallicity relationship is present.

Figure 15.3. The metallicity-age distribution for our S stars. The sample has been re-sampled to give a similar number of stars for each age bin. In spite of a large metallicity spread for old stars, a clear age-metallicity relationship is present.

Finally, thanks to our accurate age reconstruction, we are able to investigate (for the first time using evolved stars) the age-metallicity relationship in the Solar neighborhood.

The first striking point is that the young (i.e. aged <1 Gyr) stars have a small spread in [Fe/H] (a = 0.09), comparable to the measurement errors. In spite of a larger spread in the metallicity distribution of older stars, a clear age-metallicty relationship is present in our sample, as shown in Figure 15.3, which shows the age-metallicity relationship for the S stars. The relationship would indicate a very modest increase in [Fe/H] over the last 3-4 Gyr, together with an increase of ~0.3-0.5 dex in the previous 10 Gyr. This result seems at odds with the results obtained by Nordstrom et al. (2004) for a much larger sample of main-sequence stars. It is beyond the scope of this paper to discuss this discrepancy; however, we note that determination of the age of main-sequence stars might be very critical, as pointed out e.g. by Pont and Eyeer (2004), who found a clear age-metallicity relationship from their analysis of main-sequence stars, once ages had been reviewed critically.

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