Astron. Astrophys. 334, 221-238 (1998)

5. Summary

We have performed spectroscopic abundance analyses of seven recently announced extrasolar planetary system candidates. The metallicities are less than solar for CrB and HD 114762, approximately solar for 70 Vir and 47 UMa, and greater than solar for Cnc, 16 Cyg B, and 51 Peg (also And and Boo, from Paper I). We have also estimated the projected stellar rotational velocities and employed them, along with other data, to estimate the most likely masses of the substellar companions of these stars. The companions of 70 Vir and HD 114762 fall in the brown dwarf mass range. The companions of And, 16 Cyg B, and 51 Peg have the smallest masses and are the best extrasolar planet candidates at this time.

The most surprising, and potentially most important, findings in this study are the high metallicities of Cnc and 51 Peg (and And and Boo). These results are consistent with the planetary orbital migration model, whereby a gas giant migrates to within a few hundredths of an AU of the parent star, the material between the planet and the star presumably being accreted onto the latter. If this is confirmed by future observations, then Galactic chemical evolution models will have to be revised accordingly.

Our results for Cnc are anomalous. A straight-forward interpretation of the data implies an age greater than that of the universe. We tentatively interpret the Cnc system as a nearly pole-on spectroscopic binary. Careful line-profile analysis should be capable of testing this interpretation, and detailed analysis of its M dwarf companion will allow us to compare the metallicities of the pair.

To improve the mass estimates of the companions, future research should be directed at refining the estimates through the use of high-resolution high-S/N spectroscopy, especially for 16 Cyg A and B. Additional theoretical work needs to be done on the planet migration process, the role of metallicity in the planet formation process, and stellar models with metal-rich envelopes. Also, we encourage observers to direct their planetary search efforts at the known SMR dwarfs and subgiants. Finally, we also encourage spectroscopic abundance analyses to be done on new systems as they are announced.

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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