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

1. Introduction

Between October 1995 and July 1997 eight candidate extra-solar planetary systems were announced by groups searching for radial velocity variations among solar-type stars: 51 Peg (Mayor & Queloz 1995 and Marcy et al. 1997); 47 UMa (Butler & Marcy 1996); 70 Vir (Marcy & Butler 1996); Cnc, Boo, And, (Butler et al. 1997); 16 Cyg B (Cochran et al. 1997); CrB (Noyes et al. 1997). To these we can add HD 114762 (Latham et al. 1989). These historic discoveries are the fruits of about 15 years of searching by several groups (Campbell et al. 1988; Cochran & Hatzes 1994; Walker et al. 1995), which had not, until now, found evidence of planetary mass companions. With these finds, we finally have empirical data to test planet formation models. One aspect of planet formation that has not been discussed in much detail is the dependence of planet formation on the metallicity of the protostellar cloud. With nine planetary-system candidates (and one certain case, the Solar System), we can begin to address this question.

The primary purpose of this study is to determine accurately the metallicities of the photospheres of Cnc, CrB, 16 Cyg A and B, 51 Peg, 47 UMa, 70 Vir, and HD 114762 (the stars And and Boo have been analyzed by Gonzalez 1997; Paper I); in addition, we determine the abundances of about 15 other elements including lithium. Also, we compare the abundance patterns of 16 Cyg B with its companion, 16 Cyg A, which does not display regular radial velocity variations (Cochran et al. 1997). While others have performed separate abundance analyses on all these stars already, we have analyzed them together as a homogenous group for the first time, therefore minimizing the relative systematic errors, and, unlike most previous studies, we make use of high-quality spectra. In addition, we employ modern model atmospheres. As a secondary goal, we also derive estimates of for these stars (except 16 Cyg A and B), and, combining them with other data, employ them to place constraints on the masses of the companions (assuming the radial velocity variations are not intrinsic to the stars). We end with a discussion of the metallicity trends among the parent stars of the extrasolar planets.

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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