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Astron. Astrophys. 356, 590-598 (2000)

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1. Introduction

After the discovery of the first extra-solar planet orbiting the solar twin star 51 Peg by Mayor & Queloz (1995), more than 25 planetary candidates including a 3-planet system (Butler et al. 1999) have now been indirectly detected by high-precision radial-velocity monitoring of solar-type stars of the solar neighbourhood (see Marcy et al. 1999 (MCM99); Queloz et al. 2000and Vogt et al. 2000for recent reference updates). The unexpected characteristics of several of the new systems (small separations, large eccentricities, etc), have challenged our conventional views on planetary formation. Amongst the most interesting problems is the open question of the formation of hot Jupiters , giant planets very close to their parent stars. Several scenarios are competing to explain these systems. In the continuation of the conventional picture requiring the presence of an "ice" core for the runaway growth of giant planets in the outer regions of the protostellar nebulae (Pollack & Bodenheimer 1989; Boss 1995; Lissauer 1995), a first approach invokes an inwards migration motion of the forming planet due to planet-disk interactions (Goldreich & Tremaine 1980; Lin & Papaloizou 1986; Ward 1997) which stops because of star-planet tidal interactions, mass transfer from the planet to the star (Lin et al. 1996; Trilling et al. 1998) or lack of material in the very inner region of the system (magnetospheric cavity). Other scenarios involving gravity interactions between giant planets (Weidenschilling & Marzari 1996; Rasio & Ford 1996) were also proposed to bring outer giant planets close to their stars. Another recent approach based on in situ formation (Wuchterl 1996, 1997, 1999; Bodenheimer et al. 1999) is gathering an increasing support.

In this context, the future statistical improvements expected from ongoing large surveys will prove to be decisive. Among the latter, our ambitious planet-search programme with the CORALIE echelle spectrograph on the 1.2-m Euler Swiss telescope at La Silla (Chile) is briefly described in the next section. The results from this long-term systematic survey will be presented in a series of numbered publications directly following new extra-solar planet discoveries, so that complementary studies like detailed abundance analyses, photometric transit or dust disk searches can be undertaken rapidly. Within about one year the CORALIE spectrograph has already made a decisive contribution to the discovery of the planetary companion around the M4 dwarf Gl 876 (Delfosse et al. 1998; simultaneously discovered by Marcy et al. 1998) and allowed us to detect 3 extra-solar planets: the companion of Gl 86 described in Queloz et al. (2000, Paper I) and the two new candidates presented in this paper, namely those around HD 75289, the lightest extra-solar planet found to date around a solar-type star, and HD 130322. During the preparation of this publication, a new CORALIE planetary candidate was announced around the star HD 192263 (Santos et al. 1999a). It is described in details in Santos et al. (2000, Paper III).

With periods of 3.51 and 10.72 days, respectively, the planets presented here belong to the hot Jupiter family and thus increase the number of planets with small orbital separations, an accumulation in the distribution that, although favoured by observational biases, is nevertheless statistically significant (MCM99).

The parent star descriptions, radial-velocity observations, orbital solutions and inferred planetary characteristics for these two new candidates are presented in Sects. 3 and 4, respectively. Individual radial-velocity measurements will be made available in electronic form at the CDS in Strasbourg. In Sect. 5, we discuss the other sources of possible radial-velocity variations and rule them out by use of bisector analysis and photometry. Finally, Sect. 6 brings a summary of the results and a short discussion.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000