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ASTRONOMY and ASTROPHYSICS the CORALIE Survey for Southern Extra-Solar Planets II

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ASTRONOMY and ASTROPHYSICS the CORALIE Survey for Southern Extra-Solar Planets II Astron. Astrophys. 356, 590–598 (2000) ASTRONOMY AND ASTROPHYSICS The CORALIE survey for southern extra-solar planets II. The short-period planetary companions to HD 75289 and HD 130322? S. Udry1, M. Mayor1, D. Naef1,F.Pepe1, D. Queloz1,2, N.C. Santos1, M. Burnet1, B. Confino3, and C. Melo1 1 Observatoire de Geneve,` 51 ch. des Maillettes, 1290 Sauverny, Switzerland 2 Jet Propulsion Laboratory, Mail-Stop: 306-388, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 3 St-Luc Observatory, 3961 St-Luc, Switzerland Received 2 December 1999 / Accepted 28 January 2000 Abstract. Doppler measurements from the high-resolution picture requiring the presence of an “ice” core for the run- CORALIE spectrometer of the stars HD 75289 and HD 130322 away growth of giant planets in the outer regions of the pro- reveal 2 new extra-solar giant planets. They are the second and tostellar nebulae (Pollack & Bodenheimer 1989; Boss 1995; third planetary candidates discovered with CORALIE in less than Lissauer 1995), a first approach invokes an inwards migra- 1 year, since its installation on the 1.2-m Euler Swiss telescope tion motion of the forming planet due to planet-disk inter- at La Silla. The minimum mass of the planet around HD 75289 actions (Goldreich & Tremaine 1980; Lin & Papaloizou 1986; (m2 sin i = 0.42 MJup) is the lightest found to date for a solar- Ward 1997) which stops because of star-planet tidal interac- type star companion. The orbital period is 3.5 days and the tions, mass transfer from the planet to the star (Lin et al. 1996; orbit is circular. The second candidate, a Jupiter-mass planet, Trilling et al. 1998) or lack of material in the very inner orbits HD 130322 in 10.72 days, on an almost circular trajec- region of the system (magnetospheric cavity). Other sce- tory. Due to their small orbital periods, both planets belong to narios involving gravity interactions between giant planets the hot Jupiter family, increasing thus the already large number (Weidenschilling & Marzari 1996; Rasio & Ford 1996) were of known Jupiter sized planets with orbits much smaller than in also proposed to bring outer giant planets close to their the solar system. stars. Another recent approach based on in situ formation (Wuchterl 1996, 1997, 1999; Bodenheimer et al. 1999) is gath- Key words: techniques: radial velocities – binaries: spec- ering an increasing support. troscopic – stars: individual: HD 75289 – stars: individual: In this context, the future statistical improvements expected HD 130322 – stars: activity – planetary systems 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 tele- scope at La Silla (Chile) is briefly described in the next section. 1. Introduction The results from this long-term systematic survey will be pre- After the discovery of the first extra-solar planet orbiting the sented in a series of numbered publications directly following solar twin star 51 Peg by Mayor & Queloz (1995), more new extra-solar planet discoveries, so that complementary stud- than 25 planetary candidates including a 3-planet system ies like detailed abundance analyses, photometric transit or dust (Butler et al. 1999) have now been indirectly detected by high- disk searches can be undertaken rapidly. Within about one year precision radial-velocity monitoring of solar-type stars of the CORALIE spectrograph has already made a decisive con- the solar neighbourhood (see Marcy et al. 1999 (MCM99); tribution to the discovery of the planetary companion around Queloz et al. 2000 and Vogt et al. 2000 for recent reference the M4 dwarf Gl 876 (Delfosse et al. 1998; simultaneously dis- updates). The unexpected characteristics of several of the covered by Marcy et al. 1998) and allowed us to detect 3 extra- new systems (small separations, large eccentricities, etc), have solar planets: the companion of Gl 86 described in Queloz et al. challenged our conventional views on planetary formation. (2000, Paper I) and the two new candidates presented in this Amongst the most interesting problems is the open ques- paper, namely those around HD 75289, the lightest extra-solar tion of the formation of hot Jupiters, giant planets very close planet found to date around a solar-type star, and HD 130322. to their parent stars. Several scenarios are competing to ex- During the preparation of this publication, a new CORALIE plain these systems. In the continuation of the conventional planetary candidate was announced around the star HD 192263 (Santos et al. 1999a). It is described in details in Santos et al. Send offprint requests to: [email protected] (2000, Paper III). ? Based on observations collected with the CORALIE echelle spec- With periods of 3.51 and 10.72 days, respectively, the plan- trograph on the 1.2-m Euler Swiss telescope at La Silla Observatory, ets presented here belong to the hot Jupiter family and thus ESO Chile S. Udry et al.: The CORALIE survey for southern extra-solar planets. II 591 Fig. 1. Apparent magnitude and distance distributions of the CORALIE planet-search sample. 80% of the stars are brighter than mV =9the faintest ones being mainly K dwarfs for which our technique is the most efficient increase the number of planets with small orbital separations, – Removal of the fainter cool dwarfs, by a colour-dependent an accumulation in the distribution that, although favoured distance cut off (dmax = f(B − V )), taking into account by observational biases, is nevertheless statistically significant that our cross-correlation technique needs less signal for K (MCM99). dwarfs1. The parent star descriptions, radial-velocity observations, orbital solutions and inferred planetary characteristics for these The final sample is presented in Fig. 1 which shows the stel- two new candidates are presented in Sects. 3 and 4, respectively. lar apparent magnitude and distance distributions. We note that Individual radial-velocity measurements will be made available ∼80% of the selected stars are brighter than mV =9and that in electronic form at the CDS in Strasbourg. In Sect. 5, we dis- faint objects are mostly K dwarfs for which our technique is the cuss the other sources of possible radial-velocity variations and most efficient. rule them out by use of bisector analysis and photometry. Finally, For the planet search itself, large amplitude binaries and Sect. 6 brings a summary of the results and a short discussion. potentially intrinsic variable stars (large activity index and fast rotators, Saar et al. 1998, Santos et al. 1999b, see Sect. 5) have to be avoided. The stability of a planetary system may be perturbed 2. The CORALIE planet-search sample and even destroyed by the close proximity of a stellar compan- ion. Moreover, the simultaneous determinations of the large am- The diversity of characteristics presented by the new planetary plitude and planetary orbits would require an additional observa- systems (e.g. MCM99) makes clear that existing ideas of plan- tional effort. Most large-amplitude systems were detected with etary formation need serious revision. Enlarging the number of the CORAVEL spectrographs (Baranne et al. 1979) at a lower detected systems is now necessary to obtain proper statistical precision level (300m s−1). Long-period binaries showing only distributions of orbital elements, planetary masses, etc, funda- linear drifts in radial velocity remain nevertheless good candi- mental for a better understanding of the formation and evolution dates for the planet search as the gravitational perturbation of of these systems. the stellar companion is weak and the long-period orbital effect Since the summer of 1998, a large high-precision radial- on the radial velocities can be easily corrected (e.g. Gl 86, Pa- CORALIE velocity programme is being carried out with the per I). The CORAVEL measurements also provide us with the echelle spectrograph on the 1.2-m Euler Swiss telescope stellar rotational velocity (Benz & Mayor 1984) often used as an at La Silla. Velocities are measured by numerically cross- activity indicator (Saar et al. 1998). The fast rotators and large correlating high-resolution spectra with a template appropriate amplitude binary systems are collected in low-priority subpro- for G–K stars. Further information on the method as well as grammes used as observing backup programmes; however, they technical and instrumental details and a description of the first still belong to the global volume-limited sample to be used for discoveries of the programme are given in Baranne et al. (1996) statistical purpose. We are left in the end with about 1000 high- and in Queloz et al. (1999, 2000). priority candidates for the core of the planet-search programme The overall sample consists of about 1650 solar-type stars itself. selected according to distance in order to obtain a well-defined The CORALIE survey offers a unique opportunity to explore volume-limited set of dwarf stars. As very briefly described in the relation between activity and other stellar characteristics like Udry et al. (1999), the global sample selection was performed rotational velocity, rotational period or intrinsic radial-velocity applying the following criteria: and photometric variations (Santos et al. 1999b). Because of its size and the quality of the measurements, – Selection of stars closer than 50 pc (π ≥ 20 mas) with pre- the above defined planet-search programme will also signif- cise parallaxes (σ ≤ 5 mas) and F8 ≤ Spectral type < M1 π icantly improve the data available for stellar binary systems, in the HIPPARCOS catalogue. reactualizing the Duquennoy & Mayor (1991) type of work. In – Photometric cut off of giant stars (2.5 mag off a “mean” main sequence). This allows us to remove most evolved stars 1 For a given signal-to-noise ratio, the photon-noise error inversely while keeping stars with misclassified luminosity classes scales with the depth of the cross-correlation function which is maxi- like the two candidates presented here (see below).
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