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The HARPS Search for Southern Extra-Solar Planets

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The HARPS Search for Southern Extra-Solar Planets The HARPS search for southern extra-solar planets. III. Three Saturn-mass planets around HD 93083, HD 101930 and HD 102117 C. Lovis, M. Mayor, François Bouchy, F. Pepe, D. Queloz, N. C. Santos, S. Udry, W. Benz, Jean-Loup Bertaux, C. Mordasini, et al. To cite this version: C. Lovis, M. Mayor, François Bouchy, F. Pepe, D. Queloz, et al.. The HARPS search for southern extra-solar planets. III. Three Saturn-mass planets around HD 93083, HD 101930 and HD 102117. Astronomy and Astrophysics - A&A, EDP Sciences, 2005, 437 (3), pp.1121-1126. 10.1051/0004- 6361:20052864. hal-00017981 HAL Id: hal-00017981 https://hal.archives-ouvertes.fr/hal-00017981 Submitted on 17 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A&A 437, 1121–1126 (2005) Astronomy DOI: 10.1051/0004-6361:20052864 & c ESO 2005 Astrophysics The HARPS search for southern extra-solar planets III. Three Saturn-mass planets around HD 93083, HD 101930 and HD 102117 C. Lovis1, M. Mayor1, F. Bouchy2,F.Pepe1,D.Queloz1,N.C.Santos3,1, S. Udry1,W.Benz4, J.-L. Bertaux5, C. Mordasini4, and J.-P. Sivan2 1 Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: [email protected] 2 Laboratoire d’Astrophysique de Marseille, Traverse du Siphon, 13013 Marseille, France 3 Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal 4 Physikalisches Institut Universität Bern, Sidlerstrasse 5, 3012 Bern, Switzerland 5 Service d’Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France Received 11 February 2005 / Accepted 16 March 2005 Abstract. We report on the detection of three Saturn-mass planets discovered with the HARPS instrument. HD 93083 shows radial-velocity (RV) variations best explained by the presence of a companion of 0.37 MJup orbiting in 143.6 days. HD 101930 b has an orbital period of 70.5 days and a minimum mass of 0.30 MJup. For HD 102117, we present the independent detection of a companion with m2 sin i = 0.14 MJup and orbital period P = 20.7 days. This planet was recently detected by Tinney et al. (2004, ApJ, submitted). Activity and bisector indicators exclude any significant RV perturbations of stellar origin, reinforcing the planetary interpretation of the RV variations. The radial-velocity residuals around the Keplerian fits are 2.0, 1.8 and 0.9 m s−1 respectively, showing the unprecedented RV accuracy achieved with HARPS. A sample of stable stars observed with HARPS is also presented to illustrate the long-term precision of the instrument. All three stars are metal-rich, confirming the now well- established relation between planet occurrence and metallicity. The new planets are all in the Saturn-mass range, orbiting at moderate distance from their parent star, thereby occupying an area of the parameter space which seems difficult to populate according to planet formation theories. A systematic exploration of these regions will provide new constraints on formation scenarios in the near future. Key words. stars: individual: HD 93083 – stars: individual: HD 101930 – stars: individual: HD 102117 – stars: planetary systems – techniques: radial velocities – techniques: spectroscopic 1. Introduction characterization of long-period planets and complex planetary systems. The low-mass and long-period regions of the exo- The HARPS instrument is the new ESO high-resolution (R = planet parameter space will therefore be under close scrutiny in 115 000) fiber-fed echelle spectrograph, mainly dedicated to the coming months and years. This will improve our knowledge planet search programmes and asteroseismology. It has already of the planet distribution in the mass-period diagram and will proved to be the most precise spectro-velocimeter to date, allow comparisons with theoretical predictions (see for exam- reaching an instrumental RV accuracy of ∼1ms−1 (Mayor ple Armitage et al. 2002; Ida & Lin 2004; Alibert et al. 2005). et al. 2003; Pepe et al. 2004; Santos et al. 2004a), and even In this paper we present the discovery of three plane- better on a short-term basis. This opens a new field in the tary companions to the stars HD 93083, HD 101930 and search for extrasolar planets, allowing the detection of com- HD 102117. Interesting characteristics of these planets include panions of a few Earth masses around solar-type stars. Indeed, a mass in the Saturn-mass regime and below, and an orbital dis- the increase of the planet frequency towards very low masses is tance to the star of 0.1–0.5 AU. According to recent planet for- confirmed by the recent discovery of Neptune-mass exoplanets mation scenarios (Ida & Lin 2004), they are situated in a region (McArthur et al. 2004; Santos et al. 2004a; Butler et al. 2004). of the mass-period diagram that seems difficult to populate. Moreover, the combination of CORALIE (Queloz et al. 2000) The systematic exploration of this mass and distance regime is and HARPS data, extending over several years, will allow the therefore an important test for the core-accretion and migration Based on observations made with the HARPS instrument on theories of planet formation. the ESO 3.6 m telescope at La Silla Observatory under programme This paper is structured as follows. Section 2 describes ID 072.C-0488(E). the observations, the data reduction process and discusses the Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20052864 1122 C. Lovis et al.: The HARPS search for southern extra-solar planets. III. long-term precision of the HARPS measurements. The physi- cal properties of the parent stars are presented in Sect. 3, fol- lowed by the RV measurements and orbital solutions in Sect. 4. We discuss in the last section the characteristics of these new planets in the context of the already-known properties of exo- planets and highlight some theoretical questions that might be answered by radial-velocity surveys in the near future. 2. Observations and measurement precision HD 93083, HD 101930 and HD 102117 are all members of the HARPS high-precision RV sample. The aim of this sur- veyistoobtainRV measurements with photon errors below 1ms−1 to detect very low-mass extrasolar planets. The stars in this sample have been selected from the CORALIE planet Fig. 1. Radial velocities for a sample of 12 stable stars from the HARPS high-precision programme, observed over about 700 days. search database for being non-evolved, having low projected −1 For each star, the mean RV has been subtracted. The overall rms of rotational velocity (v sin i < 3kms ) and exhibiting low ac- −1 ff 1.81 m s is made of di erent contributions, among which photon − − − tivity levels (log RHK < 4.7). These criteria should elimi- noise (∼0.8 m s 1), calibration noise (∼0.8 m s 1), guiding errors nate most stars showing large intrinsic RV variations and select (0.2–0.6 m s−1) and stellar acoustic modes (0.5–2.0 m s−1). Seasonal only very quiet, chromospherically inactive stars, so that stel- averages have been computed (bottom part). The absence of long-term lar RV jitter does not hide possible planetary signals down to drifts and the dispersion of only 0.64 m s−1 illustrate the long-term sta- 1–2 m s−1. Obviously, the photon noise error on the radial ve- bility of ThAr reference lamps. locity must also remain below 1 m s−1. For each HARPS spec- trum we compute the photon-limited, ultimate RV precision us- – The mean photon error, computed according to Bouchy ing the formulae given by Connes (1985) and Bouchy et al. −1 −1 et al. (2001), is 0.8 m s ; (2001). HARPS typically delivers 0.5–1 m s photon-limited – the zero-point wavelength calibration, given by nightly / ∼ −1 accuracy at a S N ratio of 100 per pixel (0.8 km s ) at 550 nm ThAr reference spectra, is presently determined with an ac- on solar-type stars. The exact number mainly depends on the curacy of ∼0.8 m s−1; depth of absorption lines and the projected rotational velocity. – guiding errors are responsible for 0.2–0.6 m s−1 of extra The radial velocities for HD 93083, HD 101930 and dispersion, due to the non-optimal tuning of the guiding HD 102117 have been obtained with the standard HARPS re- software. This issue has now been solved and the guiding duction pipeline, based on the cross-correlation with a stellar noise should remain below 0.3 m s−1 in the future; template, the precise nightly wavelength calibration with ThAr – the largest contribution to the global RV dispersion spectra and the tracking of instrumental drifts with the simul- comes from the star itself, mainly due to acoustic modes taneous ThAr technique (Baranne et al. 1996). Of particular (p-modes). For our 12 stars we estimate this contribution interest is the fact that the nightly instrumental drifts always to be 0.5–2.0 m s−1, depending on spectral type and evolu- −1 remain below 1 m s for HARPS due to the high instrumental tionary status (see Mayor et al.
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