The first high-precision radial velocity search for extra-solar planets |
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| Authors: | Gordon AH Walker |
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| Institution: | 1. Laboratoire Matériaux Polymères Interfaces Environnement Marin (MAPIEM EA 4323), Université du Sud Toulon Var, ISITV, BP 56, 83162 La Valette du Var Cedex, France;2. CNES (Centre National d''Etudes Spatiales), 18 Av. Edouard Belin, 31401 Toulouse Cedex 9, France;1. Museum für Naturkunde Berlin Invalidenstraße 43, 10115 Berlin, Germany;2. Observatoire de la Côte d’Azur, Nice, France;3. Deutsches Zentrum für Luft und Raumfahrt, Berlin, Germany;4. Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway;1. Laboratory of Metallic and Semiconducting Materials (LMSM), Université de Biskra, BP 455 RP, 07000 Biskra, Algeria;2. School of Physics and Astronomy, Nottingham Nanotechnology and Nanoscience Center, University of Nottingham, Nottingham, NG7 2RD, UK |
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| Abstract: | The reflex motion of a star induced by a planetary companion is too small to detect by photographic astrometry. The apparent discovery in the 1960s of planetary systems around certain nearby stars, in particular Barnard’s star, turned out to be spurious. Conventional stellar radial velocities determined from photographic spectra at that time were also too inaccurate to detect the expected reflex velocity changes. In the late 1970s and early 1980s, the introduction of solid-state, signal-generating detectors and absorption cells to impose wavelength fiducials directly on the starlight, reduced radial velocity errors to the point where such a search became feasible. Beginning in 1980, our team from UBC introduced an absorption cell of hydrogen fluoride gas in front of the CFHT coudé spectrograph and, for 12 years, monitored the radial velocities of some 29 solar-type stars. Since it was assumed that extra-solar planets would most likely resemble Jupiter in mass and orbit, we were awarded only three or four two-night observing runs each year. Our survey highlighted three potential planet hosting stars, γ Cep (K1 IV), β Gem (K0 III), and ? Eri (K2 V). The putative planets all resembled Jovian systems with periods and masses of: 2.5 years and 1.4 MJ, 1.6 years and 2.6 MJ, and 6.9 years and 0.9 MJ, respectively. All three were subsequently confirmed from more extensive data by the Texas group led by Cochran and Hatzes who also derived the currently accepted orbital elements.None of these three systems is simple. All five giant stars and the supergiant in our survey proved to be intrinsic velocity variables. When we first drew attention to a possible planetary companion to γ Cep in 1988 it was classified as a giant, and there was the possibility that its radial velocity variations and those of β Gem (K0 III) were intrinsic to the stars. A further complication for γ Cep was the presence of an unseen secondary star in an orbit with a period initially estimated at some 30 years. The implication was that the planetary orbit might not be stable, and a Jovian planet surviving so close to a giant then seemed improbable. Later observations by others showed the stellar binary period was closer to 67 years, the primary was only a sub-giant and a weak, apparently synchronous chromospheric variation disappeared. Chromospheric activity was considered important because κ1 Cet, one of our program stars, showed a significant correlation of its radial velocity curve with chromospheric activity.? Eri is a young, magnetically active star with spots making it a noisy target for radial velocities. While the signature of a highly elliptical orbit (e = 0.6) has persisted for more than three planetary orbits, some feel that even more extensive coverage is needed to confirm the identification despite an apparent complementary astrometric acceleration detected with the Hubble Space Telescope.We confined our initial analyses of the program stars to looking for circular orbits. In retrospect, it appears that some 10% of our sample did in fact have Jovian planetary companions in orbits with periods of years. |
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