Academia.eduAcademia.edu

Two new planets from the Anglo-Australian planet search

2001, The Astrophysical …

Precise Doppler measurements from the Anglo-Australian Telescope (AAT) UCLES spectrometer reveal periodic Keplerian velocity variations in the stars HD 160691 and HD 27442. HD 160691 has a period of 743 days, a semiamplitude of 54 m s~1, and a high eccentricity, e \ 0.62, typical of extrasolar planets orbiting beyond 0.2 AU. The minimum (M sin i) mass of the companion is 1.97 and the M J , semimajor axis is 1.65 AU. HD 27442 has a 415 day period, a semiamplitude of 32 m s~1, and an eccentricity of 0.058. The minimum mass is 1.43 and the semimajor axis is 1.18 AU. This is the Ðrst M J , extrasolar planet orbiting beyond 0.2 AU that is in a circular orbit similar to solar system planets. The photon-limited precision of AAT/UCLES measurements is 3 m s~1 as demonstrated by stable stars and Keplerian Ðts to planet-bearing stars. In addition, we present conÐrmation of four previously announced planets.

THE ASTROPHYSICAL JOURNAL, 555 : 410È417, 2001 July 1 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A. TWO NEW PLANETS FROM THE ANGLO-AUSTRALIAN PLANET SEARCH1 R. PAUL BUTLER,2,3 C. G. TINNEY,3 GEOFFREY W. MARCY,4 HUGH R. A. JONES,5 ALAN J. PENNY,6 AND KEVIN APPS7 Received 2000 December 25 ; accepted 2001 March 7 ABSTRACT Precise Doppler measurements from the Anglo-Australian Telescope (AAT) UCLES spectrometer reveal periodic Keplerian velocity variations in the stars HD 160691 and HD 27442. HD 160691 has a period of 743 days, a semiamplitude of 54 m s~1, and a high eccentricity, e \ 0.62, typical of extrasolar planets orbiting beyond 0.2 AU. The minimum (M sin i) mass of the companion is 1.97 M , and the J semimajor axis is 1.65 AU. HD 27442 has a 415 day period, a semiamplitude of 32 m s~1, and an eccentricity of 0.058. The minimum mass is 1.43 M , and the semimajor axis is 1.18 AU. This is the Ðrst J extrasolar planet orbiting beyond 0.2 AU that is in a circular orbit similar to solar system planets. The photon-limited precision of AAT/UCLES measurements is 3 m s~1 as demonstrated by stable stars and Keplerian Ðts to planet-bearing stars. In addition, we present conÐrmation of four previously announced planets. Subject headings : planetary systems È stars : individual (HD 160691, HD 27442) 1. systems is richer and more complicated than previously imagined. Future breakthroughs will require greater measurement sensitivity. In particular, the most pressing questions at the moment are does the planet mass function continue to rise through the Jupiter to the Neptune-mass range, and what fraction of stars have ““ solar systemÈlike ÏÏ planets, i.e., Jupiter and Saturn analogs in circular orbits beyond 4 AU ? Doppler precision of 3 m s~1 or better is required to address these questions. The Anglo-Australian Planet Search, therefore, has the speciÐc goal of achieving a long-term precision of 3 m s~1. In ° 2 we show that this goal has been reached. Section 3 reports the stellar characteristics and Doppler velocities of the host stars for two giant planets that have emerged from this survey. Section 4 presents velocities and orbital solutions from the Anglo-Australian Telescope (AAT) data for four planets previously announced from the Keck, CORALIE, and ESO planet surveys and is followed by a discussion of our results. INTRODUCTION All D50 extrasolar planets discovered over the last 5 yr have come from precision Doppler surveys of nearby dwarf stars ranging in spectral type from late F through M4 (Mayor & Queloz 1995 ; Marcy & Butler 1998, 2000 ; Noyes et al. 1997 ; Cochran et al. 1997 ; Vogt et al. 2000 ; Kurster et al. 2000 ; Udry et al. 2000 ; Fischer et al. 2001). With one exception (Marcy, Butler, & Vogt 2000), all the published planets have Doppler velocity amplitudes greater than 30 m s~1. With a measurement error of D10 m s~1, Doppler velocity amplitudes of 30 m s~1 are the smallest that can be easily detected. Detection of planets with smaller amplitudes, such as solar system analogs and Neptune-to-Saturn mass planets in short period orbits, requires measurement precision of 3 m s~1 or better (Butler & Marcy 1997 ; Butler et al. 2001). For the case of massive planets orbiting within 3 AU, the D50 known planets constitute a useful statistical sample. The substellar companion mass function abruptly rises at 5 M and continues to rise down to the detection limit near J 1 M . About 7% of nearby stars have massive planets with J periods less than 5 yr, including D0.75% with ““ 51 orbital PegÈlike ÏÏ planets in 3È5 day circular orbits (Cumming, Marcy, & Butler 1999 ; Butler et al. 2001). Although existing planet hunting techniques are still in their infancy, two primary results have emerged over the last 5 yr. Extrasolar planets are common, and the architecture of planetary 2. THE ANGLO-AUSTRALIAN DOPPLER SURVEY The Anglo-Australian Planet Search began observations in 1998 January and is currently surveying 200 stars. Initial results from this work have been published by Tinney et al. (2001). The AAT program stars have been chosen to be among the brightest, chromospherically inactive dwarf and subgiant stars ranging in spectral type from late F through early M. Most of the stars are south of declination [20¡ to prevent overlap with the Keck planet survey (Vogt et al. 2000), and most of the stars are brighter than V \ 7.5, consistent with achieving signal-to-noise ratio (S/N) Z 200 with exposure times of 10 minutes or less. This S/N is required to achieve photon-limited precision of 3 m s~1 (Butler et al. 1996). High-resolution spectra, R D 45,000, are taken with the UCLES echelle spectrometer (Diego et al. 1990) on the 3.9 m AAT. These spectra span the wavelength range from 4820 to 8550 AŽ . An iodine absorption cell (Marcy & Butler 1992) provides wavelength calibration from 5000 to 6000 AŽ . The spectrometer point-spread function is derived from the 1 Based on observations obtained at the Anglo-Australian Telescope, Siding Spring, Australia. 2 Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015-1305 ; paul=dtm.ciw.edu. 3 Anglo-Australian Observatory, P.O. Box 296, Epping, NSW 1710, Australia. 4 Department of Astronomy, University of California, Berkeley, CA 94720 ; and Department of Physics and Astronomy, San Francisco State University, San Francisco, CA 94132. 5 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK. 6 Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK. 7 Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QJ, UK. 410 TWO NEW PLANETS FROM PLANET SEARCH 411 detailed shapes of the embedded iodine lines (Valenti, Butler, & Marcy 1995 ; Butler et al. 1996). Similar systems on the Lick 3 m and the Keck 10 m telescopes currently provide photon-limited precision of 3 m s~1 (Butler et al. 1996 ; Butler & Marcy 1997 ; Vogt et al. 2000). No nightly corrections (Walker et al. 1995) are applied to the Lick, Keck, or AAT systems. Figure 1 shows AAT velocities of four stable dwarf stars with spectral types ranging from late F to early G. The rms of the measured velocities for these stars range from 3.8 to 4.9 m s~1, consistent with measurement uncertainties of 3È4 m s~1 and intrinsic stellar variability of 2È3 m s~1 (Saar, Butler, & Marcy 1998). Of these stars, the F8 dwarf HD 196378 (HR 7875) was previously reported to be a velocity variable. Kurster et al. (1999) report, ““ Our best candidate so far for having an orbiting planet is the F8V star /2 Pav (\HR 7875).ÏÏ They report a velocity amplitude of 39.5 m s~1 and a probable period of 42.5 days. Our velocities would have revealed this planet, but we have failed to detect it. Figure 2 shows AAT velocities of four stable mid-G dwarfs, while Figure 3 shows four stable stars ranging from late G to early K. These stars exhibit a velocity rms ranging from 2.3 to 5.0 m s~1, consistent with measurement uncertainty of D3 m s~1. Of these stars, the G8 V star q Ceti (HD FIG. 2.ÈAAT Doppler velocities of stable G dwarfs FIG. 1.ÈAAT Doppler velocities of stable late F and early G dwarfs. These observations span the 2.8 yr of the AAT Planet Search Project. 10700, HR 509) is frequently used as a velocity standard by precision velocity groups because it is bright, chromospherically inactive, and visible from both the northern and southern hemispheres. By the late 1980s the CanadaFrance-Hawaii Telescope precision velocity survey had shown an rms of 13 m s~1 for this star (Campbell, Walker, & Yang 1988 ; Walker et al. 1995). Measurements taken since 1994 November from the Lick precision velocity program show an rms of 4.6 m s~1 (Butler et al. 1996), while the ESO precision velocity program has reported an rms of 14 m s~1 for this star (Kurster et al. 2000). After 3 yr, the AAT velocities for this star have an rms of 3.9 m s~1, as shown in Figure 3. Slowly rotating, chromospherically inactive, mainsequence dwarf stars ranging in spectral type from early G to mid-M have been shown to be intrinsically stable at the 3 m s~1 level (Butler & Marcy 1997 ; Saar et al. 1998 ; Vogt et al. 2000 ; this paper). In addition, Fischer et al. (2001) have shown several class IV subgiants are stable at the 6 m s~1 level. In Figure 4 we present four stable subgiants from the AAT survey. These stars are stable at the 3È4 m s~1 level. With intrinsic stability at this level, it is possible to search for planets around subgiants using the precision Doppler technique. In addition, the stability of such stars makes these stars excellent targets for stellar seismology campaigns, especially as they are predicted to have larger ampli- 412 BUTLER ET AL. FIG. 3.ÈAAT Doppler velocities of stable late G and K dwarfs tudes than main-sequence dwarfs. Bedding et al. (2001) have recently detected a 17 minute oscillation in HD 2151 using the AAT Planet Search hardware and software. They took 1200 velocity measurements over Ðve nights with an rms of 3.3 m s~1. On a timescale of hours the precision was 2.2 m s~1. 3. STELLAR CHARACTERISTICS AND ORBITAL SOLUTIONS 3.1. Stellar Characteristics of HD 160691 A total of 140 observations of HD 160691 (HR 6585, HIP 86796, GL 691, k Ara) have been made by Hipparcos (Perryman et al. 1997), yielding a distance of 15.3 pc and a V magnitude of 5.20. The resulting absolute magnitude is M \ 4.28. The star is photometrically stable within HipV parcos measurement error, with photometric scatter of 0.002 mag. The Bright Star Catalog (Hoffleit & Jaschek 1982) assigns a spectral type of G3 IVÈV, in reasonable agreement with the Hipparcos spectral type of G5 V. The star is chromospherically inactive, with log R@(HK) \ [5.02 (Henry et al. 1996). Its chromospherically inferred age is D6 Gyr. Combining Hipparcos astrometry of HD 160691 with the SIMBAD radial velocity ([9.0 km s~1) yields an extremely low space velocity with respect to the local standard of rest : U, V , W \ [4, ]3, ]3 km s~1. Vol. 555 FIG. 4.ÈAAT Doppler velocities of stable G subgiants Like many of the planet-bearing stars, HD 160691 is extremely metal-rich. The [Fe/H] derived from highresolution spectroscopy is 0.28 ^ 0.04 (Favata, Micela, & Sciortino 1997), in good agreement with our photometric estimate of ]0.29. The lithium line at 6707.8 AŽ was not detected in high-resolution (R \ 100,000) spectra (Favata, Micela, & Sciortino 1996). The mass of HD 160691 estimated from B[V , M , and [Fe/H] is 1.08 ^ 0.05 M . Bol _ 3.2. Doppler V elocities and Orbital Fit for HD 160691 The 21 Doppler velocity measurements of HD 160691 obtained between 1998 November and 2000 November are listed in Table 1 and shown graphically in Figure 5. The best-Ðt Keplerian yields an orbital period of 743.5 days, a velocity amplitude of 53.6 m s~1, and an eccentricity of 0.62. The minimum (M sin i) mass of the planet is 1.97 M , and J the semimajor axis is 1.65 AU. The rms to the Keplerian Ðt is 2.98 m s~1, yielding s2 \ 0.98. l 3.3. Stellar Characteristics of HD 27442 A total of 107 observations of HD 27442 (HR 1355, HIP 19921, v Ret) have been made by Hipparcos, yielding a distance of 18.2 pc and a V magnitude of 4.55. The resulting absolute magnitude is M \ 3.25. The star is photoV metrically stable within Hipparcos measurement error, with photometric scatter of 0.003 mag. The Bright Star Catalog assigns this star a spectral type of K2 IVa. No. 1, 2001 TWO NEW PLANETS FROM PLANET SEARCH 413 TABLE 2 VELOCITIES FOR HD 27442 FIG. 5.ÈDoppler velocities for HD 160691. The rms to the best-Ðt Keplerian (solid line) is 3 m s~1. The period is 743.5 days, and the semiamplitude is 53.6 m s~1. Assuming the mass of HD 160691 is 1.08 M , the _ minimum (M sin i) mass of the companion is 1.97 M , and the semimajor J axis is 1.65 AU. The orbital eccentricity is 0.62, similar to other planets orbiting beyond 1 AU. On the basis of measured equivalent widths of Fe lines observed at high resolution (R D 60,000), Randich et al. (1999) determined the metallicity of HD 27442 to be [Fe/ H] \ 0.22. This is somewhat more metal-rich than earlier estimates, including Elgaroy, Engvold, & Lund (1999), who report [Fe/H] \ 0.00 based on averaging several previous estimates, and the photometric estimate of Eggen (1993), who found [Fe/H] \ 0.06. On the basis of evolutionary tracks, Randich et al. (1999) Ðnd the mass of HD 27442 to be 1.2 ^ 0.1 M and the age to be 10 Gyr, consistent with _ subgiant status. They also report an upper limit for the equivalent width of Li at 3 mAŽ . JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 831.0816 . . . . . . . . 1,118.1404 . . . . . . 1,525.9634 . . . . . . 1,527.0333 . . . . . . 1,630.9035 . . . . . . 1,745.3341 . . . . . . 1,767.3337 . . . . . . 1,768.3132 . . . . . . 1,828.1498 . . . . . . 1,830.0084 . . . . . . 1,856.1407 . . . . . . 1,857.0932 . . . . . . 1,919.0796 . . . . . . 1,921.1039 . . . . . . [47.5 [11.5 2.1 [7.5 [44.4 [23.1 [13.5 [17.3 13.2 11.8 19.6 20.1 0.0 3.6 2.4 2.2 2.0 2.4 2.2 2.3 2.8 2.5 2.5 3.2 2.6 3.9 2.7 2.3 January. These measurements are graphically shown in Figure 6. The best-Ðt Keplerian yields an orbital period of 415.2 days, a velocity amplitude of 32.5 m s~1, and an eccentricity of 0.058. The minimum (M sin i) mass of the planet is 1.35 M , and the semimajor axis is 1.16 AU. The J rms to the Keplerian Ðt is 2.96, yielding s2 \ 1.65. This is l 3.4. Doppler V elocities and Orbital Fit for HD 27442 Table 2 lists the 14 Doppler velocity measurements of HD 27442 obtained between 1998 January and 2001 TABLE 1 VELOCITIES FOR HD 160691 JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 1,118.8874 . . . . . . 1,119.9022 . . . . . . 1,120.8870 . . . . . . 1,121.8928 . . . . . . 1,236.2864 . . . . . . 1,410.8977 . . . . . . 1,412.9773 . . . . . . 1,413.8981 . . . . . . 1,630.3042 . . . . . . 1,683.0926 . . . . . . 1,684.1320 . . . . . . 1,718.1184 . . . . . . 1,742.9096 . . . . . . 1,743.9240 . . . . . . 1,745.0440 . . . . . . 1,766.9330 . . . . . . 1,767.9689 . . . . . . 1,827.8973 . . . . . . 1,828.8866 . . . . . . 1,829.8890 . . . . . . 1,855.9058 . . . . . . [4.3 [4.8 [4.7 [4.3 [20.1 [40.8 [41.2 [33.8 46.2 49.0 46.2 33.8 21.1 30.7 19.4 11.3 13.4 0.6 6.4 0.0 [1.1 3.3 2.9 2.8 2.9 4.0 2.8 5.5 2.8 3.3 3.8 4.0 3.6 3.2 3.8 3.2 3.1 3.2 3.0 3.8 3.5 4.6 FIG. 6.ÈDoppler velocities for HD 27442. The rms to the best-Ðt Keplerian (solid line) is 2.9 m s~1. The period is 415.2 days, and the semiamplitude is 32.5 m s~1. Assuming the mass of HD 27442 is 1.2 M , the _ this minimum (M sin i) mass of the companion is 1.35 M . The orbit of planet is similar to the Earth in both semimajor axis,J a \ 1.16 AU, and eccentricity, e \ 0.058. This is the only known planet orbiting beyond 0.15 AU that is in a circular orbit, similar to solar system planets. TABLE 3 ORBITAL PARAMETERS Parameter HD 160691 HD 27442 Orbital period P (days) . . . . . . . . . . . . Velocity amplitudeK (m s~1) . . . . . . Eccentricity e . . . . . . . . . . . . . . . . . . . . . . . . u (deg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Periastron time (JD) . . . . . . . . . . . . . . . . M sin i (M ) . . . . . . . . . . . . . . . . . . . . . . . . . J a (AU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . rms (m s~1) . . . . . . . . . . . . . . . . . . . . . . . . . 743 (10) 53.6 (2) 0.62 (0.05) 305 (5) 2,451,626.8 (5) 1.97 (0.14) 1.65 (0.12) 2.98 415.2 (5) 32.5 (2) 0.058 (0.05) 347 (20) 2,451,849.3 (4) 1.35 (0.11) 1.16 (0.11) 2.96 414 BUTLER ET AL. the only planet found to date orbiting beyond 0.15 AU in an orbit as circular as solar system planets. Orbital parameters for both HD 160691 and HD 27442 are listed in Table 3. 4. ORBITAL SOLUTIONS FOR HD 134987, HD 13445, HD 75289, AND HD 17051 Keplerian orbital parameters for four previously announced planet candidates have been conÐrmed by our AAT observations. The high precision of these data will be a powerful tool in the search for additional planetary companions to these stars. 4.1. HD 134987 The planet orbiting HD 134987 (HR 5657, G5 V) was announced from the Keck survey in 1999. As outlined in Vogt et al. (2000), this star is similar to 51 Pegasi in spectral type, enhanced metallicity, and low chromospheric activity. A total of 11 observations of this star have been made by the AAT between 1998 April and 2000 July. These observations are listed in Table 4 and shown graphically in Figure 7. The AAT derived orbital parameters are consistent with Vogt et al. (2000). 4.2. HD 13445 A planet orbiting the nearby star HD 13445 (GL 86, HIP 10138, K1 V) was announced by the CORALIE team in TABLE 4 VELOCITIES FOR HD 134987 JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 917.2282 . . . . . . . . 1,003.0032 . . . . . . 1,213.2775 . . . . . . 1,276.0475 . . . . . . 1,382.9573 . . . . . . 1,413.8811 . . . . . . 1,630.2677 . . . . . . 1,683.0609 . . . . . . 1,706.0960 . . . . . . 1,717.9564 . . . . . . 1,742.9340 . . . . . . 0.0 [14.6 [12.1 [19.2 75.2 30.9 89.3 25.5 [0.6 [4.8 [8.2 5.6 7.2 4.7 6.5 5.4 2.5 4.1 5.0 5.9 4.3 3.5 FIG. 7.ÈDoppler velocities for HD 134987 (G5 V). The solid line is a Keplerian orbital Ðt with a period of 264.6 days, a semiamplitude of 53.7 m s~1, and an eccentricity of 0.37, yielding a minimum (M sin i) of 1.63 M for the companion. The rms of the Keplerian Ðt is 2.7 m s~1. These resultsJ are consistent with Vogt et al. (2000). Vol. 555 1999 (Queloz et al. 2000). In addition to the short-period Keplerian orbit, they report a long-term linear trend of [0.36 m s~1 day~1. The rms to their Keplerian Ðt plus linear trend is 7 m s~1. This star has been observed 23 times as part of the AngloAustralian Survey. These observations are listed in Table 5. The Ðrst observation was made in 1998 January, and the observations span 3 yr. As shown in Figure 8, these observations conÐrm the CORALIE result (Queloz et al. 2000). The rms to our Ðt of a Keplerian plus linear trend is 3.66 m s~1, yielding s2 \ 0.86, slightly better than expected based l on our estimated measurement error. TABLE 5 VELOCITIES FOR HD 13445 JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 831.0350 . . . . . . . . 1,211.9651 . . . . . . 1,213.9815 . . . . . . 1,214.9298 . . . . . . 1,235.9312 . . . . . . 1,236.9078 . . . . . . 1,383.2736 . . . . . . 1,387.3139 . . . . . . 1,411.2467 . . . . . . 1,413.2313 . . . . . . 1,414.3164 . . . . . . 1,473.0974 . . . . . . 1,525.9320 . . . . . . 1,526.9613 . . . . . . 1,743.3292 . . . . . . 1,745.2853 . . . . . . 1,828.1337 . . . . . . 1,829.0121 . . . . . . 1,829.9880 . . . . . . 1,856.1052 . . . . . . 1,918.9660 . . . . . . 1,919.9811 . . . . . . 1,921.0019 . . . . . . 83.5 338.2 398.5 349.9 [277.2 [333.5 0.0 356.6 [406.7 [255.3 [88.1 [403.8 79.6 207.6 [459.4 [204.5 228.2 213.2 150.5 [160.9 [209.3 [41.7 97.0 4.0 5.7 5.3 4.7 5.2 5.5 4.8 4.0 4.9 4.0 4.2 4.3 4.9 4.8 6.5 5.3 5.2 5.4 6.3 6.2 4.6 4.8 5.0 FIG. 8.ÈPhased Doppler velocities for HD 13445 (K1 V). The solid line is a Keplerian orbital Ðt with a period of 15.764 days, a semiamplitude of 379 m s~1, and an eccentricity of 0.046, yielding a minimum (M sin i) of 4.04 M for the companion. The rms of the Keplerian Ðt is 3.7 m s~1. A J linear trend of [108.1 m s~1 yr~1 has been removed from these velocities. These results are consistent with Queloz et al. (2000), with the possible exception of the linear trend. No. 1, 2001 TWO NEW PLANETS FROM PLANET SEARCH 415 TABLE 7 VELOCITIES FOR HD 17051 FIG. 9.ÈPhased Doppler velocities for HD 75289 (G0 V). The solid line is a Keplerian orbital Ðt with a period of 3.508 days, a semiamplitude of 56 m s~1, and an eccentricity of 0.01, yielding a minimum (M sin i) of 0.45 M for the companion. The rms of the Keplerian Ðt is 4.8 m s~1. These resultsJ are consistent with Udry et al. (2000). Our derived orbital parameters agree with the CORALIE results within measurement error, with the possible exception of the linear trend. The simultaneous best Ðt to the AAT velocities gives a linear trend of [0.296 ^ 0.005 m s~1 day~1, about 20% smaller than that, [0.36 m s~1 day~1, found by Queloz et al. (2000) from their CORALIE data. With their lower precision CORAVEL data, they Ðnd a steeper linear trend of [0.5 m s~1 day~1 spanning 1980 to the present. The trend for HD 13445 implies the presence of an additional companion with a period much longer than 10 yr and an amplitude greater than 1 km s~1. It could be a low-mass stellar companion (Queloz et al. 2000). 4.3. HD 75289 The CORALIE team (Udry et al. 2000) have announced a ““ 51 PegÈlike ÏÏ planet orbiting the star HD 75289 (HR 3497, HIP 43177, G0 V). Their observations were carried out between 1998 November and 1999 October and yield an rms to a Keplerian Ðt of 7.5 m s~1. This star has been observed 13 times as part of the Anglo-Australian Planet Search. The Ðrst observation was made in 1998 January, and the observations span 3 yr. These velocities are listed in Table 6. As shown in Figure 9, these observations conÐrm the CORALIE result. The rms velocity residual to our Keplerian Ðt is 4.82 m s~1, yielding s2 \ 1.13. l JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 1,118.1146 . . . . . . 1,235.9437 . . . . . . 1,383.2779 . . . . . . 1,413.2245 . . . . . . 1,473.1000 . . . . . . 1,525.9348 . . . . . . 1,526.9763 . . . . . . 1,743.3364 . . . . . . 1,745.2936 . . . . . . 1,828.1295 . . . . . . 1,856.1093 . . . . . . 1,856.9355 . . . . . . 1,918.9703 . . . . . . [55.1 39.5 [71.5 [72.3 0.0 47.9 76.1 [32.3 [50.1 66.7 62.5 44.5 [2.7 4.4 5.0 7.4 6.2 4.8 4.6 5.9 5.9 5.0 5.6 5.3 8.6 5.4 4.4. HD 17051 In 1998 June the ESO Precise Radial Velocity Survey announced a planet orbiting HD 17051 (• Hor, HR 810, HIP 12653, G0 V) with a 600 day orbit (Kurster et al. 1999 ; see also Glanz 1998). On the basis of the same data, they later announced the planet had a period of 320 days (Kurster et al. 2000). Their data set consists of 95 measurements taken between late 1992 and early 1998. They report a Keplerian semiamplitude of 67 m s~1. The rms velocity residual to their Keplerian Ðt is 27 m s~1, while they estimate their measurement error to be 17 m s~1. They attribute the di†erence to stellar activity. A stellar Doppler ““ jitter ÏÏ of 20 m s~1 would account for the di†erence between the internal measurement error and the observed scatter to a Keplerian Ðt. The Anglo-Australian Planet Search began observing HD 17051 in 1998 November. A total of 13 observations TABLE 6 VELOCITIES FOR HD 75289 JD ([2,450,000) Radial Velocity (m s ~1) Error (m s~1) 830.1656 . . . . . . . . 914.9334 . . . . . . . . 1,212.1495 . . . . . . 1,213.1426 . . . . . . 1,214.2518 . . . . . . 1,236.9418 . . . . . . 1,274.0100 . . . . . . 1,275.9947 . . . . . . 1,631.0085 . . . . . . 1,717.9152 . . . . . . 1,856.2491 . . . . . . 1,919.1969 . . . . . . 1,920.1472 . . . . . . [0.8 [39.8 40.3 [39.9 33.8 15.6 31.7 [15.2 [25.4 [7.1 27.1 0.0 77.9 4.6 4.3 6.1 5.1 6.3 5.8 8.2 4.1 4.9 5.5 6.3 5.4 5.2 FIG. 10.ÈDoppler velocities for HD 17051 (G0 V). The solid line is a Keplerian orbital Ðt with a period of 312 days, a semiamplitude of 63 m s~1, and an eccentricity of 0.15, yielding a minimum (M sin i) mass of 2.13 M for the companion. The rms of the Keplerian Ðt, 10.4 m s~1, is twice the J internal measurement error, consistent with the observed chromospheric activity and youth of this star. These results are consistent with Kurster et al. (2000). 416 BUTLER ET AL. Vol. 555 TABLE 8 ORBITAL PARAMETERS Parameter HD 134987 HD 13445a HD 75289 HD 17051 Orbital period P (days) . . . . . . . . . . . . . Velocity amplitude K (m s~1) . . . . . . Eccentricity e . . . . . . . . . . . . . . . . . . . . . . . . u (deg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Periastron time (JD) . . . . . . . . . . . . . . . . M (M )b . . . . . . . . . . . . . . . . . . . . . . . . . . Star _ M sin i (M ) . . . . . . . . . . . . . . . . . . . . . . . . . . J a (AU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . rms (m s~1) . . . . . . . . . . . . . . . . . . . . . . . . . . 264.6 (5) 53.7 (4) 0.37 (0.12) 345 (10) 2,451,628.8 (4) 1.05 1.63 0.82 2.73 15.764 (0.005) 379 (1) 0.046 (0.002) 260 (3) 2,451,225.1 (0.2) 0.8 4.04 0.114 3.66 3.508 (0.001) 56 (1) 0.014 (0.005) 0 2,451,214.5 (0.001) 1.15 0.46 0.047 4.82 312 (5) 63 (4) 0.15 (0.05) 309 (5) 2,451,492.8 (4) 1.03 2.13 0.91 10.4 a Additional slope is [108.1 ^ 2 m s~1 yr~1. b Stellar masses from discovery papers. have been made through 2001 January, as listed in Table 7 and shown in Figure 10. The solid line in Figure 10 is the best-Ðt Keplerian to the measured Doppler velocities. Within measurement error, we conÐrm the orbital parameters of Kurster et al. (2000). The rms to the Keplerian Ðt to the AAT data is 10.4 m s~1, about twice the estimated measurement error. As Kurster et al. (2000) note, HD 17051 is a young, chromospherically active G0 V star, consistent with the measured log R@(HK) \ [4.65 (Henry et al. 1996), and a rotation period of 8 days (Saar & Osten 1997 ; Saar et al. 1997). Saar et al. (1998) Ðnd the Doppler ““ jitter ÏÏ corresponding to this level of activity to be 10 m s~1, consistent with the rms to the Keplerian Ðt to the AAT data. Table 8 lists the orbital parameters for these four stars based on the AAT measurements. 5. DISCUSSION The two newly detected planets announced here both orbit beyond 1 AU. HD 160691 is typical of the examples of such planets discovered to date, moving on a highly elliptical orbit. In contrast, HD 27442 is in an Earth-like orbit, with a semimajor axis of 1.16 AU and an eccentricity of 0.058 ^ 0.05 (consistent with circular). This is the Ðrst planet to be discovered orbiting beyond 0.15 AU in a circular orbit like the planets of our solar system. The AAT data provide the Ðrst conÐrmation of orbital parameters of four planets recently announced from the Keck, CORALIE, and ESO planet searches. In addition, the AAT data appear to rule out the claimed planet around HD 196378. Two broad strategies are being pursued by the various groups carrying out precision velocity surveys. Several groups are carrying out surveys with precision of 10È20 m s~1. There are several advantages to this strategy. Relatively low S/N (D70) spectra, or small wavelength coverage (D50 AŽ ), are required to achieve this level of precision, and data reduction strategies are simpliÐed. A precision of 10 m s~1 allows the reliable detection of planets that induce amplitudes of 30 m s~1 or larger. Since only these largeamplitude signals are detectable, chromospherically active stars with associated Doppler ““ jitter ÏÏ of 10È20 m s~1, such as HD 17051, remain viable candidates. The other strategy is to pursue much higher precision, 3 m s~1 or better. This strategy carries several penalties, including the need for large wavelength coverage (Z1000 AŽ ), high S/N (Z200), and complex data analysis. The payo† for such a strategy is the ability to detect lower mass planets in short-period orbits and Jupiter-like planets in distant ([4 AU) orbits. To illustrate this, consider the problem of detecting a Jupiter analog. Jupiter induces a Doppler velocity variation in the Sun with an amplitude of 12.5 m s~1. However, the mean expectation value for sin i of n/4 reduces this to a typical amplitude of 10 m s~1 in a Doppler velocity survey. The top panel of Figure 11 shows synthetic observations of a Jupiter analog with a measurement uncertainty of 5 m s~1. The solid line is a best-Ðt Keplerian to this data. The result is an unconvincing 2 p detection with no constraint on orbital eccentricity. The lower panel shows the same situation but for 2 m s~1 measurement uncertainties. In this case a solid 5 p detection is made, and the eccentricity is determined to within ^0.05. The eccentricity of a 3 p detection is poorly constrained to within ^0.2. Without knowledge of the orbital eccentricity, it is not possible to categorize a Jupiter-mass companion at 5 AU as a solar system analog. True solar system analogs must reside in circular orbits. The Anglo-Australian Planet Search has been surveying the 200 brightest dwarf and subgiant stars ranging in spectral type from late F to early M and south of declination [20¡ since 1998 January. Long-term photon-limited precision of 3 m s~1 has been achieved, unique among the southern hemisphere precision velocity surveys. The long- FIG. 11.ÈSimulated Jupiter signal observed with a precision of 5 and 2 m s~1. Solid lines are best-Ðt Keplerians to the simulated data sets. With measurement precision of 5 m s~1, an unreliable 2 p detection is obtained with no information on the orbital eccentricity. With precision of 2 m s~1, a solid 5 p detection is made, and the eccentricity is determined to within ^0.05. No. 1, 2001 TWO NEW PLANETS FROM PLANET SEARCH term goal of this project is to maintain and improve this precision for another decade to allow for the detection of true solar system analogs, Jupiter-mass planets orbiting beyond 4 AU. We gratefully acknowledge the support and encouragement of the director of the Anglo-Australian Observatory, Brian Boyle. The superb technical support at the Anglo-Australian Telescope has been critical to the success of this projectÈin particular we acknowledge E. Penny, 417 R. Paterson, D. Sta†ord, F. Freeman, S. Lee, J. Pogson, and G. Scha†er. We acknowledge support by NSF grant AST 99-88087 and travel support from the Carnegie Institution of Washington (to R. P. B.), by NASA grant NAG5-8299 and NSF grant AST 95-20443 (to G. W. M.), and by Sun Microsystems. We thank the Australian and UK Telescope assignment committees (ATAC and PATT) for allocations of telescope time. This research has made use of the SIMBAD database, operated at the Centre de Donnees de Strasbourg, France. REFERENCES Kurster, M., Endl, M., Els, S., Hatzes, A. P., Cochran, W. D., Dobereiner, Bedding, T. R., et al. 2001, ApJ, 549, L105 S., & Dennerl, K. 2000, A&A, 353, L33 Butler, R. P., & Marcy, G. W. 1997, in ASP Conf. Ser. 134, Brown Dwarfs Kurster, M., Hatzes, A. P., Cochran, W. D., Dennerl, K. Dobereiner, S., & and Extrasolar Planets, ed. R. Rebolo, E. L. Martin, & M. R. Zapatero Endl, M. 1999, in ASP Conf. Ser. 185, Precise Stellar Radial Velocities, Osorio (San Francisco : ASP), 162 ed. J. B. Hearnshaw & C. D. Scarfe (San Francisco : ASP), 154 Butler, R. P., Marcy, G. W., Fischer, D. A., Vogt, S. S., Tinney, C. G., Jones, Marcy, G. W., & Butler, R. P. 1992, PASP, 104, 270 H. R. A., Penny, A. J., & Apps, K. 2001, in ASP Conf. Ser., Planetary ÈÈÈ. 1998, ARA&A, 36, 57 Systems in the UniverseÈObservation, Formation and Evolution, ed. ÈÈÈ. 2000, PASP, 112, 137 A. Penny, P. Artymowicz, A. M. Lagrange, & S. Russell (San Francisco : Marcy, G. W., Butler, R. P., & Vogt, S. S. 2000, ApJ, 536, L43 ASP), in press Mayor, M., & Queloz, D. 1995, Nature, 378, 355 Butler, R. P., Marcy, G. W., Williams, E., McCarthy, C., Dosanjh, P., & Noyes, R. W., Jha, S., Korzennik, S. G., Krockenberger, M., Nisenson, P., Vogt, S. S. 1996, PASP, 108, 500 Brown, T. M., Kennelly, E. J., & Horner, S. D. 1997, ApJ, 483, L111 Campbell, B., Walker, G. A. H., & Yang, S. 1988, ApJ, 331, 902 Perryman, M. A. C., et al. 1997, A&A, 323, L49 Cochran, W. D., Hatzes, A. P., Butler, R. P., & Marcy, G. W. 1997, ApJ, Queloz, D., et al. 2000, A&A, 354, 99 483, 457 Randich, S., Gratton, R., Pallavicini, R., Pasquini, L., & Carretta, E. 1999, Cumming, A., Marcy, G. W., & Butler, R. P. 1999, ApJ, 526, 890 A&A, 348, 487 Diego, F., Charalambous, A., Fish, A. C., & Walker, D. D. 1990, Proc. Saar, S. H., Butler, R. P., & Marcy, G. W. 1998, ApJ, 498, L153 SPIE, 1235, 562 Saar, S. H., Huovelin, R. A., Osten, R. A., & Shcherbakov, A. G. 1997, Eggen, O. J. 1993, AJ, 106, 80 A&A, 326, 741 Elgaroy, O., Engvold, O., & Lund, N. 1999, A&A, 343, 222 Saar, S. H., & Osten, R. A. 1997, MNRAS, 284, 803 Favata, F., Micela, G., & Sciortino, S. 1996, A&A, 311, 951 Tinney, C. G., Butler, R. P., Marcy, G. W., Jones, H. R. A., Penny, A. J., ÈÈÈ. 1997, A&A, 323, 809 Vogt, S. S., Apps, K., & Henry, G. W. 2001, ApJ, 551, 507 Fischer, D. A., Marcy, G. W., Butler, R. P., Vogt, S. S., Frink, S., & Apps, Udry, S., et al. 2000, A&A, 356, 590 K. 2001, ApJ, 551, 1107 Valenti, J., Butler, R. P., & Marcy, G. W. 1995, PASP, 107, 966 Glanz, J. 1998, Science, 280, 2037 Vogt, S. S., Marcy, G. W., Butler, R. P., & Apps, K. 2000, ApJ, 536, 902 Henry, T. J., Soderblom, D. R., Donahue, R. A., & Baliunas, S. L. 1996, AJ, Walker, G. A. H., Walker, A. R., Irwin, A. W., Larson, A. M., Yang, S. L. S., 111, 439 & Richardson, D. C. 1995, Icarus, 116, 359 Hoffleit, D., & Jaschek, C. 1982, Yale Bright Star Catalog (4th ed. ; New Haven : Yale Univ. Press)