arXiv:astro-ph/0202457v2 26 Mar 2002 eeteouinmcaim.Temnmmms fthe from of ranges dif- companions to minimum planetary related detected The be mechanisms. might which evolution pe- 2001) orbital ferent Mayor and & eccentricity dis- distribution (Udry as riod been widely-spread such parameters have a orbital show candidates of orbits planetary Their 80 covered. about date To Introduction 1. fteobtlpae nyamnmmmass minimum sin a projection only of plane, angle orbital the technique the constrain of to this us Because allow parameters. not provides does orbital It most measurements. with radial-velocity us precise of that M 0.53 only of mass a oee ymaso the of of means Four by planet, covered lightest Saturn. The of 2001). al. mass (Jorissen et the sub-saturnian have half candidates known about the to down masses pcrgaho h .- ue ws eecp tL Silla La at telescope Swiss Euler Chile 1.2-m ESO Observatory, the on spectrograph [email protected] inidcdb lntr opno nisprn parent its on companion planetary varia- a radial-velocity by induced The tion determined. be can companion edopitrqet to requests offprint Send ⋆ DI ilb netdb adlater) hand by inserted be will (DOI: Astrophysics & Astronomy h ehiu hc nele l hs icvre is discoveries these all underlies which technique The ae nosrain olce ihteCRLEechelle CORALIE the with collected observations on Based w hr-eidStrincmain oH 017adH 16 HD and 108147 HD to companions Saturnian short-period Two h OAI uvyfrsuhr xr-oa lnt VII. planets extra-solar southern for survey CORALIE The Abstract. Accepted / Received Sau CH–1290 Maillettes, des ch. Gen`eve, 51 de Observatoire e words. Key by produced effects o perturbation modification residual a well the as significantly wh discuss We technique, velocity. cross-correlation radial weighted computed the paper this in ntuetlacrc fCRLEcmie ihtesimulta s m the 3 with me than combined by better discovered CORALIE level were of Doppler candidates accuracy the new instrumental of two means The by precision. planets ment Saturn-mass companion. of second detection a The of presence the possibly lntr systems planetary n t iiu asi .4ta fStr.Isobti chara is orbit Its Saturn. compani of The that days. 1.34 6.4 is is of mass period mass minimum orbital its minimum its and The and planets. Saturn discovered of mass ever lightest the to .Pp,M ao,F aln,D af .Qeo,NC Santos N.C. Queloz, D. Naef, D. Galland, F. Mayor, M. Pepe, F. epeettedsoeyo w aunms opnost HD to companions Saturn-mass two of discovery the present We ehius ailvlcte tr:idvda:H 1081 HD individual: : – velocities radial techniques: Sat CORALIE Myre l 2000). al. et (Mayor − 1 aucitno. manuscript nmn bevdojcstepeiini o iie ypho by limited now is precision the objects observed many On . .Pp,e-mail: Pepe, F. : HD pcrgahadhas and spectrograph 34 ,hsbe dis- been has c, 83443 m ∼ 2 sin 0Jupiter 10 i fthe of i slrei t iiu mass minimum its if large is tbeisrmnainadb ffiin esrmn and techniques. measurement reduction efficient data by and the and efficient bias instrumentation by this improved stable reduce be to must order precision In Mayor measurement 2001). & Udry al. on (e.g. et function bias Jorissen mass 2001; detection the a of end produces low-mass and the difficult low-mass more of detection high-mass is the planets to hand, other sensitive the On particularly companions. technique the makes see etr aeyaot1ms m 1 about precision namely term better, short even the that is proven have 2001) Carrier & rm(ulze l 00 dye l 00.Tgte with Together 2000). al. et pro- Udry ELODIE 2000; radial-velocity al. high-precision et (Queloz since large gram out, carrying a are 1998, we Silla summer La at telescope Swiss Euler ta.20a vrtm clslne hn3yas h re- The . 3 than on longer measurements scales asteroseismology time cent over 2001a) al. et h iiain r o eemndmil yphoton by influence. mainly astroclimatic determined residual now and noise are limitations The ftedt euto otaehv otiue oobtain to contributed improvement have overall software Recent an reduction 2001). data the Mayor known of the & of (Udry half about candidates of discovery the lowed c ed oa mrvmn ntepoo os fthe of noise photon the in improvement an to leads ich elrcasrto lines. absorption telluric h ueia rs-orlto akwihreduces which mask cross-correlation numerical the f en,Switzerland verny, sn the Using nt D184 risisprn tri 09days 10.9 eccentricity, in high star a parent by its cterized orbits 108147 HD to on ehiu ead ihrda-eoiymeasure- radial-velocity high demands technique h opno oH 676i fol .7the 0.77 only of is 168746 HD to companion the n fteCRLEehlesetorp.The spectrograph. echelle CORALIE the of ans eu hrrfrnetcnqehsrahda reached has technique ThAr-reference neous ntenrhr hemisphere, northern the in instrumental 7–sas niiul D184 stars: – 168746 HD individual: stars: – 47 CORALIE 017adH 676 ohbelong Both 168746. HD and 108147 .Ur,adM Burnet M. and Udry, S. , rcso below precision cel pcrgaho h 1.2-m the on spectrograph echelle o os.W present We noise. ton m 2 e sin 0 = − 1 i . 0 indicating 50, .. vr1night. 1 over r.m.s slre hsfact This large. is ∼ CORALIE α s m 3 coe 1 2018 31, October e (Bouchy A Cen 8746 − 1 (Queloz a al- has ⋆ 2 F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII.

The present paper describes the discovery of two extract the radial-velocity information content in an op- Saturn-mass companions to the stars HD 108147 and timized way. Deep and sharp lines, for example, are not HD 168746. HD 168746 b, with its of only weighted sufficiently, although they contain more radial- 0.77 MSat, is one of the four sub-saturnian planets discov- velocity information than broad and weak lines. The chal- ered to date. On the other hand, HD 108147 b distinguishes lenge was therefore to combine the simplicity and robust- itself by a high eccentricity, fueling the discussion of the ness of the cross correlation with a more efficient informa- origin of such high eccentricity in the case of short-period tion extraction. companions. Before the orbital parameters and the stel- The solution presented here assumes that the absorp- lar characteristics of these two objects described in detail tion lines of the stellar spectrum used to compute the ra- in the second part of the paper, we discuss additional im- dial velocity have all about the same FWHM, which is provements made recently in the data reduction, and more a good approximation since broad lines are not included precisely, in the way of extracting the radial-velocity infor- in our standard numerical mask. The lines have different mation obtained from the high-resolution spectra recorded relative depths which are averaged in the resulting CCF. with CORALIE. Lines with large relative depth contain intrisically more radial-velocity information than weak lines, however. Our goal is therefore to build up a weighted cross-correlation 2. Improving the radial-velocity precision on function CCF w which accounts for the correct weight of CORALIE data each spectral line contained in the mask:

2.1. The weighted cross-correlation w CCF (vR) = CCFi(vR) wi (5) i · The technique of the numerical cross correlation used to X determine the precise is described in detail In order to understand the origin of wi the following ex- in Baranne et al. (1996) and will not be discussed here. We ample shall be presented: The contribution to the global only recall that the measured spectrum is correlated with a CCF of a spectral line l with relative depth cl = 0.5 numerical mask consisting of 1 and 0 value-zones, with the and a continuum level of Sl = I0 is exactly the same non-zero zones corresponding to the theoretical positions as for a line m with relative depth cm = 1 but a con- and widths of the stellar absorption lines at zero velocity. tinuum level at Sm = I0/2. The cross-correlation signal The cross-correlation function (CCF) is constructed by CCFi ci Si is indeed of equal amplitude in both cases, ∝ · shifting the mask as a function of the Doppler velocity: namely CCFl = CCFm I0/2. The noise on each point of the cross-correlation function∝ is however proportional CCF (vR) = S(λ) M(λvR ) dλ (1) to the square root of the real spectral signal Si, and thus · Z √2 times larger in the CCFl. If we would fit each cross-

= S(λ) Mi(λvR ) dλ (2) correlation function CCFi independently we would get an · i precision on the fitted position which is √2 larger for the Z X line m. Thus, the weight of the line l should be greater. = S(λ)Mi(λvR ) dλ = CCFi(vR), (3) In general, for a given amplitude of the cross- i i X Z X correlation signal CCFi, the noise on each point of the where CCF – and thus on the resulting Gaussian fit – is σ2 v i R CCFi 1 ∝ 1 c Si . The weight we have to give to the single v − ci ci λ R = λ vR . (4) ∝ ∝ 1 s 1+ c CCFi must therefore be equal to wi = 2 = ci, where ci is σi the relative depth of each spectral line i. Eq. (5) becomes In this equation S(λ) is the recorded spectrum, and then: M(λvR ) represents the Doppler-shifted numerical mask w which can be expressed as the sum of masks Mi each CCF (vR) = CCFi(vR) ci (6) corresponding to a stellar absorption line i. The result- i · X ing CCF (vR) is a function describing somehow a flux- weighted “mean” profile of the stellar absorption lines = I(λ) (Mi(λvR ) ci) dλ . (7) · i · transmitted by the mask. For the radial velocity value of Z X the star, we take the minimum of the CCF, fitted with a By means of a synthetic K0 dwarf spectrum we have com- Gaussian function. puted the relative depth ci (and thus the weight) for each The cross-correlation technique has proven to be very absorption line contained in the numerical mask. We have robust and simple, and to deliver excellent results. The modified the reduction software to take into account the great advantage of the cross-correlation with a numeri- relative weight of each line when computing the CCF. cal mask is that it does not need any high signal-to-noise Finally, we have compared the old and the new code by reference spectra to compute the precise radial velocity. numerical simulation on a set of 100 spectra issued from However, it has been shown in the past (Bouchy et al. a high signal-to-noise spectrum on which statistical noise 2001; Chelli 2000) that in terms of photon noise the tech- has been added. The result was a reduction of 1.25 of the nique could still be improved. In fact, the CCF does not measured radial velocity dispersion which is equivalent to F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII. 3 an hypothetical increase in signal-to-noise on the spec- 8 trum of the same value. We have also applied the old and 6 the new algorithm on the radial-velocity data of HD 108147 and HD 168756. The results of this comparison will be pre- 4 sented below. 2

2.2. Telluric lines: reduced contamination effects 0

Telluric absorption lines superimposed to the stellar spec- −2 trum form absorption bands, especially in the red-visible −4 wavelength region. The position of the telluric lines rela- −6 tive to the stellar lines changes as a function of the relative Error on stellar radial velocity [m/s] radial velocity between and star, while the strength −8 of the telluric lines depends strongly on the astroclimatic conditions. When cross-correlating the recorded spectrum −10 −30 −20 −10 0 10 20 30 with the numerical mask a parasitic signal is produced by Earth radial velocity [km/s] the casual correlation of a line in the mask with a telluric line in the spectrum. This might introduce an asymmetry Fig. 1. The plot shows the error on the measured radial in the CCF and consequently an error on the measured velocity produced by telluric lines as a function of Earth stellar radial velocity. In principle, the cross-correlation radial velocity. For this purpose a real stellar spectrum has mask used in the ELODIE and CORALIE data reduction been Doppler-shifted by a radial-velocity value ranging from 30kms−1 to +30kms−1 and superimposed with a has been “cleaned” from telluric lines: all stellar lines for − which the wavelength lies close (at a couple of line widths) zero-velocity telluric absorption spectrum. The obtained to that of a telluric line have been removed from the nu- radial velocity has then been compared to the star velocity merical mask. Because of the quite large annual Earth mo- obtained on the original stellar spectrum, the error being tion, however, the telluric lines change their relative posi- the difference between them. The solid line shows the re- tion with regard to the stellar lines by about 30kms−1 sults using the original numerical mask while the dashed ( 0.6 A˚ at 6000 A).˚ These means that every± stellar line line represents the results with the new clean mask lying± within 30kms−1 from a telluric line is potentially ± affected by a telluric line more or less strongly in some 3. A Saturn-mass planet in eccentric orbit around period of the . The superposition of these effects can HD 108147 then result in an increased “noise” or even in an erroneous periodic signal on the stellar velocity with a period of one 3.1. Stellar characteristics of HD 108147 year. Fig. 1 shows simulations made on real ELODIE spec- HD 108147 (HIP 60644) is a F8/G0 dwarf in the tra. . Its magnitude is V = 6.99 while the In order to reduce this effect we have cleaned the origi- HIPPARCOS catalogue (ESA 1997) lists a nal mask carefully from all stellar lines potentially affected B V = 0.537. The precise astrometric parallax is π = by telluric lines. Although the photon-noise was slightly 25.−95 0.69mas corresponding to a distance of about increased – since some regions of the stellar spectra were 38.57 pc± from the . The derived , not used anymore in the cross correlation – quite impres- MV = 4.06, is typical for a G0 dwarf. sive results have been obtained. In our simulations and as Stellar parameters such as effective temperature shown by the dashed curved in Fig. 1, the error produced Teff = 6265K, log g = 4.59, as well as by the telluric lines has been reduced at least by a factor [Fe/H] = +0.2 have been derived in the detailed LTE spec- of 3. troscopic analysis carried out by Santos et al. (2001). The Although in case of the CORALIE instrument the as- obtained is slightly higher than the average troclimatic conditions are in average more convenient than value for stars of the CORALIE sample, like most of the on ELODIE, and therefore the effect of the telluric lines stars with giant planets, and is very close to the mean is expected to be less important, we have decided to ap- value of [Fe/H] of stars with planets (Santos et al. 2001). ply the new numerical mask on the radial velocity data Using the evolutionary tracks of the Geneva models given of HD 108147 and HD 168756 presented in this paper. The by Schaerer et al. (1993), Santos et al. (2001) compute a corresponding results are discussed below. M =1.27 M⊙. The stellar mass is higher than Finally, we should also mention that the improvement the “typical” mass of G0 dwarfs and can be explained by of the mask presented in this section does not apply only the high metallicity of the star. to newly acquired data. It will allow us to re-correlate Fig. 2 shows the Ca ii H absorption line region of the any spectrum obtained with ELODIE or CORALIE until CORALIE spectrum at λ3968.5 A.˚ The emission flux in the present and improve the existing radial-velocity data the core of the Ca ii H line corrected for the photospheric base. flux provides us with the chromospheric activity index 4 F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII.

Table 1. Observed and inferred stellar parameters for HD 108147 and HD 168746. Photometric and astrometric data have been extracted from the HIPPARCOS catalogue (ESA 1997) while spectroscopic data are from Santos et al. (2001)

Parameter HD 108147 HD 168746 Spectral Type F8/G0 G5 V 6.99 7.95 B − V 0.537 0.713 π [mas] 25.95 ± 0.69 23.19 ± 0.96 Distance [pc] 38.57 ± 1 43.12 ± 1.8 MV 4.06 4.78 L/L⊙ 1.93 1.10 [Fe/H] 0.2 ± 0.06 −0.06 ± 0.05 M/M⊙ 1.27 ± 0.02 0.88 ± 0.01 Teff [K] 6265 ± 40 5610 ± 30 log g [cgs] 4.59 ± 0.15 4.50 ± 0.15 v sin i [km s−1] 5.3 1.0 ′ log(RHK) −4.72 – ′ Prot(RHK)[days] 8.7 – ′ age(RHK) [Gyr] 2.17 –/old star

Fig. 2. λ3968.5 A˚ Ca ii H absorption line region of the summed CORALIE spectra. Upper panel: HD 108147. presence of a planetary companion. The measured varia- Lower Panel: HD 168746. Both objects show low chromo- tion cannot be produced by stellar activity: The Geneva spheric activity. Strong activity would result in an emis- photometry data show very low dispersion of 1 mmag and sion peak in the center of the absorption line no CCF-bisector variation (Queloz et al. 2001b) has been measured at the ms−1 level. Therefore the planetary ex- planation seems to be the most likely. S (Santos et al. 2000) from which we derive the activ- Cor The best-fit Keplerian orbit to the data is shown ity indicator log (R′ )= 4.72. This value is typical for HK in Fig. 3. It yields a precisely-determined orbital pe- stars with a low chromospheric− activity level (Henry et al. riod P of 10.901 0.001 days and a large eccentricity 1996). Using the calibration given by Donahue (1993) and e =0.50 0.03. The± semi-amplitude of the radial-velocity quoted in Henry et al. (1996) we compute for this star an variation± is K = 36 1ms−1. The weighted r.m.s. of the age of approximately 2 Gyrs, while the rotational period data to the Keplerian± fit is 9.2 m s−1. The complete set resulting from the calibration given by Noyes et al. (1984) of orbital elements with their uncertainties are given in is of 8.7 days. The star is not seen as photometrically vari- Table 2. able in the HIPPARCOS data, confirming again the low activity level. We have estimated the projected rotational Using the best-fit orbital parameters and the mass of − min- velocity of the star to be v sin i = 5.3 kms 1 by means HD 108147 given above we derive for the companion a imum mass m sin i = 0.40 M (which is about only of the CORALIE cross-correlation function (CCF) (Queloz 2 Jup et al. 1998). The relatively high could 1.34 times the mass of Saturn). Because of the many data cause a small jitter on the radial-velocity data, which are points and the long observation period the orbit is deter- however expected to be in the order of only few ms−1 mined very accurately. This allows us to determine the (Saar & Donahue 1997; Santos et al. 2001). The observed minimum mass of the companion with accuracy of bet- and inferred stellar parameters are summarized in Table 1. ter than 4%, provided that we do not consider the major error source, namely the uncertainty on the mass of the primary. From the orbital parameters and the star mass 3.2. CORALIE orbital solution for HD 108147 we get also the separation of the companion to its parent star which is a =0.104 AU. The surface equilibrium tem- The precise radial velocity data of HD 108147 have perature of the planet at such a distance is estimated to been collected by our group during the period of time be about 890 K, following Guillot et al. (1996). from March 1999 to February 20021. The result of this HD 108147b belongs to the so-called hot (or campaign is a set of 118 data points having a mean better: hot Saturn!) category of extra-solar planets. The photon-noise error on the individual measurements of −1 close location to its parent star makes the planet a good εi =7.8 m s . A periodic variation of the radial velocity candidate for a photometric transit search. The photomet- couldh i be detected on this data, which clearly indicates the ric monitoring described in a forthcoming paper (Olsen et 1 The discovery was announced in May 2000, see al., in prep.) did unfortunately not show any indication www.eso.org/outreach/press-rel/pr-2000/pr-13-00.html for a transit. F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII. 5

Table 3. Comparison of the different data reductions on HD 108147. The r.m.s of the data to the Keplerian fit is reduced from 11.9 to 9.2ms−1 if the weighted cross cor- relation and the clean numerical mask are used

Cross correlation Obtained r.m.s [m s−1] Standard 11.9 Clean mask only 10.7 Weighted only 10.1 Weighted & clean mask 9.2

tating. Closer analysis of the residuals (Fig. 3, bottom) might indicate however the presence of a possible second companion with an P of 587 34 days and ± a minimum mass of m2 sin i =0.42 MJup. A best-fit solu- tion considering a second companion reduces the r.m.s to the Keplerian to 8.0 m s−1 while the reduced χ passes from 1.440 to 1.290. In order to confirm or reject this possibility we will continue to collect additional high-precision data during the next observational seasons. We plan to moni- tor the radial velocity of this object using the CORALIE Fig. 3. Phase-folded radial-velocity measurements ob- spectrograph, but also taking advantage of the superior 2 tained with CORALIE for HD 108147. The error bars rep- performances of the HARPS spectrograph (Pepe et al. resent photon-noise errors only. On the lower panel the 2000) which is expected to reach a radial-velocity preci- −1 residuals of the measured radial velocities to the fitted or- sion of 1 m s . bit are plotted as a function of time. They show a tiny indication for the presence of a long-period, second com- 3.4. Results of the improved cross correlation panion of HD 108147 As mentioned previously, the radial velocity data have Table 2. CORALIE best Keplerian orbital solutions de- been computed by using the weighted cross correlation rived for HD 108147 and HD 168746, as well as inferred and the new numerical mask. For comparison we have planetary parameters nevertheless computed the radial velocity data also us- ing the standard on-line data reduction. The results are Parameter HD 108147 HD 168746 summarized in Table 3. P [days] 10.901 ± 0.001 6.403 ± 0.001 In the error budget of our best single-orbit solution T [JD] 2451591.6 ± 0.1 2451994.7 ± 0.4 about 6.6 m s−1 arise from sources other that photon ± ± e 0.498 0.025 0.081 0.029 noise. The r.m.s obtained with the standard algorithm −1 ± ± γ [km s ] 5.026 0.001 -25.562 0.001 was 11.9 m s−1 and with the weighted cross correlation ω [deg] 318.95 ± 3.03 16.30 ± 20.88 10.1 m s−1. Considered this, the weighted cross correlation K [m s−1] 36 ± 1 27 ± 1 reduced the radial velocity dispersion arising from photon Nmeas 118 154 σ(O-C) [m s−1] 9.2 9.8 noise by a factor 1.31, confirming the simulation results. −1 a1 sin i [Mm] 4.634 2.364 On the other hand, the r.m.s of 11.9 m s obtained with −9 −9 −1 f(m) [M⊙] 3.337·10 1.284·10 the original mask passes to 10.7 m s with the new clean m1 [M⊙] 1.27 0.88 mask. Thus, we estimate that the telluric lines add a dis- −1 m2 sin i [MJup] 0.40 0.23 persion on the stellar radial velocity of roughly 5 m s . a [AU] 0.104 0.065 Teq [K] 890 900 3.5. The eccentricity of the orbit around HD 108147 Considered the short orbital period, the eccentricity of the 3.3. A planetary system around HD 108147 ? orbit around HD 108147 is surprisingly high. According to the commonly believed migration scenario for the forma- The weighted r.m.s. of the data to the Keplerian fit is tion of hot , the giant planets like HD 108147 can- 9.2 m s−1 and the reduced χ is 1.440, indicating an inter- not be formed in situ at a distance of only 0.1 AU from nal error for the single measurements of 6.4ms−1. The the parent star. On the other hand the orbit of planets residual dispersion is 6.6 m s−1 and is only partially com- having suffered a strong migration are expected to be cir- posed of instrumental errors, which are in the order of −1 −1 3ms (Queloz et al. 2001a). The remaining 6 m s could 2 HARPS will be installed on the ESO 3.6-m telescope and be due to stellar jitter, since the star is fairly rapidly ro- become operational at the beginning of 2003 6 F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII. cularized by the tidal interaction with the accretion disk (Goldreich & Tremaine 1980). Another mechanism must therefore be at the origin of its high eccentricity. One suit- able explanation is provided by the gravitational interac- tion between multiple giant planets in the early stages of the system formation (Weidenschilling & Marzari 1996; Rasio & Ford 1996; Lin & Ida 1997). As a result one planet could have been projected on a short-period orbit with a high eccentricity, leaving different possibilities concerning the destiny of its scattering partner. Eccentric orbits can however also be produced by additional companions on longer-period orbits, which perturb the short-period or- bit of the inner companion by tidal forces (Mazeh et al. 1997). The companion can be either planetary or stellar and would be seen in many cases only as a slow drift of the gamma-velocity γ. Finally, additional scenarios to explain high eccentricities were proposed recently: For example the migration of a single Jupiter-mass planet through a planetesimal disk (Levison et al. 1998), or, the simulta- neous formation and migration of two or more planets through the accretion disk, e.g locked in a resonant sys- tem (Murray et al. 2002). The numerical simulations in Fig. 4. Phase-folded radial-velocity measurements ob- the latter paper show that a large variety of possible re- tained with CORALIE for HD 168746. The error bars rep- sults can be obtained, depending on the initial planetary resent photon-noise errors only. The lower panel shows the mass(es), disk properties and time scales. residuals of the measured radial velocities to the fitted or- In summary, various mechanisms can explain short- bit as a function of time period eccentric orbits. In almost all of them interaction with planetary or stellar companions plays a fundamental role, and the large eccentricity of the orbit might be an 4.2. CORALIE orbital solution for HD 168746 indication for their presence. Therefore it is very likely to During the period spanning from May 1999 to September find a second, long-period companion around HD 108147. 20013 we have recorded 154 precise radial-velocity data As mentioned above the long-term follow up of this object points of HD 168746. Because of the magnitude of the star might confirm or reject this possibility. the mean photon-noise error on the individual measure- −1 ments is relatively high, namely of εi =9.7 m s . Due 4. A Sub-Saturn mass planet around HD 168746 to the large amount of data we couldh neverthelessi iden- 4.1. Stellar characteristics of HD 168746 tify easily the presence of a companion with a very low minimum mass. Because of the photometric stability of HD 168746 (HIP 90004) is a G5 dwarf of magnitude HD 168746 and the absence of line-bisector variations we V = 7.95 located at the boundary between the Scutum and can exclude that the measured radial-velocity variation is the Cauda . The HIPPARCOS cat- produced by stellar activity. alogue lists a color index B V = 0.713. The catalogue Fig. 4 shows the best fitted Keplerian orbit for the − indicates also an astrometric parallax of π = 23.19mas companion of HD 168746. We deduce an orbital period P (ESA 1997) corresponding to a distance of about 38.57pc of 6.403 0.001days and a rather small eccentricity of between the star and the Sun. The resulting absolute mag- e =0.08 ±0.03. The semi-amplitude of the radial-velocity ± − nitude is MV = 4.78. variation is only K = 27 1ms 1. The period and the The spectroscopic analysis by Santos et al. (2001) indi- semi-amplitude are well constrained± thanks to the large cates an effective temperature Teff = 5610 K, log g =4.50, amount of data points covering many periods. The com- and [Fe/H] = 0.06. The combination of the precise spec- plete set of orbital elements with their uncertainties are − troscopic parameters with the evolution models provide a given in Table 2. stellar mass of M = 0.88 M⊙. The emission flux in the The best-fit orbital parameters and the mass of core of the Ca ii H shows no evidence for chromospheric HD 168746 yield for its companion a minimum mass of activity (see Fig. 2). Since the star is too faint it has not m2 sin i = 0.23 MJup, which is only 0.77 times the mass ′ been possible to compute a reliable value for RHK from the of Saturn. The accuracy obtained on this mass estimation CORALIE spectra. Therefore it was not possible to derive is better than 3%, if we again do not consider the uncer- neither the age nor Prot. Evolutionary tracks suggest for tainty on the mass of the primary. The separation of the this star an age of at least several Gyrs. 3 The discovery was announced in May 2000, see www.eso.org/outreach/press-rel/pr-2000/pr-13-00.html F. Pepe et al.: The CORALIE survey for southern extra-solar planets VII. 7 companion to its parent star is a =0.065AU. The surface We are grateful to the staff from the Geneva and Haute- equilibrium temperature of the planet at such a distance Provence Observatories who have built and maintain the new is about 900 K. Similarly to HD 108147, a dedicated search 1.2-m Euler Swiss telescope and the CORALIE echelle spec- for a possible photometric transit of the planets in front of trograph at La Silla. We thank the Geneva University and HD 168746 remained unsuccessful (Olsen et. al, in prep.). the Swiss NSF (FNRS) for their continuous support for this project. Support to N.S. from Funda¸c˜ao para a Ciˆencia e The radial velocity data were computed, as for Tecnologia (Portugal) is gratefully acknowledged. HD 108147, using the weighted correlation and the cleaned numerical mask. We confirm a reduction of the mea- sured dispersion: The weighted r.m.s. of the data to the −1 Keplerian fit decreased from the original 11.2 m s to the References final 9.8 m s−1, with a reduced χ of 1.5. The CORALIE individual radial-velocity measure- Baranne, A., Queloz, D., Mayor, M., et al. 1996, A&AS, ments presented in this paper are available in electronic 119, 373 form at CDS in Strasbourg. Bouchy, F. & Carrier, F. 2001, A&A, 374, L5 Bouchy, F., Pepe, F., & Queloz, D. 2001, A&A, 374, 733 Chelli, A. 2000, A&A, 358, L59 5. Concluding remarks Donahue, R. A. 1993, Ph.D. Thesis, New Mexico State University The companions to HD 108147 and HD 168746 possess ESA. 1997, The HIPPARCOS and TYCHO catalogue, minimum masses of 1.34 and 0.77 M , respectively. The Sat ESA-SP 1200 latter is one of the lightest extra-solar planet candidates Goldreich, P. & Tremaine, S. 1980, ApJ, 241, 425 found to date. Both objects have been detected by means Guillot, T., Burrows, A., Hubbard, W. B., Lunine, J. I., of the CORALIE spectrograph. CORALIE and its twin & Saumon, D. 1996, ApJ, 459, L35 instrument ELODIE are particularly efficient for the de- Henry, T. J., Soderblom, D. R., Donahue, R. A., & tection of light planets: Seven of the ten lightest candi- Baliunas, S. 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