A&A 415, 391-402 (2004)
DOI: 10.1051/0004-6361:20034250
M. Mayor1 - S. Udry1 - D. Naef1 - F. Pepe1 - D. Queloz1 - N. C. Santos1,2 - M. Burnet1
1 - Observatoire de Genève, 51 ch. des Maillettes, 1290
Sauverny, Switzerland
2 - Centro de Astronomia e Astrofísica da Universidade de Lisboa,
Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018
Lisboa, Portugal
Received 2 September 2003 / Accepted 25 September 2003
Abstract
This paper summarizes the information gathered for 16 still
unpublished exoplanet candidates discovered with the CORALIE echelle
spectrograph mounted on the Euler Swiss telescope at La Silla
Observatory. Amongst these new candidates, 10 are typical extrasolar
Jupiter-like planets on intermediate- or long-period
(100 < P 1350 d) and fairly eccentric
(0.2
e
0.5) orbits (HD 19994, HD 65216,
HD 92788, HD 111232, HD 114386, HD 142415, HD 147513,
HD 196050, HD 216437, HD 216770). Two of these stars are in
binary systems. The next 3 candidates are shorter-period planets
(HD 6434, HD 121504) with lower eccentricities among which we find
a hot Jupiter (HD 83443). More interesting cases are given by the
multiple-planet systems HD 82943 and HD 169830. The former is a
resonant P2/P1 = 2/1 system in which planet-planet
interactions are influencing the system evolution. The latter is
more hierarchically structured.
Key words: techniques: radial velocities - techniques: spectroscopic - stars: activity - stars: planetary systems
For more than 5 years the CORALIE planet-search programme in the southern hemisphere (Udry et al. 2000a) has been ongoing at the 1.2-m Euler Swiss telescope - designed, built and operated by the Geneva Observatory - at La Silla Observatory (ESO/Chile). During these 5 years, CORALIE has allowed the detection (or has contributed to the detection) of 38 extra-solar planet candidates. This substantial contribution together with discoveries from various other programmes have provided a sample of more than 115 exoplanets that now permits us to point out interesting statistical constraints for the planet formation and evolution scenarios (see e.g. Santos et al. 2003b; Mayor 2003; Udry et al. 2003d, for reviews on different aspects of the orbital-element distributions or primary star properties).
The majority of our CORALIE exoplanet candidates have
been published in a series of dedicated papers, the latest among them reporting the detection
of the shortest-period Hot Jupiter discovered by radial-velocity
surveys around HD 73256 (Udry et al. 2003c) and the
very interesting case of HD 10647
(Udry et al. 2003a), a star with a high IR excess indicative of the
presence of a debris disk. The present paper of this series describes
the CORALIE exoplanets that have not been published
yet. This subsample includes candidates announced several months ago,
rapidly after their detection to allow follow-up observations. It also
includes some candidates with very long periods or that are members of
multi-planet systems requiring a delay in their final analysis. Also,
some of the new candidates correspond to very recent detections.
Table 1:
Observed and inferred stellar parameters for the
stars hosting planets presented in this paper. Definitions and
sources of the quoted values are given in the text. The age and
rotational period estimates are based on calibrations of the
activity indicator (Donahue 1993; Noyes et al. 1984),
whose reference source is also indicated: (S) for this paper following
Santos et al. (2000), (H) for Henry et al. (1996) and (B) for
Butler et al. (2002). The applied analyses and uncertainty estimates
can be found in the quoted references.
The paper is organized as follows. In the next section we summarize
the primary star properties. The radial-velocity measurements and
inferred orbital solutions will be presented in Sect. 3. In the last
section we summarize the results and provide some concluding remarks.
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Figure 1:
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The CORALIE planet-search targets have been selected from the Hipparcos catalogue (ESA 1997). Our planet-star subsample benefits thus from the photometric and astrometric information gathered by the satellite.
A high-resolution spectroscopic abundance study was performed for most
of these stars by N.C. Santos in his study demonstrating the
metallicity enrichment of stars with planets with regard to comparison
"single'' stars analysed in a homogeneous way (Santos et al. 2003b,2001, and
references therein). This study provides
precise values of the effective temperatures, metallicities and
gravity estimates, using a standard local thermodynamical equilibrium
(LTE) analysis. These values have also been updated by using better
recent oscillator strengths ()
in the procedure
(Santos et al. 2003a). From calibrations of the width and surface of
the CORALIE cross-correlation functions (CCF;
described in Santos et al. 2002), we also derive the stellar projected
rotational velocity
and [Fe/H] when not available from the
spectral analysis. Like the majority of stars hosting planets, most
of our candidates show a significant metal enrichment compared to the
Sun, with the noticeable exception of HD 6434 (the
second most deficient star hosting a planet known to date) and
HD 111232.
From the colour index, the measured
and the
corresponding bolometric correction, we estimate the star
luminosities
and we then interpolate the masses and ages in the grid
of Geneva stellar evolutionary models with appropriate metal
abundances (Schaerer et al. 1993; Schaller et al. 1992). For the brightest star in
the sample we estimate, following Santos et al. (2000), an activity
indicator from the reemission flux in the Ca II H absorption
line. In some cases, this value is also available in the literature
(mainly in Henry et al. 1996). Such an indicator is then used to
derive calibrated estimates of the stellar rotational periods and ages
(Donahue 1993; Noyes et al. 1984). Table 1 gathers the
photometric, astrometric, spectroscopic information and inferred
quantities available for our sample of stars hosting exoplanet
candidates.
The activity indicator is, however, not always available in the
literature or cannot be estimated from our spectra because of the star
faintness (HD 65216, HD 114386).
Calcium reemission can then be visually checked on the co-addition of
the CORALIE best-S/N spectra. For our subsample, this
is shown in Fig. 1 for the
3968.5 Å
Ca II H absorption line region. A prominent reemission
feature is clearly visible for HD 114386 and
HD 147513, the latter presenting, moreover, a strong
activity indicator. However, these stars are only slowly rotating and
no large influence on the radial-velocity measurements is then
expected (Santos et al. 2000; Saar et al. 1998). Hints of reemission are also
observed for HD 65216, HD 83443,
HD 142415, and HD 216770 but again
is fairly low for these stars for which only a moderate
jitter might be expected.
Some stars in this sample deserve a few more comments:
- HD 6434 (HIP 5054): The star is classified as subgiant. Taking into account its low metallicity, the derived absolute magnitude, effective temperature and gravity estimates also support the slightly evolved status of the star.
- HD 19994 (HIP 14954, HR 962, GJ 128A): The star is known
to have a close physical M dwarf companion (GJ 128 B) a few
arcseconds away (100 AU; Hale 1994). Planets in
binary systems are important for our understanding of planet formation
because they seem to present different mass and orbital properties
than planets orbiting single stars
(Zucker & Mazeh 2002; Eggenberger et al. 2003; Udry et al. 2003d). The rotational
velocity of HD 19994 is fairly large
(
= 8.1 km s-1) thus, even if the star is not
clearly active, some small radial-velocity jitter might be expected.
Contamination from the secondary could also be a concern although the
6.4 mag difference between the stars makes it very unlikely.
- HD 82943 (HIP 47007): A fair amount of the easy-to-burn element 6Li has been detected in the spectra of this star (Israelian et al. 2003,2001) suggesting a planet engulfment in the late stages of the system formation, after the star has reached the Main Sequence. The not too deep convective zone of this G0 dwarf allows then for the survival of this element.
- HD 111232 (HIP 62534): The high velocity
(V = 104.4 km s-1) and low metallicity ([Fe/H] = -0.36)
of the star indicate that it probably belongs to the thick disk
population. HD 111232 is a proposed binary in the
Hipparcos catalogue (ESA 1997). It is one of the so-called problem stars for which an astrometric acceleration solution is
provided (flagged "G'' in the H59 field of the main catalogue).
However, no companion with
3.0 was found close to
the star (within 1.08
)
by speckle interferometry
(Mason et al. 1998).
- HD 121504 (HIP 68162): A visual companion ( CPD -55:5793) is observed at a separation of 34.2
(Dommanget & Nys 1994). It is an A2 star of magnitude V = 9.17 with
different proper motions than HD 121504. The pair is
thus not physical. No companion was found close to the star by speckle
interferometry (Mason et al. 1998).
- HD 142415 (HIP 78169): A Rosat-All-Sky-Survey X-ray source
(1RXS J155740.7-601154) is observed close to the star, at
5
.
No bright companion was found close to the star
by speckle interferometry measurements (Mason et al. 1998).
- HD 147513 (HIP 80337, HR 6094, GJ 620.1A): The star was proposed to be a barium dwarf by Porto de Mello & da Silva (1997). A common proper motion white dwarf (WD; V = 10), at a projected distance of 5360 AU, could support the explanation of the origin of the barium feature by the process of mass transfer in a binary system, in which the secondary component accreted matter from the evolved primary, now the WD (see e.g. Jorissen et al. 1998, and references therein). To account for the observed large separation, Porto de Mello & da Silva (1997) invoke a possible ejection of the WD from an originally quadruple system also including HD 147513. In such a case, this would be the first known case of a planet orbiting a solar-type star in a binary system with the stellar companion in the last stage of its life. The influence on the planet evolution of the mass transfer between two stars is still poorly studied. The mentioned authors also include the star in the 0.3 Gyr old Ursa Major kinematical group. Although some doubts (but no rejection) were cast on the membership (King et al. 2003), it is worth noticing that the activity level, activity-derived age and metallicity (Table 1) correspond well to the Ursa Major group. Finally, no bright companion was found closer-in to the star by speckle interferometry measurements (Mason et al. 1998).
The radial-velocity measurements presented in this paper were obtained with the CORALIE echelle spectrograph mounted on the 1.2-m Euler Swiss telescope at La Silla Observatory (ESO, Chile). CORALIE is a similar but improved version of the ELODIE spectrograph at the Haute-Provence Observatory (CNRS, France). Details on the instrument design, reduction procedures as well as radial-velocity estimates based on simultaneous thorium-lamp measurements and cross-correlation technique can be found in Baranne et al. (1996).
Table 2:
CORALIE best Keplerian orbital solutions
as well as inferred planetary parameters for the 1-planet systems.
Span is the time interval in days between the first and last
measurements. (O-C) is the weighted rms of the residuals
around the derived solutions. mask is the template used in the
cross-correlation scheme for the radial-velocity estimate
(see Sect. 3). Det.Ref. is the reference to the planet detection
announcement (conference or press release).
The typical precision obtained with CORALIE is
3 m s-1 for bright stars (Queloz et al. 2001b).
However, due to the small size of the telescope, our sample stars
(Udry et al. 2000a) are mainly photon-noise limited. For the
derivation of the orbital solution an instrumental error of
3 m s-1 is quadratically added to photon noise. Then we only
take into account the spectra with good signal-to-noise, typically
corresponding to radial-velocity uncertainties below 10 to
15 m s-1, depending on the star. Also, to improve the Doppler
information extraction from the spectra, a new weighted
cross-correlation scheme was developed (Pepe et al. 2002a). It is
applied for the velocity estimates of most of the stars of this
paper
.
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Figure 2: Phase-folded radial-velocity measurements obtained with CORALIE for HD 6434, HD 19994, HD 65216, HD 92788, HD 111232 and HD 114386, superimposed on the best Keplerian planetary solution (top panel in each diagram). The residuals as a function of Julian Date are displayed in the lower panels. |
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Figure 3: Phase-folded radial-velocity measurements obtained with CORALIE for HD 121504, HD 142415, HD 147513, HD 196050, HD 216437 and HD 216437, superimposed on the best Keplerian planetary solution (top panel in each diagram). The residuals as a function of Julian Date are displayed in the lower panels. |
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To simplify the presentation of our results for the single-planet
systems, the standard orbital parameters derived from the best fitted
1-Keplerian solution to the data are gathered in Table 2.
Useful inferred planetary parameters and further information on the
number of measurements used, their time coverage (Span) and the
residuals (weighted rms) around the solutions ((O-C)) are
given as well. Finally, we also indicate the references for the
detection announcements of these candidates
(Det.Ref.). The corresponding phase-folded radial-velocity curves
are displayed in Figs. 2 to 4 (top panel in each
diagram). The residuals drawn as a function of the Julian date are
displayed in the figures as well (bottom panels).
Simultaneous independent discoveries have been made for 3 of our candidates: by Fischer et al. (2001) for HD 92788 and by Jones et al. (2002) for HD 196050 and HD 216437. Their orbital solutions are very similar to ours (Table 2).
For most of the systems, the residuals measured around the derived
solutions are compatible with the typical individual radial-velocity
uncertainties. Some of them are, however, slightly larger
(10 m s-1). In these cases, a slight level of activity
can be invoked to explain the additional noise. HD 114386 is a good example with its clear activity level
testified by the visible reemission in the Ca II H absorption
line (Fig. 1). Another illustration is given by the similar
early G dwarfs HD 121504 and HD 142415. They present light rotation and an activity level that
can explain the somewhat large measured residuals. It is interesting
to note here the similar values of
,
and
(O-C) for the two candidates.
In order to emphasize a relation between the residuals to the
Keplerian solutions and stellar activity, we compared in a systematic
way for all the candidates the obtained residuals with the shape of
the spectral lines, estimated by the bisector inverse slope of our
cross-correlation functions (BIS; Queloz et al. 2001a).
Unfortunately, at this level of velocity variation
(10 m s-1) and at the S/N of the CORALIE spectra, no definitive conclusion can be drawn. The same
non-conclusive result was also obtained for HD 73256
which did not show any clear relation between the BIS parameter and
the residuals around the 2.5-d solution, although the activity-induced
radial-velocity jitter was undubitably emphasized by the simultaneous
variations of the stellar photometric signal and the residuals
(Udry et al. 2003a). We also searched in a systematic way, through
Fourier analysis, for additional periodicities in the residuals around
the derived solutions. Nothing significant was found for these
candidates. Some of them deserve, however, further comments (see the
following subsections).
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Figure 4: Phase-folded radial-velocity measurements obtained with CORALIE for HD 83443, superimposed to the best Keplerian planetary solution (top). The residuals as a function of Julian Date are displayed in the lower panel. |
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Amongst the 13 single-planet systems presented in this subsection, 10
(HD 19994, HD 65216, HD 92788, HD 111232, HD 114386,
HD 142415, HD 147513, HD 196050, HD 216437, HD 216770)
are on intermediate or long-period orbits
(100 < P 1350 days) with medium eccentricities
(0.2
e
0.5). Such properties are shared by the
bulk of the presently known exoplanets
. The
remaining 3 shorter-period planets (HD 6434,
HD 83443, HD 121504) present, on the
other hand, a lower eccentricity. The eccentricity-period trend is
also observed for stellar binaries. This similitude between the two
populations was often brought up as a question for different planet
and binary formations. Nevertheless, clear differences exist
(see e.g. Mayor & Udry 2000; Halbwachs et al. 2003, for a more complete
discussion). Also the mass-period
relation of exoplanets, checked for statistical significance by
Zucker & Mazeh (2002) and further discussed in Udry et al. (2003d), is
apparent in our subsample; the two shortest-period systems host (by
far) the lightest planets, with minimum masses below 0.4
,
whereas the longest periods (P
1000 d) are
associated with planet minimum masses of 1.82, 3.02 and 6.8
.
Since the discovery of the planet, we have doubled the number of
measurements available for HD 6434, gathering a total
of 130 good spectra over more than 1500 days. The star relative
faintness and low metallicity lead to a typical radial-velocity
photon-noise uncertainty on individual measurements of only
8 m s-1. However, the large number of measurements
allows us to derive precise Keplerian orbital elements for the system
(Table 2). The very low planet minimum mass inferred from
the orbital solution (
= 0.39
)
gives us a
first example of a very light planet orbiting a deficient star.
Although the star is found to be neither active nor rapidly rotating,
a concern is brought by the not so different values of the rotational
period (18.6 day) estimated from the activity indicator and the
orbital period (22 day). The uncertainty on the orbital period is
very small, but the calibrated
carries an intrinsic
uncertainty. The a posteriori verification that the shape of the
spectral lines, estimated by the bisector inverse slope of our
cross-correlation functions (BIS; Queloz et al. 2001a), does not
vary with the radial velocity provides an indication against activity
to be the source of the observed radial-velocity variation. The BIS
itself is even constant with a rms of 8.6 m s-1, at the
photon-noise level. Moreover, the radial-velocity variation is stable
over more than 68 cycles. However, rough simulations by
Santos et al. (2003c) raised the possibility of radial-velocity
variations without noticeable change in the BIS for slowly rotating
stars. In such cases, only simultaneous velocity and photometric
measurements can trace the intrinsic origin of the radial-velocity
variations, as e.g. for HD 192263
(Santos et al. 2003c) or for the HD 73256 residuals
(Udry et al. 2003c). No indication of a 22-d periodicity is present in
the Hipparcos photometric data for HD 6434 and the
Geneva photometry finds the star stable at a 3 mmag level.
The residuals around the Keplerian solution (8.1 m s-1) are
slightly larger than individual photon-noise errors (median at
6 m s-1). They seem mainly due to velocities around
JD = 2 451 600 presenting a large dispersion. Taking into account
the fairly large rotation of the star and its binarity status, this
could possibly come from an enhanced stellar activity level at that
moment, although the BIS parameter does not show anything special at
that time. Nothing else particular is visible in the residuals.
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Figure 5:
Fourier transform of the residuals around the 2.98-d orbital solution
of HD 83443 b Top: for radial-velocities in
the 2 451 500 ![]() ![]() ![]() ![]() |
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HD 83443 was first announced at the Manchester IAU
Symp. 202 to host a resonant 2-planet system with periods
P1 = 2.985 d and P2 = 29.85 d (Mayor et al. 2000). At
that time the available 93 high-S/N CORALIE spectra
allowed us to clearly find the 2 periods of variation. The two
periodic signals were highly significant. In particular, the false
alarm probability for the 30-d period in the data was very low
(
), close to the
detection limit
(Fig. 5, top). Moreover, these periodicities were not
present in the velocities of contemporary observed constant stars with
similar photometric characteristics (Udry et al. 2002). This rules
out instrumental effects as the source of the observed variations. A
new reduction of the data using the weighted cross-correlation scheme
also confirms the results presented in Manchester.
However, about 2 years later, Butler et al. (2002) trying to confirm the 2-planet system did not detect the 30-d second-planet signal in their Keck+AAT radial-velocity data. We then also verified that it had disappeared in our more recent data as well (Udry et al. 2002). The corresponding peak in the Fourier transform of the radial velocities is no longer present (Fig. 5, bottom).
The origin of this transient signal is not clear yet. An appealing
possibility is to attribute the effect to activity as the
corresponding period is compatible with the activity-related stellar
rotation estimate (within the uncertainties related to the
calibration). As mentioned above, we tried for this star as well to
see a relation between BIS and the residuals around the 2.98-d
Keplerian solution but without success at the precision of our
CORALIE data. No 30-d periodicity is found in the BIS
data either. However, if existing, such a relation between residuals
and line shape should easily come out of the HARPS
data that are already providing a preliminary commissioning
orbital solution for HD 83443 at a
1.5 m s-1 precision level (Mayor 2003).
Looking at the orbital solution given in Table 2, we can note that the ecccentricity derived for the planet is not significantly different from 0.
With its 2.98-day period, HD 83443 b was a good candidate for photometric transit search. The photometric uvbyobservations were obtained with the Strömgren Automatic Telescope (SAT) at ESO La Silla, Chile (Olsen et al. in prep.). Unfortunately, no transit was detected.
Table 3:
CORALIE simulataneously derived 2-Keplerian orbital solutions as well as inferred planetary parameters for the multi-planet systems
HD 82943 and HD 169830. Parameter
definitions are the same as in Table 2. Note that for
HD 82943, the e and
elements are not well
defined in the sense that there exist other solutions with very different
e and
estimates but with equivalent
(O-C) values
(see text).
We mentioned above that the somewhat large residuals obtained for this
system may probably be related to the stellar activity-induced jitter
(
= -4.57). We also could see in the temporal
distribution of the residuals some indications of an additional
radial-velocity drift. If real, this drift is very small. A combined
Keplerian + linear drift model yields a drift value of
3 m s-1 y-1, without changing noticeably the
planetary orbital parameters. Furthermore, the combined fit does not
improve much the quality of the solution: the
(O-C) decreases
only from 11.6 to 11.2 m s-1. The drift is thus not considered
as significant and not included in the solution given in
Table 2. Future measurements will confirm or rule out this
potential drift.
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Figure 6: Top: temporal radial-velocity measurements obtained with CORALIE for the 2-planet systems HD 82943 (left) and HD 169830 (right), superimposed to the best simultaneously-derived 2-Keplerian planetary solutions. Bottom: residuals as a function of Julian Date. |
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The Keplerian solution derived for HD 142415 yields a
period of 386.3 1.6 days, not too far from 1 year. As we have
points covering the maximum and close (on both sides) to the minimum
of the phased radial-velocity curve over 4 full cycles, this proximity
to an annual variation is not so much of a concern for the reality of
the planetary signal. The trouble resides rather in the difficulty of
covering the whole phase interval. As a result the minimum of the
curve is highly undersampled and the best fitted orbital solution
tends to be very eccentric, increasing the radial-velocity
semi-amplitude in an artificial way. This effect is even worsened by
the stellar activity induced jitter that makes the orbital elements
more difficult to determine. We have thus arbitrarily fixed the
eccentricity of the solution to e = 0.5, a plausible value when
looking at the sequence of the radial-velocity measurements. Note,
however, that solutions with e between 0.2 and 0.8 would still be
acceptable.
The phase coverage of our 31 CORALIE radial velocities is not good enough to obtain a constrained value of the planet eccentricity. We thus fixed this eccentricity to e = 0.3, a value minimizing the residuals around the derived solution.
The star hosts a very interesting 2-planet system. Between JD = 2 451 184 and JD = 2 452 777, we have gathered 142 good CORALIE spectra of the star that allow us to derive a 2-Keplerian approximate solution with periods Pb = 435.1 d and Pc = 219.4 d. The whole set of orbital parameters are given in Table 3 and the solution and velocities are displayed in Fig. 6 (left). The particular values of the periods made us miss the short-period minimum of the curve when the star was behind the Sun. The long-period planet ( HD 82943 b) was thus announced first (ESO 2000), about 1 year before the detection of the secondary planet signature (HD 82943 c; ESO 2001).
The Pi/Pj = 2/1 resonant systems are very important because, when the planet orbital separations are not too large, planet-planet gravitational interactions become non-negligible during planet "close'' encounters, and will noticeably influence the system evolution on a timescale of the order of a few times the long period. The radial-velocity variations of the central star will then differ substantially from velocity variations derived assuming the planets are executing independent Keplerian motions. We observe a temporal variation of the instantaneous orbital elements. In the most favourable cases, the orbital-plane inclinations, not otherwise known from the radial-velocity technique, can be determined since the amplitude of the planet-planet interaction directly scales with their true masses.
In the case of multiple planets, only approximate analytic solutions of the gravitational equations of motion exist, and one must resort to numerical integrations to model the data. Several studies have been conducted in this direction for the Gl 876 system (Nauenberg 2002; Laughlin & Chambers 2001; Rivera & Lissauer 2001), similar to HD 82943 in the sense that it also hosts two 2/1 resonant planets at fairly small separations. The results of the Newtonian modeling of the Gl 876 system have validated the method, improving notably the determination of the planetary orbital elements, but the time coverage of the measurements is still too small for the method to provide strong constraints on the plane inclinations. The valley of the acceptable solutions is still very shallow, although including the correct answer provided by HST astrometric observations (Benedict et al. 2002). Further radial-velocity measurements will undoubtedly improve the situation.
Our correct modeling of HD 82943, taking into account the planet-planet interactions, is under study and will be presented in a forthcoming paper with a more detailed description of the system behaviour (Correia et al. in prep.).
Our present approximate solution for the system (Table 3) yields residuals at the level of the photon noise of the radial-velocity measurements. The inferred two planetary masses are very similar. Although in a 2/1 resonance, the two orbits do not seem to be aligned. However, we have to warn the reader not to take too litterally the results described here because:
- As mentioned above, due to the planet-planet interactions, the orbital elements are time dependent;
- Due to the non-optimum phase coverage, the planet eccentricities
and s are badly (if at all) constrained by the data. The
small uncertainties given in the table only relate to the given local
solution in parameter space. There are, however, other local solutions
with almost equivalent
minimum values. For instance, a
solution with the very different values of the eccentricities
ec = 0.4 and eb
0 only increases the residuals from
6.8 to 6.9 m s-1. In this case, as eb
0,
is completely non-determined and it is then possible to
find for these eccentricities an aligned configuration with
=
= 110 deg and the same level of residuals
(6.9 m s-1), leaving furthermore Pi and Ki almost
unchanged.
We will now continue to follow closely this system, accumulating measurements to improve in the future the derived solution.
A 230-d period planet orbiting HD 169830 was first described in Naef et al. (2001). The orbital solution was derived from the 35 CORALIE spectra available at that time. After the second maximum of the radial-velocity curve, we noticed an additional trend in the data pushing us to follow the star more closely. We have now gathered 112 good spectra that allow us to simultaneously derive a complete 2-Keplerian orbital solution for the system (Table 3 and Fig. 6, right).
Unlike HD 82943, this system is not resonant but more hierarchically structured. The separation between the 2 planets stays always fairly large, the 2-Keplerian model is then supposed to provide a good approximation of the system evolution on a fairly long time. However, the time span of our velocity measurements barely covers the long period variation. The corresponding planetary orbit is thus not completely constrained and a large uncertainty is still observed for the long period.
This 2-planet system is the first to be announced after the proposition by Mazeh & Zucker (2003b) of a possible correlation between mass ratio and period ratio for adjacent planets in multi-planet systems. It is interesting to note that the new system agrees with the proposed correlation (Mazeh & Zucker 2003a).
We have described in this paper 16 still unpublished exoplanet candidates discovered with the CORALIE echelle spectrograph mounted on the 1.2-m Euler Swiss telescope at La Silla Observatory. Amongst these new candidates:
- Ten are typical extrasolar Jupiter-like planets on intermediate- or
long-period (100 < P 1350 d) and fairly eccentric
(0.2
e
0.5) orbits (HD 19994,
HD 65216, HD 92788, HD 111232, HD 114386, HD 142415,
HD 147513, HD 196050, HD 216437, HD 216770). They resemble the bulk of
extra-solar planets found to date.
- Two of these planets (HD 19994, HD 147513) are orbiting one component of a multiple-star system. Such planets seem to present different orbital and mass characteristics than the other single-star planets (Zucker & Mazeh 2002; Eggenberger et al. 2003). The companion to HD 147513 is even a white dwarf, the evolution to which has probably also influenced the planet evolution through mass transfer between the two stars.
- Three candidates are shorter-period planets ( HD 6434, HD 121504, HD 83443) with lower eccentricities (the latter being a hot Jupiter).
- More interesting cases are given by the multiple-planet systems HD 82943 and HD 169830. HD 82943 is a resonant Pb/Pc = 2/1 system in which planet-planet interactions are influencing the system evolution. HD 169830 is non-resonant and more hierarchically structured, and therefore less affected by this kind of interaction.
From a more global point of view, our candidates follow the period-eccentricity and period-mass trends observed for the whole sample of known extra-solar planets. They follow as well the trend for stars hosting planets to be more metal rich than normal stars of the solar neighbourhood (Gonzalez et al. 2001; Santos et al. 2003b,2001; Laws et al. 2003). Only 3 amongst the 15 stars are metal deficient with regards to the Sun whereas almost all the others present high [Fe/H] values.
We emphasize the difficulty encountered to fully constrain multi-planet systems. Such a task, involving many free parameters, requires a good phase coverage and a fair number of measurements, even for the simplest cases. As a consequence, studies on multi-planet system stability should not rely too closely on the given orbital parameters. The published solutions will probably change in the future (some will notably change) as more measurements become available. A substantial advance in this domain will be brought by the the new HARPS spectrograph mounted on the ESO 3.6-m telescope at La Silla (Pepe et al. 2002b) available since October 2003. With the very high precision achieved for radial-velocity measurements and the quality of the spectra, HARPS is now providing us with an unequalled tool to characterize multi-planet systems and/or disentangle activity-induced jitter from orbital radial-velocity variations.
Acknowledgements
We are grateful to the staff from the Geneva Observatory which is maintaining the 1.2-m Euler Swiss telescope and the CORALIE echelle spectrograph at La Silla. In particular, many thanks to Luc Weber for his continuous improvement of the CORALIE spectrograph softwares and to Bernard Pernier for his efforts in maintaining the CORALIE database and for his contribution to a large number of observations. We thank the Geneva University and the Swiss NSF (FNRS) for their continuous support for this project. Support from Fundação para a Ciência e Tecnologia, Portugal, to N.C.S., in the form of a scholarship is gratefully acknowledged. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.