S. Udry1 - M. Mayor1 - J. V. Clausen2 - L. M. Freyhammer3 - B. E. Helt2 - C. Lovis1 - D. Naef1 - E. H. Olsen2 - F. Pepe1 - D. Queloz1 - N. C. Santos1,4
1 - Observatoire de Genève, 51 ch. des Maillettes, 1290
Sauverny, Switzerland
2 -
Niels Bohr Institute for Astronomy, Physics and Geophysics;
Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen
Ø, Denmark
3 -
Royal Observatory of Belgium, Ringlaan 3, 1180 Brussel and Vrije Universiteit
Brussel, Pleinlaan 2, 1050 Brussels, Belgium
4 -
Centro de Astronomia e Astrofísica da Universidade de Lisboa,
Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018
Lisboa, Portugal
Received 14 April 2003 / Accepted 27 May 2003
Abstract
Recent radial-velocity measurements obtained with the
CORALIE spectrograph on the 1.2-m Euler Swiss
telescope at La Silla unveil the presence of a new Jovian-mass Hot
Jupiter around HD 73256. The 1.85-
planet moves on an extremely short-period (P = 2.5486 d),
quasi-circular orbit. The best Keplerian orbital solution is
presented together with an unsuccessful photometric
planetary-transit search performed with the SAT Danish telescope at
La Silla. Over the time span of the observations, the photometric
follow-up of the candidate has nevertheless revealed a
-d photometric periodicity corresponding to the
rotational period of the star. This variation as well as the
radial-velocity jitter around the Keplerian solution are shown to be
related to the fair activity level known for HD 73256.
Key words: techniques: radial velocities - techniques: photometric - stars: binaries: spectroscopic - stars: individual: HD 73256 - stars: activity - stars: planetary systems
The increasing timebase of the radial-velocity surveys searching for extra-solar planets allows the different planet-hunter teams to announce new planetary candidates on longer and longer periods. This growing period-interval coverage is very important. With the enlargement of the available statistics, new properties of the planetary period distribution are emerging. They provide interesting new constraints for the migration scenarios (for a recent review see Udry et al. 2003).
On the other hand short-period planets, easier to detect because of
the larger reflex motions induced on the primaries and the better
phase coverage for orbital-element determinations, were rapidly
detected. An almost complete census of these systems is available in
well-covered surveys. In the case of our CORALIE
planet-search programme (Udry et al. 2000), the number of targets is
very large (1650) and there are still candidates with only few
measurements. From this survey, we present here a new short-period
planet orbiting the star HD 73256, detected thanks to
very recent radial-velocity observations.
The parent star description, radial-velocity measurements, orbital solution and inferred planetary characteristics for this new candidate are presented in Sects. 2 and 3. Hot Jupiters are promising candidates for photometric transit searches so, in Sect. 4, we describe our photometric observations aiming first to detect a potential planetary transit and then to check the star photometric stability. In Sect. 5, we discuss photometric and line bisector measurements in relation to the activity-induced radial-velocity jitter observed on top of the Keplerian orbital variation. Finally, Sect. 6 gives a summary of the results and some concluding remarks.
HD 73256 ( HIP 42214) is a G8/K0 dwarf in
the southern Pyxis constellation. The HIPPARCOS
catalogue (ESA 1997) lists a visual magnitude V = 8.08, a
colour index B-V = 0.782, and a precise astrometric parallax
= 27.38
0.77 mas that sets the star at a distance of
36.5 pc from the Sun. Its absolute magnitude is then estimated to be
MV = 5.27, slightly overluminous for a typical G8 dwarf. This is
probably due to the enhanced metallicity content of the star (see
below).
From a high-resolution spectroscopic abundance study of HD 73256, we have determined precise values of its effective
temperature (
= 5570
50 K), metallicity
([Fe/H] = 0.29
0.05) and gravity
(
= 4.66
0.10), using a standard local
thermodynamical equilibrium (LTE) analysis (see Santos et al. 2000c, for error
estimates). From calibrations of the width and surface
of the CORALIE cross-correlation functions (CCF;
described in Santos et al. 2002) we can also derive estimates of
= 3.22 km s-1 and [Fe/H] = 0.27
. The high metal content is a recurrent property
of stars hosting planets (for a review see e.g. Santos et al. 2003a, and references
therein).
From the colour index and
we derive a bolometric
correction BC = -0.122 (Flower 1996).
The star luminosity is then estimated to be L = 0.69
.
Note however that Flower's calibrations do not take metallicity into
account. According to the tracks of the Geneva evolution models with
appropriate metal abundance (Schaerer et al. 1993), the position of the
star in the HR diagram indicates a mass
1.05
.
This mass is higher than
typical values for G8/K0 dwarfs because of the high metallicity of the
star.
Table 1: Observed and inferred stellar parameters for HD 73256. Definitions and sources of the quoted values are given in the text.
![]() |
Figure 1:
![]() |
Open with DEXTER |
The models also suggest a completely unconstrained age close to 1 Gyr
for the star, in agreement with its measured high activity level.
HD 73256 belongs to the sample surveyed by
Henry et al. (1996) for Ca II H and K chromospheric emission. It
is found to be fairly active with an index
= -4.49. Chromospheric emission is also
directly visible on the coaddition of our CORALIE spectra
(Fig. 1). Following the calibration by Donahue (1993),
this activity index value points towards a young stellar age around
830 Myr.
From the relation between the activity index and stellar rotation
period (Noyes et al. 1984), we derive a period of rotation
13.9 days for HD 73256. Assuming
that the orbital and rotation axes coincide
, a "statistical'' equatorial
velocity
can be derived from the radius of the star. The
orbital plane inclination is then obtained from the measured projected
rotational velocity
= 3.22 km s-1. Using the simple
relation between stellar luminosity, radius and effective temperature
L =
and with the stellar parameter
values given above, the radius is estimated to be
0.89
.
This leads to a value
3.26 km s-1, very close to the quoted
(
0.98). The true mass of the planet
is thus not expected to be very different from the derived minimum
mass. The short period and favourable inclination make HD 73256 a good candidate for a photometric transit search (see
Sect. 4).
The observed and inferred stellar parameters are summarized in Table 1. Due to the fair activity level of the star some radial-velocity jitter is expected on a typical timescale of the order of the rotational period. This is discussed in Sect. 5.
![]() |
Figure 2: Phase-folded radial-velocity measurements obtained with CORALIE for HD 73256 (top). Error bars represent photon-noise errors. The still large residuals around the solution (bottom) show some structure and may be explained by activity-induced jitter (see Sect. 5). For clarity of the diagram, the measurement on JD 2 451 964 is not displayed in the figure. |
Open with DEXTER |
The first observation with CORALIE of HD 73256 started in February 2001 (JD = 2 451 964.67). However, almost 2 years elapsed before we observed the star again. Because of the clear radial-velocity difference between the 2 measurements, the star was then intensively followed during more than 2 months. Like this, we have gathered 40 precise radial velocities. The photon-noise errors of individual measurements are typically of 4-5 m s-1 despite the relative faintness of the star for our 1.2-m telescope.
The best Keplerian model reproducing the observations yields an
accurately constrained orbital period of
days,
a non-significant eccentricity e = 0.029
0.02, and a
semi-amplitude K = 269
8 m s-1 of radial-velocity
variation. Uncertainties are estimated through Monte-Carlo
simulations. The phase-folded radial-velocity curve is displayed in
Fig. 2 (top).
Using the derived 1.05
mass for HD 73256,
the best-fit parameters lead to a companion minimum mass
and a separation
a = 0.037 AU between the star and the planet. This inferred
separation is the 3
smallest known to date amongst hot
Jupiters, after the 2 OGLE
(Udalski et al. 2002b,a) candidates recently proposed
(Konacki et al. 2003; Dreizler et al. 2003) but for which stronger
radial-velocity confirmations are, however, required. At such a small
distance the planet is strongly heated by its parent star. From
recent models of irradiated planets with condensed dust atmospheres,
Barman et al. (in prep.) estimate the planet day-side temperature
to be around 1500 K (see also Baraffe et al. 2003, for intrinsic temperature
estimates). The complete set of orbital elements with
their uncertainties and the inferred planetary parameters are given in
Table 2.
Table 2: CORALIE best Keplerian orbital solution derived for HD 73256 as well as inferred planetary parameters. Uncertainties are estimated through Monte-Carlo simulations.
The measured weighted rms around the solution is
= 14.8 m s-1, a high value compared to the
individual photon-noise error. Moreover, some structure is clearly
apparent in the residuals drawn as a function of the Julian date
(Fig. 2, bottom).
The possible periodicity of the residuals is discussed in
Sect. 5 in relation with the activity and rotational period
of the star.
With its 2.55-day period, the HD 73256 system is a good candidate for photometric transit search. Furthermore, as discussed above, a favourable geometry could be expected from activity indicator and rotational velocity considerations. We have thus rapidly launched an intensive campaign of high-precision differential photometry in order to detect a possible planetary transit.
The photometric uvby observations were obtained with the Strömgren Automatic Telescope (SAT) at ESO La Silla, Chile.
Details on the standard observational and reduction strategy can be
found in Clausen et al. (2001). HD 72673,
HD 72954AB, and HD 71583 were used
as comparison stars.
Continuous differential observations were carried out, on two nights,
for several hours around the predicted transit times
(Fig. 3). Typical rms errors of one magnitude difference
are 0.003-0.004 (ybv) and 0.005-0.006 (u). Unfortunately, no
transit indication is found. This implies an orbital inclination
smaller than i 82.5
(i.e.
0.99).
However, the data show different magnitude levels from one night to
the other (different symbols in the figure). The star is also known to
be active. Thus, in order to check for photometric variability,
HD 73256 was also regularly monitored, over several
additional nights, to cover the complete interval of orbital phases.
The result is shown in Fig. 4 (top) displaying the observed
differential magnitudes ()
as a function of the Julian
date. A periodic variation is clearly visible over the time span of
the observations. A Fourier transform of the data
yields a period of 13.97 days (Fig. 5), in very good
agreement with the rotational period derived from the activity index
(13.9 days). The photometric variation can thus be interpreted as due
to spots on the surface of the star. Such spots not only perturb the
observed stellar luminosity over a few rotational periods but also
induce a spurious radial-velocity jitter, observable as a
radial-velocity variation of similar periodicity (see
Sect. 5.2 below).
![]() |
Figure 3: SAT differential photometric data ( HD 73256- HD 72673, instrumental system) around the orbital phase of the potential transit (vertical line), in four different colours of the Strömgren photometry, for different nights of observation (different symbols). Typical rms errors of one magnitude difference are 0.003-0.004 (ybv) and 0.005-0.006 (u). The horizontal lines represent the mean magnitude differences (all observations). No indication of a transit is found. |
Open with DEXTER |
![]() |
Figure 4:
Top: SAT photometric observations of
HD 73256 - HD 72673 in the y band
(instrumental system). The typical uncertainty on ![]() ![]() |
Open with DEXTER |
Most extra-solar giant planets have been detected through variations
of the observed radial-velocity of solar-type stars. Such variations
may also be induced by stellar activity related phenomena (like spots)
over a few rotational periods. The corresponding radial-velocity
amplitude can reach a few tens of m s-1(Santos et al. 2000b; Saar & Donahue 1997; Saar et al. 1998). Moreover, this amplitude
increases with the rotational velocity of the star. For
HD 73256,
is not very high, so we do not
expect much effect from activity. Nevertheless, the measured fair
activity level of the star calls for an a posteriori check to
assert the orbital solution as the best explanation for the
observations. First, the photometric measurements vary with a
timescale much longer than the orbital period suggesting different
origins for the radial-velocity and photometric variations. This can
be further checked by controlling the shape of the spectral lines,
expected to vary in the case of spot-induced radial-velocity
variations. A powerful diagnostic is directly available from our
spectra by computing the bisector inverse slope (BIS) of the
cross-correlation functions used for the radial-velocity determination
(Queloz et al. 2001). In case of phased variations of radial
velocities and bisector slopes, we expect a tight correlation between
these two quantities. This is not observed for the 2.55-day period
phasing of the data (Fig. 6). Also, no indication of
variation of the photometric and BIS data is seen in the Fourier space
at the position of the orbital period (Fig. 5). We thus can
exclude activity as the source of the observed main radial-velocity
variation. A planetary companion is the best explanation.
![]() |
Figure 5:
Fourier transforms of radial-velocity residuals around the Keplerian
solution ( top), SAT photometric ![]() ![]() |
Open with DEXTER |
We have noticed in previous sections that the star presents periodic patterns in the photometric data and in the residuals around the Keplerian orbital solution as well. We shall now examine the possibility for these variations to be explained by stellar activity-induced spots.
First, we show that the two quantities present variations of similar
timescales (13.97 days), furthermore compatible with the rotation
period deduced from the activity index (13.9 days). This is readily
visible in Fig. 4 where available simultaneous photometric
points and radial velocities (here the residuals around the Keplerian
model) are plotted as a function of the Julian date. Fourier
transforms of the data also yield similar behaviours in the frequency
space (Fig. 5). In Fig. 4, illustrative sine
curves with fixed photometric period are plotted on top of the
points. The phase shift between the two curves (1.53 rad) is close to
/2, exactly what is predicted for photometric and radial-velocity
variations induced by activity-related spots, as shown on
HD 166435 by Queloz et al. (2001). Very similar
results are obtained in the 4 different bands of the Strömgren
photometry.
Another approach consists in comparing the residuals around the orbital solution to the BIS measurements, in the same way as for the actual radial velocities. Figure 7 (top) presents the CCF bisector inverse slope (BIS) measurements and radial-velocity residuals, phased with the rotational period derived from the photometric data. Although the two quantities do not correlate clearly (Fig. 7, bottom), some hints of coupled variations are emphasized in the top panels by the sine curves with fixed 13.97-d period fitted to the data. We are probably reaching here the limit of the method for the available spectral resolution and radial-velocity precision, especially if the BIS signature is much smaller than the radial-velocity jitter in the case of weak rotators, as suggested by Santos et al. (2003b).
The above considerations unambiguously show the link between stellar activity and the radial-velocity spurious noise observed around the orbital solution. Important progresses are expected in the domain with the higher quality spectra and radial velocities to be provided by HARPS (Pepe et al. 2002).
![]() |
Figure 6: Top: radial velocities (RV, upper panel) and inverse bisector slope (BIS, middle panel) phased with the orbital period (P = 2.5486 d) for HD 73256. Bottom: RV vs. BIS plot (same scale) showing the independence of the two quantities. |
Open with DEXTER |
We have described here a new Hot Jupiter candidate orbiting the star
HD 73256, detected with CORALIE
as part of our large planet-search survey in the southern hemisphere
(Udry et al. 2000). The planet is on a quasi-circular orbit, very
close to its parent star. The period is P = 2.5486 days. The
inferred minimum planetary mass is 1.85
and the
star-planet separation is only 0.037 AU.
![]() |
Figure 7: Same as Fig. 6 but for the residuals around the Keplerian solution, phased with the photometric period (P = 13.97 d). |
Open with DEXTER |
Concerning activity-related noise, it is worth noticing that the
planet orbits a fairly active late-type dwarf. This is also the case
for HD 130322 (Udry et al. 2000) and HD 192263
(Santos et al. 2000a,2003b). In these examples the
radial-velocity jitter induced by activity is small compared to the
orbital radial-velocity semi-amplitude and thus does not prevent us
from detecting the planet. This illustrates the result pointed out by
Saar et al. (1998) and Santos et al. (2000b) that activity-induced
radial-velocity noise is becoming smaller when going from late F to K
dwarfs, a trend mostly due to the typical lower rotation rate of the
latters. K dwarfs thus remain suitable targets for planet-search
programmes even if they show a significant activity level.
Finally, once again the combination of photometry, CCF bisector analysis and cross-correlation technique (Queloz et al. 2001) has provided the mandatory and robust diagnostics for confidently ruling out activity as the source of the observed radial-velocity variation.
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. We thank the Geneva University and the Swiss NSF (FNRS) for their continuous support for this project. Mohamed Yacine Bouzid kindly observed HD 73256 on a few nights at SAT. The SAT group gratefully acknowledges the technical staff at Copenhagen University for its excellent support. The photometry was obtained as part of an extensive study of GK-type eclipsing binaries supported by the Danish Natural Science Research Council. L.M.F. acknowledges support from the project IUAP P5/36 financed by the Belgian State. 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.