Transiting exoplanets from the CoRoT space mission. XVIII. CoRoT-18b: a massive hot jupiter on a prograde, nearly aligned orbit

Abstract

We report the detection of CoRoT-18b, a massive hot jupiter transiting in
front of its host star with a period of 1.9000693 +/- 0.0000028 days. This
planet was discovered thanks to photometric data secured with the CoRoT
satellite combined with spectroscopic and photometric ground-based follow-up
observations. The planet has a mass M_p = 3.47 +/- 0.38 M_Jup, a radius R_p =
1.31 +/- 0.18 R_Jup, and a density rho_p = 2.2 +/- 0.8 g/cm3. It orbits a G9V
star with a mass M_* = 0.95 +/- 0.15 M_Sun, a radius R_* = 1.00 +/- 0.13 R_Sun,
and a rotation period P_rot = 5.4 +/- 0.4 days. The age of the system remains
uncertain, with stellar evolution models pointing either to a few tens Ma or
several Ga, while gyrochronology and lithium abundance point towards ages of a
few hundred Ma. This mismatch potentially points to a problem in our
understanding of the evolution of young stars, with possibly significant
implications for stellar physics and the interpretation of inferred sizes of
exoplanets around young stars. We detected the Rossiter-McLaughlin anomaly in
the CoRoT-18 system thanks to the spectroscopic observation of a transit. We
measured the obliquity psi = 20 +/- 20 degrees (sky-projected value: lambda =
-10 +/- 20 degrees), indicating that the planet orbits in the same way as the
star is rotating and that this prograde orbit is nearly aligned with the
stellar equator.

Full text

Page 1

Astronomy & Astrophysics manuscript no. article
August 16, 2011
c ? ESO 2011
Transiting exoplanets from the CoRoT space mission?
XVIII. CoRoT-18b: a massive hot jupiter on a prograde, nearly aligned orbit
G. H´ ebrard1,2, T.M. Evans3, R. Alonso4, M. Fridlund5, A. Ofir6, S. Aigrain3, T. Guillot7, J.M. Almenara8,9,
M. Auvergne10, A. Baglin10, P. Barge8, A.S. Bonomo8, P. Bord´ e11, F. Bouchy1,2, J. Cabrera12, L. Carone13,
S. Carpano5, C. Cavarroc11, Sz. Csizmadia12, H.J. Deeg9, M. Deleuil8, R.F. D´ ıaz1,2, R. Dvorak14,
A. Erikson12, S. Ferraz-Mello15, D. Gandolfi5, N. Gibson3, M. Gillon16, E. Guenther17, A. Hatzes17,
M. Havel7, L. Jorda8, H. Lammer18, A. L´ eger11, A. Llebaria8, T. Mazeh6, C. Moutou8, M. Ollivier11,
H. Parviainen9, M. P¨ atzold13, D. Queloz4, H. Rauer12,19, D. Rouan10, A. Santerne8, J. Schneider20,
B. Tingley9, and G. Wuchterl17
(Affiliations can be found after the references)
Received ; accepted
ABSTRACT
We report the detection of CoRoT-18b, a massive hot jupiter transiting in front of its host star with a period of 1.9000693 ±
0.0000028 days. This planet was discovered thanks to photometric data secured with the CoRoT satellite combined with
spectroscopic and photometric ground-based follow-up observations. The planet has a mass Mp= 3.47 ± 0.38MJup, a radius
Rp = 1.31 ± 0.18RJup, and a density ρp = 2.2 ± 0.8g/cm3. It orbits a G9V star with a mass M? = 0.95 ± 0.15M?, a radius
R? = 1.00 ± 0.13R?, and a rotation period Prot = 5.4 ± 0.4days. The age of the system remains uncertain, with stellar evolution
models pointing either to a few tens Ma or several Ga, while gyrochronology and lithium abundance point towards ages of a few
hundred Ma. This mismatch potentially points to a problem in our understanding of the evolution of young stars, with possibly
significant implications for stellar physics and the interpretation of inferred sizes of exoplanets around young stars. We detected
the Rossiter-McLaughlin anomaly in the CoRoT-18 system thanks to the spectroscopic observation of a transit. We measured the
obliquity ψ = 20◦± 20◦(sky-projected value λ = −10◦± 20◦), indicating that the planet orbits in the same way as the star is
rotating and that this prograde orbit is nearly aligned with the stellar equator.
Key words. stars: planetary systems - techniques: photometry - techniques: radial velocities - techniques: spectroscopic
1. Introduction
Out of the ∼ 550 exoplanets known to date, more than 100
transit their parent stars as seen from the Earth. This par-
ticular configuration allows numerous key studies, including
accurate radius, mass, and thus density measurements (see,
e.g., Winn 2010a for a review), atmospheric studies in ab-
sorption through transits and in emission through occulta-
tions (e.g. Vidal-Madjar et al. 2003; Wheatley et al. 2010),
dynamic analyses through possible timing variations (e.g.
Holman et al. 2010), or spin-orbit alignment measurements
thanks to the Rossiter-McLaughlin effect (e.g. Bouchy et
al. 2008). The power of these analyses incited numerous
search surveys for transiting planets. Most of them were
discovered in the last five years, and the detection rate is
still increasing.
The CoRoT space mission (COnvection ROtation and
planetary Transits, Baglin et al. 2009) was launched on
2006 December 27. Based on a 27-cm telescope and a
2.8◦× 2.8◦-field camera, it is designed to study asteroseis-
mology and detect transiting exoplanets. The satellite al-
Send offprint requests to: G. H´ ebrard (hebrard@iap.fr)
?The CoRoT space mission, launched on 2006 December 27,
has been developed and is operated by CNES, with the con-
tribution of Austria, Belgium, Brazil, ESA (RSSD and Science
Programme), Germany and Spain.
lows several thousand stars (V = 12 − 16) to be continu-
ously observed for up to 150 days with a high photometric
accuracy. CoRoT is thus well adapted to detecting tran-
siting planets with small radii, such as CoRoT-7b (L´ eger
et al. 2009; Queloz et al. 2009), or on long orbital periods,
such as CoRoT-9b (Deeg et al. 2010). It can also detect hot
jupiters, such as CoRoT-18b. We report its discovery here.
We describe in Sect. 2 the CoRoT observations and
the transit detection of the planetary candidate. Then, we
present in Sect. 3 the ground-based follow-up observations
that were needed to establish the planetary nature of the
event detected by CoRoT and also to characterize this plan-
etary system. The analysis of the whole dataset and the re-
sults are presented in Sect. 4, before conclusion in Sect. 5.
2. CoRoT observations and transit detection
CoRoT-18 was one of 4161 target stars observed by CoRoT
from 2010 March 5 to 29 as part of SRa03, the third short
run of the satellite in the Galactic anti-center direction.
The coordinates, magnitudes and identifiers of CoRoT-18
in various catalogs are given in Table 1. The finding chart
is plotted in Fig. 1. Following the method described in
Gandolfi et al. (2008), the distance, d, and interstellar ex-
tinction, AV, to CoRoT-18 were derived using the DENIS
and 2MASS magnitudes reported in Table 1 and synthetic
arXiv:1107.2032v2 [astro-ph.EP] 15 Aug 2011

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2G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
Table 1. CoRoT-18 IDs, coordinates and magnitudes.
CoRoT window ID
CoRoT ID
USNO-B1 ID
2MASS ID
CMC14
SRa03 E2 1347
315211361
0899-0092144
06324137-0001537
063241.3-000153
Coordinates
RA (J2000)
Dec (J2000)
06h32m41.s36
−00◦01?53.??71
Magnitudes
Filter
B2 (USNO-B1)
R2 (USNO-B1)
V
R (CMC14)
I (DENIS)
J (2MASS)
H (2MASS)
K (2MASS)
Mag
15.79
14.99
15.00
14.472
14.051
13.441
13.080
13.014
Error
-
-
0.10
0.048
0.030
0.024
0.031
0.030
Fig.1. The sky area around CoRoT-18 on the POSS red
image. The target is in the middle of the image, with the
overplot of the CoRoT photometric aperture mask.
colors from a model atmosphere with the same parameters
as the star (see Sect. 4.2 and Table 3 below). We found
AV = 0.15 ± 0.15 mag and d = 870 ± 90 pc, as well as
V = 15.00 ± 0.10 in the Johnson standard system.
The cadence for this target was 32seconds throughout
the CoRoT observations, resulting in 65120 exposures span-
ning the wavelength range 300-1000nm. The data were pro-
cessed by the CoRoT pipeline (Auvergne et al. 2009). The
normalized white-light curve, obtained by summing the flux
from the three channels and normalizing by the mean flux,
is shown in the top panel of Fig. 2. It clearly shows 13
planetary-like transit features, with a period of ∼ 1.9 days
and a depth of ∼ 2%. The phase-folded light curve is plot-
ted in the upper panel of Fig. 3.
To reduce computing time, the data was rebinned to
512 seconds, which is the normal observing cadence for
CoRoT exoplanet targets. This binned light curve is used
throughout the rest of this paper. We checked that this
binning does not significantly affect the parameter retrieval
Fig.2. Top: CoRoT light curve in black with the 13 tran-
sits shown in red. All epochs in this paper are given in
Heliocentric Julian Date (HJDUTC; Eastman et al. 2010).
The original light curve (shown in gray) contained a discon-
tinuity at HJD−2450000 = 5281.62, which was corrected
by subtracting 15 mmag from data taken after this date.
A linear fit to the light curve has also been used to re-
move any trend on timescales longer than the duration
of the run. The light curve overplotted in blue is binned
to one point per orbital period of the CoRoT satellite to
make sure that no systematics from the rotation period can
come through. Bottom: Lomb-Scargle periodogram of the
out-of-transit light curve (black curve in top panel) as a
function of relative improvement in χ2compared to a con-
stant flux model. It shows the stellar rotation signature at
5.4 ± 0.4 days (solid red line), as well as its first harmonic
(dashed red line). Also shown are the Gaussian fit used to
estimate the uncertainty in the stellar rotation period and
the resulting 1-σ interval (dotted red lines).
by using the formalism presented by Kipping (2010b). The
mean flux was 57366.4e−per 32-second exposure, and the
relative standard deviation of the binned light curve is
6.9×10−3, a factor 7.2 above the photon noise. This factor
reduces to 1.6 when the transits are removed and variations
on timescales longer than a day are filtered out (black line
in upper panel of Fig. 2; see below Sect. 4.1).
CoRoT-18 was one of ten objects of interest identified
soon after the end of SRa03 observations by the “alarm
mode” pipeline (Surace et al. 2008), which removes the out-
liers flagged by the main pipeline, detrends the light curves
to remove long-term trends (instrumental and stellar) using
a median filter, and searches for transits using an implemen-
tation of the box least squares (BLS) algorithm of Kov` acs,
Zucker & Mazeh (2002). A number of tests were then car-
ried out to check that the CoRoT data were compatible
with a planetary origin for the transits: full transit fits to
the white, red, green, and blue light curves using the transit
formalism of Mandel & Agol (2002), check for differences
in depth between odd- and even-numbered transits, search
for possible companion occultation at the transit antiphase,
and search for ellipsoidal variations. Because this candidate

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G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit3
Fig.3. Phase-folded CoRoT (top panel) and Euler (bot-
tom panel) transit light curves with the best fit and the
residuals. CoRoT data cover 13 transits with a 512-second
bin and the Euler data one transit with a ∼ 3-minute bin.
Both datasets are joint-fitted (see Sect. 4.1).
passed all these tests, it was put forward for follow-up ob-
servations with high priority.
3. Ground-based follow-up observations
3.1. “On-off” photometry
The point spread function of CoRoT that contains 50% of
the flux has an elongated shape of about 35??×23??. The pho-
tometry is done through an aperture of that size. Owing to
this poor spatial resolution, a deep stellar transit diluted in
the flux of other source(s) also included in the large CoRoT
aperture could mimic a shallow planetary transit. “On-off”
photometry of the transit performed from the ground with
a telescope allowing higher spatial resolution could identify
contaminated eclipsing binaries (Deeg et al. 2009).
“On-off” photometric observations of CoRoT-18 were
performed in November 2010 with the ESA Optical Ground
Station (OGS) 1-m telescope, located at Iza˜ na in Tenerife
(Spain). Alternated short exposures of 30 seconds and long
exposures of 100 seconds were taken, for a total dura-
tion of 28-min observation on-transit and 28-min obser-
vation off-transit. Seven extra sources are detected in a
radius of 30??around the main, brighter target. We per-
formed aperture photometry of the target and neighbor-
ing stars. The main target shows a transit with a depth
0.03±0.01 mag, in agreement with the transit detected with
CoRoT. The seven other targets show stable fluxes, within
precisions ranging from 0.01 to 0.39 mag, depending on the
target and the exposure time. This “on-off” observations
thus allowed us to exclude the detected transit signature
caused by an eclipsing binary diluted in the CoRoT point
spread function.
3.2. Radial velocities
We started the spectroscopic follow-up of CoRoT-18 in
October 2010 with the SOPHIE spectrograph at the 1.93-
m telescope of Haute-Provence Observatory, France. Three
measurements performed in three successive nights near ex-
treme phases (assuming a circular orbit) showed large radial
velocity variations, in phase with the CoRoT ephemeris.
The variation was on the order of 1 kms−1, indicating a
companion with a mass around 3MJup. Thus we decided
to pursue the spectroscopic observations with SOPHIE to
strengthen the detection and to characterize the planetary
system. We also used the HARPS spectrograph at the 3.6-
m ESO telescope in La Silla, Chile, and the 2.56-m FIES
spectrograph attached at the Nordic Optical Telescope in
La Palma, Spain. Having three ground-based instruments
at different longitudes was useful for reaching a good phase
coverage for this system, which has an orbital period close
to an integer number of terrestrial days. The observa-
tions were conducted up to January 2011, in good enough
weather conditions to allow satisfactory data to be secured
in reasonable exposure times for this faint target.
Both SOPHIE (Bouchy et al. 2009) and HARPS (Mayor
et al. 2003) are cross-dispersed, environmentally stabilized
echelle spectrographs dedicated to high-precision radial ve-
locity measurements. SOPHIE data were acquired in High-
Efficiency mode (resolution power R = 40000) and HARPS
data in standard HAM mode (R = 115000). The spectra
extraction was performed using the SOPHIE and HARPS
pipelines. Following the techniques described by Baranne
et al. (1996) and Pepe et al. (2002), the radial veloci-
ties were measured from a weighted cross-correlation of
the spectra with a numerical mask. We used a standard
G2 mask that includes more than 3500 lines. The result-
ing cross-correlation functions were fitted by Gaussians to
get the radial velocities and the associated photon-noise er-
rors. The full width at half maximum of those Gaussians is
12.6±0.2 kms−1, and its contrast is 27.7±0.8 % of the con-
tinuum in the case of the HARPS data. The SOPHIE data
gave similar parameters. We adjusted the number of spec-
tral orders used in the cross-correlation in order to reduce
the dispersion of the measurements. Indeed, some spectral
domains are noisy, so using them degrades the accuracy of
the radial velocity measurement. We finally used the orders
10 to 38 for SOPHIE, and 5 to 71 for HARPS.
Moonlight contamination was clearly visible in some
spectra and in some cases at a radial velocity close to that
of the target. Such contamination can affect the radial ve-
locity measurements. Following the method described in
Pollacco et al. (2008) and H´ ebrard et al. (2008), we esti-
mated and corrected the Moon contamination by using the

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4 G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
Fig.4. Upper panel: Radial velocity measurements of
CoRoT-18 with 1-σ error bars as a function of time together
with their Keplerian fit (top) and residuals of the fit. The
fit is described below in Sect. 4.4. Lower panel: Same as
above but as a function of the orbital phase. The data are
from SOPHIE (red circles), HARPS (blue diamonds), and
FIES (green squares).
second optical-fiber aperture, which is targeted on the sky
for both SOPHIE and HARPS, whereas the first aperture
points toward the target. This induces radial velocity cor-
rections up to 400 ms−1.
The FIES observations were performed in January 2011
under clear and stable weather conditions with seeing typi-
cally in the range 0.??8−1.??0. We used the 1.??3 high-resolution
fiber giving a resolving power R ? 67000. Following the
observing strategy described in Buchhave et al. (2010),
three consecutive science exposures of 20 minutes were
recorded for each observing night immediately preceded
and followed by one long-exposed ThAr spectrum. Data re-
duction and spectra extraction were performed using stan-
dard IRAF routines. Finally, the FIES radial velocity mea-
surements of CoRoT-18 were derived cross-correlating the
science spectra with the spectrum of the radial velocity
standard star HD50692 (Udry et al. 1999) observed with
the same instrument set-up.
The log of the observations and the radial velocity mea-
surements are reported in Table 2. Radial velocity measure-
ments have accuracies ranging between 23 and 98 ms−1
depending on the observing parameters. This table also
shows the bisector spans that we measured on the cross-
correlation functions in order to quantify the possible
shape variations of the spectral lines. One SOPHIE spec-
trum was too polluted by the Moon to allow any accurate
bisector measurement.
The radial velocity variations agree with Doppler shifts
caused by a planetary companion, and the transit-signal
detected from the CoRoT light curve could be interpreted
as coming from a massive hot-Jupiter. We designate it as
CoRoT-18b hereafter.
The measurements are displayed in Fig. 4, together with
their circular Keplerian fit, assuming the period and tran-
sit epoch determined by the CoRoT light curve and refined
with the photometric transit observed from the ground (see
Sect. 3.4). The photometric and radial velocity data show
good agreement. SOPHIE and HARPS radial velocities ob-
tained with different stellar masks (F0 or K5) produce vari-
ations with the same amplitude as obtained with the G2
mask, so there is no indication that their variations could
be explained by blend scenarios implying stars of differ-
ent spectral types. Similarly, the cross-correlation function
bisector spans show neither variations nor trend as a func-
tion of radial velocity (Fig. 5). This reinforces the conclu-
sion that the radial velocity variations are not caused by
spectral line profile changes due to blends.
Fig.5. Bisector span as a function of the radial velocities
with 1-σ error bars. The ranges have the same extents in
the x- and y-axes. The data are from SOPHIE (red circles),
HARPS (blue diamonds), and FIES (green squares).
3.3. Transit spectroscopy
A transit of CoRoT-18b was observed in spectroscopy on
2011 January 28. The goal was to detect the Rossiter-
McLaughlin anomaly, which is an apparent distortion of

Page 5

G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit5
the stellar lines profile because of the transit of the planet
in front of the rotating star. It allows the measurement
of the sky-projected angle between the planetary orbital
axis and the stellar rotation axis, usually denoted λ (see,
e.g., Bouchy et al. 2008). The spectroscopic transit was ob-
served with HARPS in the EGGS mode to improve the
throughput. By comparison to the HAM mode of HARPS
used in Sect. 3.2 for the orbit determination (1??diame-
ter fiber with a scrambler, allowing the resolution power
R = 115000 to be reached), the EGGS mode of HARPS
uses a larger fiber (1.??4) without scrambler. The spectral
resolution is then reduced (R = 80000) but the efficiency
gain reaches a factor ∼ 2 improvement.
The target was continuously observed during a 5.5-hour
sequence under good weather condition, with a seeing vary-
ing between 0.??7 and 1.??0. Twelve measurements were se-
cured with exposure times ranging from 1200 to 1800 sec-
onds, including five within the transit. The remaining ob-
servations obtained before and after the transit are manda-
tory for references. The radial velocities were extracted as
for HARPS/HAM data (Sect. 3.2), but using fewer orders
here for the cross-correlation (orders 18 to 69) to reduce
the dispersion of the measurements.
The HARPS/EGGS data are plotted in Fig. 6. The
Rossiter-McLaughlin anomaly is detected, with an ampli-
tude of ∼ 100 ms−1, as expected according the rotation
of the star and the depth of the photometric transit. The
red shift during the first part of the transit and the blue
shift during the second part indicate a prograde orbit. The
symmetry of the feature agrees with an aligned system.
Fig.6. Spectroscopic observation of the 2011 January 28
transit of CoRoT-18b. Top: HARPS/EGGS radial veloc-
ity measurements as a function of the orbital phase (filled
diamonds), Keplerian fit ignoring the transit (dashed line),
and final fit including the model of the Rossiter-McLaughlin
anomaly (solid line). The vertical dotted lines show the
times of mid-transit, first, second, third, and fourth con-
tacts. Bottom: Residuals of the final fit.
3.4. Transit photometry
A transit of CoRoT-18b was observed with the Euler
1.2-m telescope on 2011 January 28 between 01:08 and
05:36 UT, roughly at the same time and location as
the Rossiter-McLaughlin anomaly observed in spectroscopy
with HARPS (see Sect. 3.3). The goal was to refine the
ephemeris. A total of 83 frames were recorded on the re-
cently installed 4K×4K E2V detector, with an exposure
time of three minutes. As for the CoRoT light curve, we
checked that this binning does not significantly affect the
parameter retrieval by using the formalism presented by
Kipping (2010b). Standard calibration images were taken
on the same night. The fluxes from the target and 20 refer-
ence stars were extracted using standard aperture photom-
etry with custom IDL routines. An average reference star
was constructed by interactively selecting the stars that ex-
hibited less real or instrumental variability. Nine stars were
selected for this purpose. The final light curve was normal-
ized to the median of the flux level after the egress of the
transit. It is plotted in the lower panel of Fig. 3.
We considered shot noise as a first estimation of the ac-
curacy of the measurements, which was at the level of 10−3
for this object. The accuracy level is later re-established
by an evaluation of the reduced χ2of the fit process to
take the correlated noise present in the data into account
(see Sect. 4.1). To better estimate the accuracy of the mea-
surements, the dispersion of the data before and after the
transit is 1.7 × 10−3. The bump seen on the light curve
near phase −0.01 could hint that the planet is transiting in
front of a stellar spot; however, the amplitude of this event
is within an the order of magnitude of the correlated noise
so a spot detection cannot be claimed here.
4. Analysis
4.1. Light curves analysis
4.1.1. Initial transit fit and light curve normalization
To remove the modulations in the CoRoT light curve
caused by rotating active regions on the stellar surface (see
the top panel of Fig. 2), we fit second-order polynomials
to stretches of data on either side of each transit spanning
approximately −2 to −1 and +1 to +2 times the transit du-
ration. These sections were then normalized and kept for
further light curve fitting, while flux measurements outside
were discarded from the analysis.
A detailed investigation of the immediate surroundings
of the target (using Digital Sky Survey data) revealed that
2.0±0.1% of the flux in the photometric aperture was con-
tributed by background stars. This was subtracted from the
median out-of-transit flux before re-normalizing. The un-
certainty in the contamination fraction translates into an
effective uncertainty on the derived radius ratio due to con-
tamination of ∼ 0.0001. This is 15 times smaller than the
final uncertainties derived below, so is negligible.
The transits were then modeled using the formalism
of Mandel & Agol (2002), with quadratic limb darkening
coefficients u1 and u2 defined according to the standard
law of the form I(µ)/I(1) = 1 − u1(1 − µ) − u2(1 − µ)2,
where I(1) and I(µ) are the specific intensities at the cen-
ter of the stellar disk and at the angle θ between the
line of sight and the emergent intensity, respectively, and
µ = cos(θ). We performed an initial least-squares fit (us-

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6 G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
Table 2. Radial velocities of CoRoT-18.
HJDUTC
-2455000
484.6704
485.6621
486.6469
505.6367
525.6027†
527.5751†
529.6121†
557.5227†
577.3831†
578.3721†
580.3673†
538.7441
539.7253
547.7838
548.6767†
583.7515†
590.7061
568.4512
569.5086
570.5516
580.5441
581.5089
589.5374
589.5542
589.5721
589.5897
589.6074
589.6261
589.6467
589.6687
589.6909
589.7132
589.7343
589.7565
†: measurements corrected from Moonlight pollution.
RV
±1σ
(kms−1)
0.041
0.062
0.051
0.032
0.086
0.065
0.065
0.096
0.041
0.052
0.094
0.027
0.038
0.023
0.055
0.051
0.051
0.066
0.070
0.076
0.098
0.084
0.052
0.042
0.046
0.048
0.051
0.052
0.045
0.039
0.033
0.037
0.043
0.065
Bis. span
(kms−1)
0.029
0.184
-0.196
-0.074
0.046
-0.128
0.117
-0.209
0.037
0.165
-
-0.043
0.010
-0.011
-0.005
-0.072
-0.087
-0.049
0.024
0.031
-0.010
-0.044
0.065
-0.053
-0.107
-0.037
0.046
0.156
-0.082
0.024
-0.053
-0.168
-0.236
0.002
exp. time
(sec)
3600
3600
3600
3600
2802
3600
3600
3600
3600
3600
3600
3600
3600
3600
3600
3200
3600
3600
3600
3600
3600
3600
1200
1500
1500
1500
1500
1500
1800
1800
1800
1800
1800
1800
S/N p. pix.
(at 550 nm)
16
13
11
15
8
11
10
7
16
15
13
11
9
13
7
8
7
10
8
9
10
11
6
8
8
7
7
7
8
8
10
9
8
5
Instrument/
mode
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
SOPHIE/HE
HARPS/HAM
HARPS/HAM
HARPS/HAM
HARPS/HAM
HARPS/HAM
HARPS/HAM
FIES
FIES
FIES
FIES
FIES
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
HARPS/EGGS
(kms−1)
30.132
29.030
30.069
30.102
28.933
29.006
29.178
29.077
29.756
29.241
29.181
28.979
30.115
29.720
29.324
29.892
29.732
30.075
29.334
29.881
29.075
30.262
29.641
29.719
29.583
29.616
29.727
29.596
29.501
29.415
29.436
29.482
29.329
29.435
ing the Levenberg-Marquardt algorithm) simultaneously to
all 13 CoRoT transits, allowing the following parameters to
vary: the period P, the epochs median of transit centers T0,
the planet-to-star radius ratio Rp/R?, the impact param-
eter b, the scaled semi-major axis a/R?, and the combina-
tions of the limb darkening coefficients u1+u2and u1−u2.
We considered a circular orbit. We discuss below the impact
of possible low eccentricity on our results. The values for the
limb darkening parameters obtained from this analysis were
fully consistent with the values of u1= 0.47 and u2= 0.21
provided by Sing (2010) for a star with Teff = 5500K,
logg = 4.5, and [M/H] = −0.1 (see Sect. 4.2). For the
remainder of our analysis, we fixed the limb darkening pa-
rameters for the CoRoT light curve to these values.
4.1.2. Detailed transit fit
To estimate the photometric uncertainty of the CoRoT
data, we calculated the standard deviation of the out-of-
transit flux values, except those within 30 minutes of the
ingress and egress of our initial fit. This provided an esti-
mate of σ = 1.7mmag, which was then used to perform
a 100000-step Metropolis-Hastings Markov Chain Monte
Carlo (MCMC) analysis with the parameters P, T0, b,
a/R?, and Rp/R?allowed to vary, using our initial best-fit
values from above as the starting points. We adjusted the
jump scales for the free parameters until a step acceptance
rate of 25-35% was achieved. The photometric uncertain-
ties were then scaled up to σ = 1.9mmag to give a reduced
χ2of unity for the best-fit MCMC solution. This upwards
scaling of the photometric uncertainties can be attributed
to the presence of correlated noise in the CoRoT light curve
(Pont et al. 2006), often expressed as
σ2
N= σ2
W/N + σ2
R,(1)
where σ2
bins containing N data points, and σW and σRrepresent
the “white” and correlated “red” components of the noise,
respectively. The uncertainty on individual flux values is
then given by σ = σN for N = 1, so that σ =
The top panel of Fig. 7 shows calculated values for σ as
a function of N for the CoRoT light curve. The variation
between the mean flux values of bins containing only a sin-
gle point (N = 1) is approximately 1.9mmag, equal to the
value that produces a reduced χ2of unity for the best-fit
model. It also shows the expectation for a white-noise-only
model that is forced to pass through the point at N = 10.
For lower values of N, the white-noise-only model is not
Nis the variance between the mean flux values of
?σ2
W+ σ2
R.

Page 7

G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit7
Fig.7. The standard deviation σN of measured out-of-
transit flux values as a function of the number of points
N for the CoRoT light curve (top panel) and the Euler
light curve (bottom panel). The dark curves indicate the
best-fit models without red noise, i.e. only white noise.
able to account for all of the variation in the data. This
illustrates why it is necessary to scale the photometric un-
certainties upwards from the standard deviation of the out-
of-transit flux values.
A similar procedure was then repeated for the Euler
light curve. We first estimated the photometric uncer-
tainty to be equal to the standard deviation of the out-
of-transit measured flux values, which was calculated as
σ = 1.9mmag. We then performed a 100000-step MCMC
analysis, allowing P, T0, b, a/R?, and Rp/R?to vary. The
limb darkening parameters u1 and u2 were also allowed
to vary, but this resulted in poor chain convergence. For
this reason, we decided to set their values to u1= 40 and
u2= 30, as provided by Claret (2004) for a star with the
same parameters as above. To investigate the effect of fixing
the limb darkening parameters, we experimented with fix-
ing them to other values provided by Claret (2004) for stars
with similar properties to CoRoT-18 and then performing
a least squares fit to the light curve. In all cases, the fit-
ted value for Rp/R?changed, as expected, while the other
parameters remained constant within the uncertainties. In
particular, the fitted values for P and T0were unaffected by
varying the limb darkening parameters. This is important
because the primary purpose of the Euler light curve is to
refine the ephemeris of the orbit (see below). We also ver-
ified that the choice of limb darkening parameters for the
Euler light curve did not affect the derived values for the
other parameters in the simultaneous fitting to the CoRoT
and Euler light curves, which is described below. This was
done by ensuring that the fitted values for Rp/R?, a/R?
and b remained the same regardless of whether the Euler
light curve was included in the fit.
The bottom panel of Fig. 7 shows the variation between
binned flux values as a function of bin number for the Euler
light curve. It shows strongly correlated noise on time scales
of ∼ 10−20 minutes (between two to five exposures ), which
decreases significantly on longer timescales. This behavior
is poorly modeled by Equation 1 above, with the solid line
showing an illustrative fit for the white-noise-only case, i.e.
σR = 0. Instead, we scaled the photometric uncertainties
up to σ = 2.1mmag, which was the value required to give
a reduced χ2of unity for the best-fit model.
We next performed a joint MCMC analysis on the
CoRoT and Euler light curves. This was done by initiating
five independent chains at random locations in parameter
space approximately ∼ 5σ away from the best-fit values
determined from the initial fitting process. The period was
held fixed to the value determined from the initial best-
fit to the CoRoT light curve, but we allowed the transit
midtimes to vary for each transit in order to investigate
the possibility of transit timing variations (see Sect. 4.1.3).
Values for a/R?, Rp/R? and b were allowed to vary with
the same values across all transits, and the limb darkening
coefficients were held fixed to the values described above.
The five chains were run in parallel for 200000 steps.
The initial 40000 steps of each chain were then discarded
to allow for the initial burn-in phase as the chains set-
tled. We also checked when the χ2first dropped below the
chain’s median value, indicating the point at which the local
neighborhood of the final solution had first been reached.
Typically, this occurred within about 20000 steps, imply-
ing that the truncation we made at 40000 steps is in fact
conservative. The Gelman-Rubin statistic was then calcu-
lated using the truncated chains for each of the free param-
eters and, in all cases, found to be within approximately 1%
of unity, indicating good mixing and convergence (Gelman
& Rubin 1992). The chains were then combined to obtain
marginalized posterior distributions for each of the free pa-
rameters, with medians and 1-σ uncertainties reported in
Table 3 and examples plotted in Fig. 8. The upper and
lower 1σ uncertainties respectively refer to the upper and
lower bounds on the intervals containing 34.1% of the chain
steps on either side of the median.
A refined estimate for the orbital period was then ob-
tained by fitting a model of the form
?TB− TA
where T?
nthorbit, TAis the epoch of the first CoRoT transit and
TB is the epoch of the Euler transit, which is 173 or-
bital periods later. Then we ran two 200000-step MCMC
chains (one for TA and one for TB) and calculated the
corresponding chain for the estimated period according to
T?
0(n) = TA+ n
173
?
, (2)
0(n) denotes the transit midtime measured for the

Page 8

8G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
Fig.8. Examples of the results of the MCMC analysis with scatter plots showing the correlations between fitted param-
eters. Values corresponding to the best-fit (lowest χ2) solutions are indicated by red lines, while solid blue lines indicate
the median values obtained for each parameter, and the dashed blue lines give the upper and lower 1-σ uncertainties.
The best and median solutions coincide for several parameters.
P = (TB− TA)/173. As expected from P´ al (2009), the
correlation between TA and P chains was negative, while
the correlation between the TBand P chains was positive.
We then calculated an optimal epoch using Equation 97
of P´ al (2009), which roughly corresponds to the median
transit epoch, and find Topt = TA+ 32 × P. The new
chain for this epoch led to the following median value
for the optimal epoch, with associated 1-σ uncertainties:
T0= Topt= 2455321.72412±0.00018 HJD. By comparison
to the classical computation using for example the epoch
of the first transit, this method with the optimal epoch al-
lows the uncertainty on T0to be decreased by 2 sec, as well
as the reduction of the correlation between T0and P (see
Fig. 8). The final ephemeris values are reported in Table 3
and plotted in Fig. 8. The best-fit solution for the combined
data set has a reduced χ2of 1.01.
Finally, we estimated the effect of introducing a
small eccentricity in the fit of the light curve. Indeed,
Kipping (2010a) has shown that assuming e = 0 for an
eccentric orbit could lead to underestimated uncertainties
to a/R∗ratio and stellar density ρ∗. We derive below the
upper limit e < 0.08 at 95% confidence from the radial
velocity measurements (see Sect. 4.4). We performed new
fits of the CoRoT light curve assuming this extreme eccen-
tricity, with different possible values for the longitude ω of
the periastron. Most extreme fits could provide a/R∗and
ρ∗ values at most to 2σ lower the values derived above.
Thus we slightly decreased the final values on these pa-
rameters and increased their uncertainties to account for a
small possible eccentricity.
4.1.3. Timing analysis
The 13 successive transits observed by CoRoT and the
observation of the additional transit with Euler offer
an opportunity for transit timing variations (TTVs) re-
search (Holman & Murray 2005). Figure 9 shows the TTVs
measured by
∆T0(n) = T?
0(n) − T0,(3)
where T?
for the nthorbit and T0denotes the refined transit midtime
derived by fitting to the ephemeris equation, as described
above. We obtain a reduced χ2of 1.3 for the hypothesis that
there are no TTVs, i.e. ∆T0(n) = 0 for n = 1,...,14. When
the ninth transit is removed (visible as the most discrepant
point in Fig. 9) the reduce χ2improves to 0.98. Inspection
0(n) again denotes the transit midtime measured

Page 9

G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit9
of this particular transit reveals that neither the ingress
nor the egress was sampled, which could help explain why
the measured TTV is somewhat discrepant. This is also
the case for the fifth transit. Also, it is possible that the
uncertainties in the measured transit midtimes are slightly
underestimated because we did not account for unocculted
spots that could result in different values for the effective
radius ratio Rp/R?being measured from transit-to-transit
as the spot coverage evolves; instead, we held Rp/R?fixed
for all transits in the MCMC analysis. In either case, given
the perfect agreement of the Euler ephemeris with all of
the other CoRoT ephemerides, we conclude that there is
no evidence of TTVs in the combined data set. Given that
the characteristic uncertainty on our TTV measurements
was about 60 seconds, we obtain a 3-σ TTV upper limit
of 180 seconds.
Fig.9. Transit timing variation ∆T0 for each transit of
CoRoT-18b. The 13 transits observed with CoRoT (on the
left) were obtained about ten months before those observed
with Euler (on the right). The 5thand 9thCoRoT transits
do cover neither the ingress nor egress and therefore could
conduct to erroneous timing measurements.
4.1.4. Rotation period
The CoRoT white-light curve of CoRoT-18 shown in the
top panel of Fig. 2 displays obvious signs of rotational mod-
ulation of star spots, with at least two large active regions
present on the stellar surface at any one time. To estimate
the rotation period we first cut out a section lasting twice
the transit duration around each transit, then fit a straight
line to the out-of-transit light curve to remove any long-
term trend. We also corrected for a discontinuity near the
end of the light curve by subtracting 15mmag from all data
taken after the discontinuity. This detrending procedure is
slightly different from the one presented above in Sect. 4.1.
We estimated the rotation period by fitting sinusoids
at 1000 trial periods ranging from 1 to 15 days to the cor-
rected out-of-transit data. The resulting best-fit amplitudes
are shown as a function of trial period in the bottom panel
of Fig. 2. The dominant peak clearly corresponds to the
interval between repeat appearances of individual active
regions, hence to the rotation period. There is also signif-
icant power at the first harmonic of the rotation period.
We do not expect aliases here from the CoRoT satellite or-
bital period, because it is well below the Nyquist sampling
frequency of the data. We checked for that by binning the
light curve to one point per orbital period (upper panel of
Fig. 2) to make sure that no systematics from the satellite
rotation period can come through, and repeated the study.
The results were identical.
To refine the estimate of the rotation period, we fit a
Gaussian to the periodogram around the main peak, shown
in the lower panel of Fig. 2, and adopted the standard de-
viation of the Gaussian as our period uncertainty, yielding
Prot= 5.4 ± 0.4 days. This period is quite short for a late
G-type star: even at the age of the Hyades, typical rotation
periods for this spectral type are in the range of eight to
nine days (Kawaler 1989).
4.1.5. Planetary occultation upper limit
We derived the depth upper limit of a possible occultation
of the planet by the star by fitting a Mandel & Agol (2002)
transit model at phase 0.5, with no limb darkening and a
transit depth reduced by a factor Fp/F∗, where Fpis the
planetary flux and F∗the stellar flux. All parameters were
kept fixed to the best-fit values derived from the transit,
except for the eclipse depth, for which we obtain a best-fit
value of 0.05 ± 0.19mmag, i.e. consistent with zero. The
resulting 3-σ upper limit is 0.61mmag. This is not partic-
ularly stringent, as one can expect a planetary to star flux
ratio of 0.40mmag in the optimistic case of a geometric
albedo equal to one.
4.2. Stellar analysis and classification
To determine the planetary parameters with an as high as
possible precision we need to know the physical conditions
of its host star. Seager & Mall´ en-Ornelas (2003), among
others, have shown that ideally there is one stellar parame-
ter, the stellar density, which can be obtained from a tran-
sit light curve of sufficient photometric precision. From this
parameter, it is possible (with a number of assumptions)
to derive, through modeling, other physical parameters of
the system. Nevertheless, as pointed out in this context by
e.g. Fridlund et al. (2010), high-precision photometric and
spectroscopic measurements that has been carried out on
other exoplanet host stars, do suggest that this rarely infers
reliably to the other properties of the star – mainly because
of flaws in stellar theory (Winn et al. 2008).
We used the two sets of HARPS observations to perform
this analysis: the HARPS/HAM data (co-addition of six
spectra totaling 5.8 hours of integration, see Sect 3.2) and
the HARPS/EGGS data (co-addition of 12 spectra total-
ing 5.4 hours of integration, see Sect 3.2). Due to its lower
resolution power (80000 vs. 115000) the HARPS/EGGS
data presents the higher signal-to-noise ratio (∼ 35 in the
continuum at Hα). We analyzed both sets of observations
and find no significant differences in the stellar parameters
beyond the internal 1-σ error. Three observations are im-
mediately made while inspecting the co-added spectra. The
appearance is that of a cool star, the line profiles are rela-
tively broad (v sini∗= 8.0±1.0 kms−1), and there is a faint
absorption (equivalent width of ∼ 40m˚ A) at the location
of the Lii (6707.8˚ A) line. There is no obvious detection of

Page 10

10 G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
any Caii chromospheric emission. This v sini∗direct mea-
surement agrees with this derived from the cross-correlation
function following the Santos et al. (2002) methodology.
To determine the spectroscopic parameters, we used
the Spectroscopy Made Easy (SME, version 162, February
2011) software package (Valenti & Piskunov 1996; Valenti
& Fischer 2005). SME calculates synthetic spectra and fits
the observations to it. All the normal stellar parameters
(Teff, logg, v sini∗, [Fe/H], abundances, etc.) can be used
either as input or as free parameters to solve for. A grid
of stellar models (Kurucz models) was utilized in order to
determine the fundamental stellar parameters iteratively.
This was achieved by fitting the observed spectrum directly
to the synthesized spectrum and minimizing the discrep-
ancies using a nonlinear least-squares algorithm. SME re-
quires atomic line data in order to synthesize a spectrum.
We utilized input from the Vienna Atomic Line Database
(Kupka et al. 1999; Piskunov et al. 1995).
Using SME and a sample of more than 1000 stars,
Valenti & Fischer (2005) found uncertainties of 44K in Teff,
0.06 dex in logg, and 0.03 dex in [M/H] per measurement.
Based on the CoRoT material (stars hosting CoRoT plan-
ets or candidates that have not been positively shown to
host planets), we find slightly larger errors than Valenti &
Fischer (2005): 70−100K in Teff, depending on the signal-
to-noise in the continuum of the spectrum at the location
of the Balmer lines, 0.05 − 0.1dex in logg, depending also
on the spectral type and on which ion we used, and fi-
nally 0.05 − 0.1dex in [M/H]. However, by comparing the
measurements with model isochrones they found a larger,
systematic offset of ∼ 0.1dex and a scatter that can oc-
casionally reach 0.3dex in logg. In CoRoT-18, we find an
internal discrepancy using SME of 0.1dex. We therefore
assign 0.1dex as our 1-σ precision.
We found Teff= 5443K ± 100K from the profile of the
Balmer lines. We determined the metallicity and found con-
sistent results from different ions indicating a star of slightly
lower than Solar metallicity: [M/H] ? −0.1±0.1. The logg
was determined utilizing the lines of Cai, Mgi and Nai,
finding a consistent result of 4.4 ± 0.1. The spectroscopic
parameters of CoRoT-18 are summarized in Table 3.
4.3. Stellar evolution tracks and the age problem
Altogether, CoRoT-18 seems remarkably similar to CoRoT-
2 (Alonso et al. 2008): the two stars (CoRoT-18 vs.
CoRoT-2) have comparable effective temperatures (5440
vs. 5450 K), metallicities (−0.1 vs. 0.0), spin periods (5.4
vs. 4.5 days), and v sini∗(8.0 vs. 12 kms−1), and they are
both active, with peak-to-peak photometric variabilities of
∼ 2% and ∼ 4%, respectively. In addition CoRoT-18b and
CoRoT-2b are the only known planets (transiting or not)
orbiting a star colder than 6000 K and with a large v sini∗.
The other planet-host stars in this temperature range all
have v sini∗ values in the range [0 − 5] kms−1. All the
planet-host stars having v sini∗∼ 10 kms−1or larger are
F-stars – except CoRoT-18 and CoRoT-2.
However, the inferred stellar densities for CoRoT-18 and
CoRoT-2 (1.35±0.25 vs. 1.814+0.050
al. 2010) slightly differ. As a result, the effective tempera-
ture, metallicity, and density constraint for CoRoT-18 are
consistent with evolution tracks for solar-mass stars that
are either particularly young and still on the pre-main se-
quence or old and towards the end of the main-sequence
−0.045gcm−3– see Gillon et
evolution (see Figs. 2-4 from Guillot & Havel 2011). We
used the CESAM evolution code (Morel & Lebreton 2009)
to calculate these evolution tracks. In Fig. 10 we plot the
solutions in the stellar-mass, age parameters, with col-
ors that depend on their quadratic distance to the effec-
tive temperature-stellar density constraints. These point
towards either a young age, less than 30 Ma (at 3σ), or
an old one, more than 8 Ga at 1σ and more than 4 Ga at
3σ. The situation is thus different than for CoRoT-2, for
which a continuum of young and late-type solutions was
found (see Guillot & Havel 2011). For CoRoT-18, these so-
lutions contradict with the other age indicators.
Fig.10. Constraints obtained from stellar evolution models
for the age and mass of CoRoT-18. The colored circles cor-
respond to constraints derived from stellar evolution mod-
els matching the stellar density and effective temperature
within a certain number of standard deviations: less than
1σ (green), 2σ (blue), or 3σ (yellow).
First, as for CoRoT-2, the rapid rotation of the star
points towards a young age. Bouvier (2007) has compiled
hundreds of rotational period measurements from photo-
metric surveys of young open clusters with ages up to
625 Ma (Hyades), and used them to model the rotational
evolution of stars in several mass bins. A range of models
is needed to reproduce the data at any given age, and con-
straints are scarce beyond 200 Ma for stars with masses be-
tween 0.8 and 1M?. However, extrapolation of the models
calibrated at earlier ages suggests that such stars are not ex-
pected to retain rates ∼ 5 times faster than the Sun (which
is the case for CoRoT-18) beyond ages of 500-600Ma.
Second, the lithium equivalent width favors the latter
end of this age range. According to a recent compilation of
Li depletion measurements, also from stars in young open
clusters (Hillenbrand et al. 2009), the equivalent width of
40m˚ A measured for CoRoT-18 is typical of stars of this
spectral type at the age of Ursa Majoris (500Ma) or the
Hyades (625Ma), while typical equivalent widths for similar
stars in M34 and M7 (∼ 200Ma) are about 100m˚ A. This

Page 11

G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit11
would thus instead suggest an age of several hundred Ma
for CoRoT-18.
Thus, for now we are unable to estimate the age of
CoRoT-18, even if it seems to be young. This illustrates
the difficulty in determining the age of stars. We adopt
the stellar mass M? = 0.95 ± 0.15M?. The conservative
error bar is large enough to agree with both pre-main-
sequence case and old star main-sequence star. This im-
plies a semi-major axis a = 0.0295±0.0016, a stellar radius
R? = 1.00 ± 0.13R?, and thus a planetary radius Rp =
1.31±0.18RJup. The equilibrium temperature of the planet
assuming an isotropic zero-albedo is Teq= 1550±90K. The
rotation period 5.4±0.4 days (Sect. 4.1.4) is consistent with
the high value v sini∗= 8.0 ± 1.0 kms−1and is fast for a
late G-type star. Using R? = 1.00 ± 0.13R?, this trans-
lates into an inclination of the stellar rotation axis that is
i∗= 70◦± 20◦, so the star is seen nearly edge-on.
4.4. Radial velocities fit
We fitted the radial velocities with a Keplerian model. The
period and the epoch of the transit were fixed to the val-
ues obtained from the light curves analysis (Sect. 4.1). If
the relative accuracy of the radial velocity measurements is
high (a few tens of ms−1here in the case of CoRoT-18),
their absolute accuracy in heliocentric or barycentric frames
could be ten times less good, so a radial velocity shift was
free to vary in the fit between the three datasets used for
the orbit (SOPHIE, HARPS/HAM, and FIES), and we fi-
nally obtained one systemic radial velocity for each of the
three instruments. The HARPS/EGGS data secured during
a transit do not significantly constrain the orbit, and they
are used below for the analysis of the Rossiter-McLaughlin
anomaly. We found the upper limit e < 0.08 at 95% confi-
dence for the eccentricity of the orbit and thus assumed a
circular orbit, as usually is the case for hot jupiters. In the
case of a slightly eccentric orbit, its orientation is not well
constrained, with the longitude of the periastron included
in the range −60◦< ω < 100◦.
The final fit of the radial velocities is plotted in Fig. 4.
The derived orbital parameters are reported in Table 3,
together with error bars that were computed from χ2vari-
ations and Monte Carlo experiments. The radial velocity
variations present a semi-amplitude K = 590 ± 14 ms−1,
corresponding to a planet with a mass Mp = 3.47 ±
0.38 MJup. This assumes M?= 0.95±0.15M?for the host
star, which here is the main source of uncertainty on Mp.
The standard deviation of the residuals to the fit is
σO−C= 41.0 ms−1for the whole dataset (35.9, 44.4, and
43.0 ms−1for SOPHIE, HARPS/HAM, and FIES, respec-
tively). The reduced χ2is 1.02 for the 22 radial veloci-
ties used in the fit. We do not detect any drift over the
106-day span of the radial velocity, with an upper limit of
200 ms−1a−1at 95% confidence. We can thus exclude any
extra planet in the system with a mass higher than 3MJup
and a period shorter than 200 days.
4.5. Planetary evolution
We have seen that CoRoT-18 and CoRoT-2 are similar for
what concerns their stars. Their planets (CoRoT-18b vs.
CoRoT-2b) are also strikingly similar, in terms of orbital
periods (1.90 vs. 1.74 days), masses (3.4 vs. 3.7MJup), and
equilibrium temperatures (1550 vs. 1539K). The inferred
planetary radii are 1.31 ± 0.18RJup for CoRoT-18b and
1.466+0.042
−0.044RJupfor CoRoT-2b (Gillon et al. 2010).
CoRoT-18b
Increased atmospheric opacities
Standard model
CoRoT-2b
w/ 1029 erg/s dissipation
Fig.11. Constraints obtained on the age and radius of
CoRoT-18b. The colored circles correspond to 1σ (red), 2σ
(yellow), or 3σ (blue) solutions. The figure also plots the
same constraints for CoRoT-2b (grayscale), when not in-
cluding the effects of spots (see Czesla et al. 2009, Guillot &
Havel 2011). The evolution tracks show the progressive con-
traction of a 3.5MJupplanet with Teq∼ 1600K, in the so-
called “standard approach”, when increasing atmospheric
opacities by a factor 3.5, and when dissipating 1029ergs−1
at the center (see Guillot & Havel 2011 for a description of
the models).
Figure 11 compares the planetary radii obtained as a
function of age for the two planets, using the approach de-
scribed in Guillot & Havel (2011). To use the same ap-
proach for the two planets, when calculating the size of
CoRoT-2b, we did not account for the effect of spots (see
Czesla et al. 2009, Guillot & Havel 2011) – in the case
of CoRoT-2b, these yield a ∼ 3% increase in the inferred
radius. For CoRoT-2b, the age constraints from evolution
models matching (Teff, ρ∗, logg, [Fe/H]) are weak and 3σ
solutions are found anywhere between 20Ma to more than
15Ga). The inferred radius is extremely large for an object
of this mass and planetary evolution models require a re-
cent (∼ 20Ma) dramatic event (birth, giant impact, close
encounter, and circularization of the orbit) to explain it.
Because of its lower stellar density, solutions for CoRoT-18
are found at both extremes of the age range (10 to 25Ma
and more than 4Ga, see Sect. 4.3). The constraints on the
size of CoRoT-18 are weaker so that they cannot be used
to confirm or refute possible inflation mechanisms or pos-
sible compositions, at least as long as the cause of the age
mismatch between stellar evolution models and youth indi-
cators (see Sect. 4.3) is not found.
If the mismatch found for CoRoT-18 between the var-
ious age indicators can be linked to the poorly known
physics of young stars, it may lead to completely reconsider
the problem of the inflated size of CoRoT-2b and other ex-
oplanets orbiting young stars.

Page 12

12 G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
4.6. Rossiter-McLaughlin anomaly analysis
The radial velocity measured during the 2011 January 28
transit were fitted in order to derive the sky-projected angle
λ between the planetary orbital axis and the stellar rota-
tion axis. We applied the analytical approach developed
by Ohta et al. (2005) to model the Rossiter-McLaughlin
anomaly shape, which use here ten parameters: the stel-
lar limb-darkening linear coefficient ?, the transit param-
eters Rp/R∗, a/R∗ and i, the parameters of the circular
orbit (P, T0, and K), the HARPS/EGGS systemic radial
velocity, and finally v sini∗and λ. We adopted ? = 0.722
computed by Claret (2004) in the g?filter corresponding
to the HARPS wavelength range. The transit and orbital
parameters were determined above from the light curves
and radial velocity fits, and their uncertainties are negligi-
ble for the fit of the Rossiter-McLaughlin anomaly shape,
according the uncertainties of the HARPS/EGGS radial ve-
locities. The main parameters that play a role in this fit are
the systemic velocity, λ, and v sini∗. As these parameters
are correlated in the Rossiter-McLaughlin fit, we computed
the χ2of the fit on a three-dimensional grid scanning their
possible values.
The systemic velocity we obtained for the transit ob-
served with HARPS/EGGS, 29.550 ± 0.016, is similar to
this obtained in Sect. 4.4 for the orbit observed with
HARPS/HAM. It is constrained thanks to the observations
secured immediately before and after the transit. The con-
fidence interval contours estimated from χ2variations for
the λ and v sini∗are plotted in Fig. 12. We thus obtained
λ = −10◦±20◦and v sini∗= 8.5±2.5 kms−1. The best fit
is plotted in Fig. 6. The dispersion of the residuals to the
fit is 46.5 ms−1. This agrees with the expected error bars
on the radial velocities, and this is similar to the dispersion
of the residuals to the fit of the orbit presented in Sect. 4.4.
Fig.12. χ2isocontours for our modeling of the Rossiter-
McLaughlin effect as a function of λ and v sini∗. The dia-
mond shows the lowest-χ2value.
The Rossiter-McLaughlin
stands on a few points; however, the shape of the ra-
dial velocity variations during the transit agree with a
Rossiter-McLaughlin feature, with λ ? 0◦and the ex-
anomalydetection only
pected amplitude. As a statistical test for the Rossiter-
McLaughlin anomaly detection, we computed the χ2over
the 12 measurements secured during the transit night, and
we get 12.5 and 24.7 for the fits including or not the
Rossiter-McLaughlin anomaly, respectively. Including the
Rossiter-McLaughlin model in the fit thus implies a fac-
tor two improvement in the χ2, for basically two extra
free parameters (λ and v sini∗, which mainly constraints
the Rossiter-McLaughlin shape). We performed an F-test
which indicates there is a probability > 70% that the χ2
improvement actually is due to the Rossiter-McLaughlin
anomaly detection.
Usually only the sky-projected value λ of the obliq-
uity ψ could be measured because the inclination i∗ of
the stellar rotation axis remains unknown. Here we esti-
mated i∗ = 70◦± 20◦(Sect. 4.2), so the sky-projected λ
could be translated into the actual obliquity. We obtain
ψ = 20◦± 20◦. This value remains inaccurate, due to the
significant uncertainties on λ and i∗. It allows, however, the
conclusion that the orbit of CoRoT-18b is prograde and
nearly aligned. This additionnaly reinforces the similarity
between the CoRoT-18 and CoRoT-2 planetary systems,
since CoRoT-2b is also prograde and aligned (Bouchy et
al. 2008).
The v sini∗value obtained from this fit agrees with this
obtained in Sect. 4.2 from spectral analysis, v sini∗= 8.0±
1.0 kms−1. As discussed by, e.g., Hirano et al. (2010) and
Simpson et al. (2011), modeling the Rossiter-McLaughlin
anomaly could produce biased v sini∗measurements when
rotational broadening of the stellar lines is significantly
larger than the instrument resolution. We do not see that
effect here, possibly because of the long exposure times of
each exposure.
5. Conclusion
We reported the detection of the 18thtransiting exoplanet
detected by the CoRoT project. This giant planet was
discovered thanks to the high-accuracy, continuous pho-
tometry obtained by the CoRoT satellite and the photo-
metric and spectroscopic follow-up performed on ground-
based telescopes. CoRoT-18b is a massive hot jupiter or-
biting a faint G9V star. Its mass is Mp= 3.47±0.38MJup,
and its radius Rp = 1.31 ± 0.18RJup, implying a density
ρp = 2.2 ± 0.8g/cm3. The period of the circular orbit is
1.9000693 ± 0.0000028 days. It is known with an accuracy
better than 0.25 seconds and the mean mid-transit epoch
with an accuracy of 20 seconds. The mass of the host star
is M?= 0.95±0.15M?and its radius R?= 1.00±0.13R?.
The parameters of this system are summarized in Table 3.
The parameters of CoRoT-18b are similar to those of
CoRoT-2b (Alonso 2008; Gillon et al. 2010), and to a lesser
extent to those of CoRoT-11b (Gandolfi 2010) and CoRoT-
17b (Csizmadia et al. 2011), the other massive hot jupiters
found with CoRoT. Interestingly CoRoT-18b is found to be
either particularly young (a few tens of Ma) or old (> 4Ga)
from stellar evolution models matching the star’s effective
temperature and inferred density, but according both to
its lithium abundance and to its relatively fast rotation,
it would be expected to be modestly young (several hun-
dred Ma). This mismatch potentially points to a problem
in our understanding of the evolution of young stars, with
possibly significant implications for stellar physics and the
interpretation of inferred sizes of exoplanets around young

Page 13

G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit13
Table 3. Planet and star parameters.
Fitted transit parameters
Planet orbital period P [days]
Transit center epoch T0 [HJD]
Scaled semi-major axis a/R∗
Radius ratio k = Rp/R∗
Impact parameter b = acosi/R∗
1.9000693 ± 0.0000028
2455321.72412 ± 0.00018
6.35 ± 0.40
0.1341 ± 0.0019
0.40+0.08
−0.14
Deduced transit parameters
Orbit inclination i [◦]
Transit duration T1−4 [h]
Egress/ingress duration T1−2 = T3−4 [h]
M1/3
∗
Stellar density ρ∗ [g cm−3]
86.5+1.4
2.387 ± 0.037
0.331 ± 0.038
0.99 ± 0.06
1.35 ± 0.25
−0.9
/R∗ [Solar units]
Results from radial velocity observations
Radial velocity semi-amplitude K [ms−1]
Orbital eccentricity e
SOPHIE systemic velocity [kms−1]
HARPS/HAM systemic velocity [kms−1]
FIES systemic velocity [kms−1]
O-C residuals [ms−1]
590 ± 14
< 0.08
29.533 ± 0.016
29.572 ± 0.015
29.657 ± 0.034
41.0
Spectroscopic stellar parameters
Effective temperature Teff[K]
Surface gravity logg [dex]
Metallicity [Fe/H] [dex]
Rotational velocity v sini∗ [kms−1]
Spectral type
Star mass [M?]
5440 ± 100
4.4 ± 0.1
−0.1 ± 0.1
8.0 ± 1.0
G9V
0.95 ± 0.15
Stellar and planetary parameters
Star radius [R?]
Distance of the system [pc]
Stellar rotation period Prot [days]
Stellar inclination i∗ [◦]
Orbital semi-major axis a [AU]
Planet mass Mp [MJ ]
Planet radius Rp[RJ]
Planet density ρp [g cm−3]
Equilibrium temperature Teq [K]
1.00 ± 0.13
870 ± 90
5.4 ± 0.4
70 ± 20
0.0295 ± 0.0016
3.47 ± 0.38
1.31 ± 0.18
2.2 ± 0.8
1550 ± 90
Rossiter-McLaughlin parameters
Sky-projected obliquity λ [◦]
HARPS/EGGS systemic velocity [kms−1]
O-C residuals (HARPS/EGGS) [ms−1]
Obliquity ψ [◦]
10 ± 20
29.550 ± 0.016
46.5
20 ± 20
stars. In addition, CoRoT-18b and CoRoT-2b are the only
known planets (transiting or not) orbiting a fast-rotating,
G-type or cooler star.
The orbit of CoRoT-18b is prograde, with a spin-orbit
angle ψ = 20◦±20◦(sky-projected value λ = −10◦±20◦),
hence an obliquity in agreement with 0. Schlaufman (2010)
and Winn et al. (2010b) have shown that misaligned plan-
ets tend to orbit hot stars. With an effective temperature of
5440 ± 100K for the host star, the CoRoT-18 system sup-
ports this trend. Exceptions to this trend are increasing,
however (Moutou et al. 2011; Brown et al. 2011).
H´ ebrard et al. (2010; 2011) have hypothesized that most
of the massive planets are prograde and moderately but sig-
nificantly misaligned, whereas the less massive planets are
distributed in two thirds of the prograde, aligned systems
and one third of the strongly misaligned systems. Being pro-
grade and nearly aligned, CoRoT-18b is at the upper limit
of the low-mass range. Interestingly, a limit is also appar-
ent in the mass distribution of known transiting planets.
This is shown in Fig. 13, where one can see a decreasing
abundance of planets with increasing planetary mass up
to 4.5MJup. No transiting planets are known in the range
Mp= [4.5−7]MJup, and a few are known with Mp> 7MJup.
It is difficult to imagine that a bias is provoking the lack
of massive planets, as they are easier to detect. This differ-
ent distribution suggests that planets below 4.5MJupcould
have a different nature or history than those above 7MJup.
This is reinforced by the different obliquity distribution.

Page 14

14 G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit
Fig.13. Histogram of the number of known transit-
ing planets as a function of their mass (data from
http://exoplanet.eu).
Acknowledgements. The French teams are grateful to the CNES for
its constant support and the funding of AB, JMA, CC. IAP/OHP
team acknowledges support of French National Research Agency
(ANR-08-JCJC-0102-01). The team at the IAC acknowledges sup-
port by grants ESP2007-65480-C02-02 and AYA2010-20982-C02-02
of the Spanish Ministry of Science and Innovation (MICINN). The
German CoRoT Team (TLS and University of Cologne) acknowl-
edges DLR grants 50OW0204, 50OW0603, and 50QM1004. We are
grateful to N. Piskunov of the Uppsala Astronomical Observatory
for continuing to make SME available to us and for answering ques-
tions about its implementation and operation. SOPHIE observations
(program 10B.PNP.MOUT) were done on the 1.93-m telescope at
Observatoire de Haute-Provence (CNRS), France. HARPS observa-
tions (program 184.C-0639) were done on the 3.6-m telescope at the
ESO La Silla Paranal observatory, Chile. FIES observations (program
P42-216) were done on the Nordic Optical Telescope, operated on the
island of La Palma jointly by Denmark, Finland, Iceland, Norway, and
Sweden, in the Spanish Observatorio del Roque de los Muchachos of
the Instituto de Astrofisica de Canarias.
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1Institut
Universit´ e Pierre & Marie Curie, 98bis boulevard Arago,
75014 Paris, France e-mail: hebrard@iap.fr
2Observatoire de Haute-Provence, CNRS/OAMP, 04870
Saint-Michel-l’Observatoire, France
3Department of Physics, Denys Wilkinson Building Keble
Road, Oxford, OX1 3RH, UK
4Observatoire de l’Universit´ e de Gen` eve, 51 chemin des
Maillettes, 1290 Sauverny, Switzerland
5Research and Scientific Support Department, European
SpaceAgency,Keplerlaan1,
The Netherlands
6School of Physics and Astronomy, Raymond and Beverly
Sackler Faculty of Exact Sciences, Tel Aviv University, Tel
Aviv, Israel
7Observatoire de la Cˆ ote d’Azur, Laboratoire Cassiop´ ee, BP
4229, 06304 Nice Cedex 4, France
8Laboratoire d’Astrophysique de Marseille, 38 rue Fr´ ed´ eric
Joliot-Curie, 13388 Marseille cedex 13, France
9Instituto de Astrof´ ısica de Canarias, and Universidad
de La Laguna, Dept. de Astrof´ ısica, 38205 La Laguna,
Tenerife, Spain
10LESIA, Observatoire de Paris, Place J. Janssen, 92195
Meudon cedex, France
11Institut d’Astrophysique Spatiale, Universit´ e Paris XI, 91405
Orsay, France
d’Astrophysiquede Paris,UMR7095 CNRS,
NL-2200AG, Noordwijk,

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G. H´ ebrard et al.: CoRoT-18b, a massive hot jupiter on a prograde, nearly aligned orbit15
12Institute of Planetary Research, German Aerospace Center,
Rutherfordstrasse 2, 12489 Berlin, Germany
13Rheinisches Institut f¨ ur Umweltforschung an der Universit¨ at
zu K¨ oln, Aachener Strasse 209, 50931, Germany
14University of Vienna,
T¨ urkenschanzstr. 17, 1180 Vienna, Austria
15IAG, Universidade de Sao Paulo, Brazil
16University of Li` ege, All´ ee du 6 aoˆ ut 17, Sart Tilman,
Li` ege 1, Belgium
17Th¨ uringer Landessternwarte Tautenburg, Sternwarte 5,
07778 Tautenburg, Germany
18Space Research Institute, Austrian Academy of Science,
Schmiedlstr. 6, 8042 Graz, Austria
19Centerfor Astronomyand
Hardenbergstr. 36, 10623 Berlin, Germany
20LUTH, Observatoire de Paris, CNRS, Universit´ e Paris
Diderot; 5 place Jules Janssen, 92195 Meudon, France
Instituteof Astronomy,
Astrophysics, TU Berlin,