XX. CoRoT-20b: A very high density, high eccentricity transiting giant planet

Abstract

We report the discovery by the CoRoT space mission of a new giant planet,
CoRoT-20b. The planet has a mass of 4.24 +/- 0.23 MJ and a radius of 0.84 +/-
0.04 RJ. With a mean density of 8.87 +/- 1.10 g/cm^3, it is among the most
compact planets known so far. Evolution models for the planet suggest a mass of
heavy elements of the order of 800 ME if embedded in a central core, requiring
a revision either of the planet formation models or of planet evolution and
structure models. We note however that smaller amounts of heavy elements are
expected from more realistic models in which they are mixed throughout the
envelope. The planet orbits a G-type star with an orbital period of 9.24 days
and an eccentricity of 0.56. The star's projected rotational velocity is vsini
= 4.5 +/- 1.0 km/s, corresponding to a spin period of 11.5 +/- 3.1 days if its
axis of rotation is perpendicular to the orbital plane. In the framework of
Darwinian theories and neglecting stellar magnetic breaking, we calculate the
tidal evolution of the system and show that CoRoT-20b is presently one of the
very few Darwin-stable planets that is evolving towards a triple synchronous
state with equality of the orbital, planetary and stellar spin periods.

Full text

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arXiv:1109.3203v1 [astro-ph.EP] 14 Sep 2011
Astronomy & Astrophysics manuscript no. CoRoT20˙Astroph
September 16, 2011
c ? ESO 2011
Transiting exoplanets from the CoRoT space mission⋆
XX. CoRoT-20b: A very high density, high eccentricity transiting giant planet
M. Deleuil1, A.S. Bonomo1, S. Ferraz-Mello11, A. Erikson10, F. Bouchy4,5, M. Havel18, S. Aigrain6, J.-M.
Almenara1, R. Alonso17, M. Auvergne2, A. Baglin2, P. Barge1, P. Bord´ e3, H. Bruntt20, J. Cabrera9, Sz. Csizmadia10,
C. Damiani1, H.J., Deeg7,8, R. Dvorak9, M. Fridlund12, G. H´ ebrard4,5, D. Gandolfi12, M. Gillon13, E. Guenther14,
T. Guillot18, A. Hatzes14, L. Jorda1, A. L´ eger3, H. Lammer15, T. Mazeh16, C. Moutou, C.1, M. Ollivier3, A. Ofir21,
H. Parviainen7,8, D. Queloz17, H. Rauer10, A. Rodr´ ıguez11, D. Rouan2, A. Santerne1, J. Schneider19, L. Tal-Or16,
B. Tingley7,8, J. Weingrill??, and G. Wuchterl14
(Affiliations can be found after the references)
Received ; accepted
ABSTRACT
We report the discovery by the CoRoT space mission of a new giant planet, CoRoT-20b. The planet has a mass of 4.24 ± 0.23 MJupand a radius
of 0.84 ± 0.04 RJup. With a mean density of 8.87 ± 1.10 gcm−3, it is among the most compact planets known so far. Evolution models for the
planet suggest a mass of heavy elements of the order of 800 M⊕if embedded in a central core, requiring a revision either of the planet formation
models or of planet evolution and structure models. We note however that smaller amounts of heavy elements are expected from more realistic
models in which they are mixed throughout the envelope. The planet orbits a G-type star with an orbital period of 9.24 days and an eccentricity of
0.56. The star’s projected rotational velocity is vsini = 4.5 ± 1.0 kms−1, corresponding to a spin period of 11.5 ± 3.1 days if its axis of rotation is
perpendicular to theorbital plane. Inthe framework of Darwinian theories and neglecting stellar magnetic breaking, we calculate thetidal evolution
of the system and show that CoRoT-20b is presently one of the very few Darwin-stable planets that is evolving towards a triple synchronous state
with equality of the orbital, planetary and stellar spin periods.
Key words. stars: planetary systems - stars: fundamental parameters - techniques: photometry - techniques: radial velocities - techniques: spec-
troscopy
1. Introduction
The existence of a planet population at very short orbital dis-
tance, a < 0.1AU typically, with its wide range of orbital and
physical properties is an intriguing phenomenon. In-situ forma-
tion of such massive bodies so close to their host-star at a lo-
cation where the circumstellar material is depleted and warm,
appears indeed highly unlikely. Planet migration from further
away distances where solid material is abundant, triggered by
gravitational interactions, is invoked to account for this pop-
ulation. The exact process is still under debate but two ma-
jor mechanisms are put forward: gradual planet migration due
to interaction with the circumstellar gas disk (Lin et al., 1996;
Papaloizou et al., 2007) or planet-planet or planet-companion
star interactions combined with tidal dissipation (Rasio & Ford,
1996; Fabrycky & Tremaine, 2007; Nagasawa et al., 2008, e.g.).
Whatever the exact nature of the formation path, these planets
have undergone a significant orbital evolution since the time of
their formation. Their current properties provide valuable hints
helping to better understand their orbital evolution, especially
the planet’s orbit eccentricity and spin-orbit alignment of the
system.
⋆The CoRoT space mission, launched on December 27th 2006,
has been developed and is operated by CNES, with the contribution
of Austria, Belgium, Brazil , ESA (RSSD and Science Programme),
Germany and Spain.
Up to now, transit surveys have been more sensitive to
planets at very short orbital period. The situation has recently
started to change thanks to the extended temporal coverage of
ground-based transit surveys and the advent of space missions,
CoRoT (Baglin et al., 2009; Deleuil et al., 2011) and Kepler
(Borucki et al., 2010). As a consequence, the number of plan-
ets with orbital periods greater than a few days has significantly
increased over the past two years. While the mean eccentric-
ity for the close-in planets is close to zero, the transiting giant
planets at larger orbital distance display a much wider range in
eccentricity, a picture more consistent with the sample of plan-
ets found by radial velocity surveys. These trends favor a third
body induced migration with tidal circularization of an initial
eccentric and possibly high-obliquity orbit (Winn et al., 2010;
Matsumura et al., 2010; Pont et al., 2011, e.g.). A consequence
of this orbital evolution is the tidal destruction of the planet
which spirals down onto the star in the life time of the system
(Gonzalez, 1997; Jackson et al., 2009), a dramatic destruction
that appears being the fate of the vast majority of the transiting
planets (Matsumura et al., 2010).
In this paper, we report on the discovery of CoRoT-20b, a
new memberofthe hot-Jupiterclass population.The planet tran-
sits its G-type parent star every 9.24 days, along an orbit with
a high eccentricity. The CoRoT observations are presented in
Section 2. The accompanying follow-up observations and their
results are described in Section 3 and Section 5 for the host-star
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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
Fig.1. The 24.278-days long CoRoT-20 reduced light curve at a constant 512-sec time sampling.
analysis. The final system parameters are derived in Section 4.
We then discuss the properties of this new planet in Section 6.
We investigate its orbital evolution and fate but also its internal
structure that rises new questions on the nature of such compact
object.
2. CoRoT-20b Light curve
The planet has been discovered in one of the fields observed by
the CoRoT satellite (Baglin et al., 2009; Deleuil et al., 2011) in
the so-called anti-center direction. This field, labeled as SRa03,
was monitored for 24.278 days starting on 1 March 2010. As a
consequence of the DPU1 break down that took place on March
2009, the number of targets actually photometrically monitored
by the instrument was reduced to a maximum of 6000 stars
only. The released telemetry is further used to oversample a
much larger number of targets than it was initially possible, up
to magnitude ≃ 15. While not among the brightest stars of the
field (Table 1), CoRoT-20 benefited this new opportunity and its
observation was performed with the regular 32 sec time sam-
pling. It was also bright enoughto allow for three-colorphotom-
etry. Two transits were detected in its light curve by the Alarm
mode pipeline (Surace et al., 2008). The target was flagged as a
good planetary candidate and put among the highest priorities
for follow-up observations.
Table 1. CoRoT-20b IDs, coordinates and magnitudes.
CoRoT window ID
CoRoT ID
USNO-B1 ID
2MASS ID
GSC2.3 ID
SRa03 E2 0999
315239728
0902-0091920
06305289+0013369
Coordinates
RA (J2000)
Dec (J2000)
97.720434
0.22692
Magnitudes
Filter
Ba
Va
Jb
Hb
Kb
Mag
15.31
14.66
12.991
12.652
12.512
Error
0.023
0.026
0.027
afrom
(Deleuil et al., 2009);
bfrom 2-MASS catalog.
USNO-B1- Providedby Exo-Dat
The light curve of CoRoT-20b is displayed in Fig 1. It shows
a star rather quiet photometrically speaking, with no special
prominent feature. Three transits are clearly visible with a pe-
riod of 9.24 days and a depth slightly shallower than 1%. For the
detailed analysis, we used the light curve reduced by the CoRoT
calibration pipeline. It corrects for the main instrumental effects
such as the CCD zero offset and gain, the background light and
the spacecraft jitter (see Auvergne et al., 2009). Portions of the
light curve that were flagged by the pipeline as affected by parti-
cle impacts during the South Atlantic Anomaly crossing, were
removed and ignored in the analysis. In total, the light curve
consists of 56869 photometric measurements. It gives a corre-
sponding duty cycle of 88%.
3. Follow-up observations
A photometric time-series of the star was obtained at the Wise
observatory on November 14, 2010 in order to check whether
an unknown nearby eclipsing binary could be the source of the
transits (Deeg et al., 2009). The detection of a transit ingress ex-
cluded this configuration at the spatial resolution of Wise. The
observed time of the ingress, with first contact at 2455515.510±
0.007 HJD was then used to refine the period of CoRoT-20b, to-
wards the value quoted in Table 3. Ground-based images from
both Wise and the DSS show that the star is rather isolated
(Fig 2). This supports the very low contamination rate that was
derived for the star within the CoRoT photometric mask (see
Sec 4).
Fig.2. Image of the DSS showing CoRoT-20 and its environ-
ment. The photometric mask used for CoRoT observations is
overplotted on the target.
Radial velocity (RV) observations started during the same
season on December 9, 2010. We used the HARPS spectro-
graph (Mayor et al., 2003) mounted on the 3.6-m ESO tele-
2

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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
Fig.3. The phase-folded radial velocity measurements of
CoRoT-20. The various symbols correspond to the different
spectrographs used for the follow-up campaign: HARPS (black
circle),SOPHIE (opencircle)andFIES(opentriangle).Thebest
fit solution is over-plotted in full line
Fig.4.BisectorspanversusradialvelocityofCoRoT-20showing
no correlation.
scope (Chile) as part of the ESO large program 184.C-0639,
the SOPHIE spectrograph (Perruchot et al., 2008) on the 1.93-
m telescope at the Observatoire de Haute Provence (France)
and the FIES spectrograph on the Nordic Optical Telescope
(Frandsen & Lindberg, 1999) based on the 2.56-m Nordic
Optical telescope in La Palma (Spain) under observing program
P42-216.
We used the same instrument set-up as for previous CoRoT
candidates follow-up : high resolution mode for HARPS and
high efficiency mode for SOPHIE without acquisition of the
simultaneous thorium-argon calibration, the second fiber being
used to monitor the Moon background light (Santerne et al.,
2011). For HARPS and SOPHIE, the exposure time was set to
1 hour. We reduced data and computed RVs with the pipeline
based on the weighted cross-correlation function (CCF) using a
numerical G2 mask (Baranne et al., 1996; Pepe et al., 2002).
FIES observations were performed in high-resolution mode
with the 1.3 arcsec fiber yielding a resolving power R ≈ 67000
and a spectral coverage from 3600 to 7400Å. Three consec-
utive exposures of 1200 sec were obtained for each observa-
tion. Long-exposedThAr spectra were acquiredright beforeand
after each science spectra set, as described in Buchhave et al.
(2010). Standard IRAF routines were used to reduce, combine,
and wavelength calibrate the nightly spectra. RV measurements
were derived cross-correlatingthe science spectra with the spec-
Table 2. Log of radial velocity observations
Date HJD
vrad
kms−1
60.728
60.353
60.453
60.985
60.443
60.320
60.813
61.048
60.948
61.077
60.267
60.611
61.090
60.260
60.146
σvrad
kms−1
0.0169
0.0138
0.0119
0.0285
0.0334
0.0234
0.0385
0.0532
0.0213
0.0175
0.0183
0.032
0.060
0.036
0.034
Spectrograph
2010-12-09
2010-12-14
2011-01-12
2011-01-16
2011-01-21
2011-01-28
2011-01-16
2011-01-17
2011-02-04
2011-02-05
2011-02-06
2011-01-06
2011-01-08
2011-01-18
2011-01-19
55540.70645
55545.75533
55574.60927
55578.64264
55583.64035
55590.66159
55578.41934
55579.41057
55597.44200
55598.38017
55599.39134
55568.55091
55570.59973
55580.60213
55581.56506
HARPS
HARPS
HARPS
HARPS
HARPS
HARPS
SOPHIE
SOPHIE
SOPHIE
SOPHIE
SOPHIE
FIES
FIES
FIES
FIES
trum of the RV standard star HD50692 (Udry et al., 1999), ob-
served with the same instrument set-up as CoRoT-20.
The 15 radial velocities of CoRoT-20b are listed in Table 2
and displayed in Fig 3. They present a clear variation, in phase
with the CoRoT ephemeris and consistent with a companion in
the planet-mass regime with an eccentric orbit. We nevertheless
investigatedthe possibility of an unresolvedeclipsing binary be-
ing the source of observed transits. To that purpose, we per-
formed the line-bisector analysis of the CCFs (see Fig. 4) and
also checked that there is no dependency of the RVs variations
with the cross-correlation masks constructed for different spec-
tral types (Bouchy et al., 2009).
The Keplerian fit of the RVs was performed simultaneously
with the transit modeling (see Section 4). All the parameters of
the fit are listed in Table 3.
4. System parameters
We calculated the flux contamination from nearby stars whose
light might fall inside the CoRoT photometric mask using the
same methodology as described in Bord´ e et al. (2010). The
methodtakes into accountthe photometricmask usedto perform
the on-board photometry and all the stars in the target neighbor-
hood, including faint background stars. We found the contami-
nation being less than 0.6% and we further neglected it.
Three sections of the light curve, each centered on a tran-
sit, were locally normalizedby fitting a third-degreepolynomial.
Each section was a 5-hours interval before the transit ingress
and after its egress. The detailed physical modeling of the sys-
tem was performed by carrying out the transit modeling and
the Keplerian fit of the radial velocity measurements simulta-
neously. For the transit fit we used the formalism of Gim´ enez
(2006, 2009). The fit implies twelve free parameters : the orbital
period P, the transit epoch Ttr, the transit duration T14, the ra-
tio of the planet to stellar radii Rp/R⋆, the inclination i between
the orbital plane and the plane of the sky, the Lagrangian orbital
elements h = e sinω and k = e cosω, where e is the eccen-
tricity and ω the argument of the periastron, the radial-velocity
semi-amplitude K, the systemic velocity γreland the two off-
sets betweenSOPHIE andHARPS radialvelocities andSOPHIE
and FIES. For the transit modeling, we used a limb-darkening
quadratic law (Claret, 2003, 2004). The limb-darkening coeffi-
cients uaand ubwere taken using the tabulated values for the
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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
CoRoT bandpass from Sing (2010) for the atmospheric parame-
ters Teff, log g and metallicity derived for the central star (see
sect. 5): ua = 0.4262 ± 0.0168 and ub = 0.2434 ± 0.0108).
The two corresponding non-linear limb-darkening coefficients
are u+ = ua+ ub = 0.6696 ± 0.0201 and u− = ua− ub =
0.1828 ± 0.0201. We decided to keep these limb-darkening pa-
rameter values fixed in the transit fitting.
The fit was performed using the algorithm AMOEBA
(Press et al., 1992). The initial values of the fitted parameters
were changed with a Monte-Carlo method to find the global
minimum of the χ2. The associated 1-sigma errors were then
estimated using a bootstrap procedure described in details in
Bouchy et al. (2011).Insuchaprocedurethelimb-darkeningpa-
rameters were allowed to vary within their error bars related to
the atmospheric parameter uncertainties. The final values of the
fitted parameters and the subsequently derived system parame-
ters are given in Table 3. Fig. 5 displays the best fit compared to
the observed folded transit.
Fig.5. The phase-folded transit in the phase space. The phase
bins are 3.3 min and the error bar of each individual bin was
calculated as the dispersion of the points inside the bin, divided
by the square root of the number of points per bin. The best
model is over plotted in full line.
5. Stellar parameters
The spectroscopic analysis was done the usual way it is carried
out for the CoRoT planets: a master spectrum was created from
the co-addition of spectra collected for the radial velocity mea-
surements of the companion. We chose the HARPS spectra that
offer the highest spectral resolution. We selected those that were
not affected by the Moon reflected light at the time of the ob-
servations. Each order of the selected spectra was corrected by
the blaze, set in the barycentric rest frame and rebinned to the
same wavelength grid with a constant step of 0.01Å. The spectra
were then co-added order per order. Each order of the co-added
spectrum was then carefully normalized and the overlapping or-
ders were mergedresulting in a single 1D spectrum. This master
spectrum has a S/N of 176 per element of resolution at 5760Å in
the continuum.
A prior estimate of the atmospheric parameters Teff, log g,
chemical composition and vsini was performed by fitting the
spectrum with a library of synthetic spectra calculated using
Fig.6. Abundances of the chemical elements measured with
VWA intheHARPS co-addedspectrumofCoRoT-20.Theabun-
dances refer to the solar value. White circles correspond to neu-
tral lines, red boxes to singly ionized lines and the yellow area
represents the mean metallicity within one sigma error bar.
MARCS stellar atmosphere, including the Hα Balmer line. The
rotational broadening was estimated on a selection of isolated
spectral lines fitted by synthetic spectra convolved with vari-
ous rotational velocities. We found vsini = 4.5 ± 1.0 kms−1and
vmacro= 3.5 ± 1.0 kms−1. The detailed analysis was then carried
out using the Versatile Wavelength Analysis package (VWA)
(Bruntt et al., 2004, 2010b). A first set of weak and isolated lines
of Fei and Feii was fitted until the derived abundances of Fe
minimized the correlation with the equivalent width and the ex-
citation potentials. We found : Teff= 5880 ± 90 K, log g = 4.05
± 0.17 and vmic= 1.10 ± 0.1 kms−1which correspondsto a G2-
type dwarf.Thenthe abundancesof otherelements for which we
could find isolated spectral lines were derived (see Fig 5). We
performed an independent estimate of the surface gravity from
the pressure-sensitive lines: the Mgi1b lines, the Nai D doublet
and the Cai at 6122Å and 6262Å. We fitted the spectrum with
the aforementioned grid of synthetic spectra in regions centered
on each of the spectral lines of interest. The inferred value of the
surface gravity is log g = 4.2±0.15, a value in good agreement
with the log g derived with VWA obtained from the agreement
between the Fei and Feii abundances. We thus adopted log g=
4.2 for the surface gravity.
The mean metallicity of the star is computed as the mean of
metals with more than 10 lines in the spectrum, such as Si, Ca,
Ti, Fe, Ni (Fig.5). Thisyields a straightmeanof[M/H] = 0.14±
0.05.Theerroron[M/H]dueto theuncertaintyonTeff, logg and
microturbulenceis 0.11dex,which we must addquadraticallyto
get [M/H] = 0.14± 0.12 (Bruntt et al., 2010a).
We also checked for any indicators of age. We found no hint
ofstellar activity in theCaii H andK lines.However,the Lii line
is clearly detected at 6708Å (see Fig. 7). We measured an equiv-
alent widthof Weq= 44mÅ anddetermineda lithiumabundance
of2.97.FollowingSestito & Randich(2005),thisleads toanage
in the range 100 Myr up to 1 Gyr, depending on the star’s initial
rotation velocity.
The modeling of the star in the HR diagram was carried out
in the (Teff,M1/3
account. It resulted in the final estimates of the star’s fundamen-
tal parametersgivenin Table3: M⋆= 1.14± 0.08 M⊙, R⋆= 1.02
± 0.05 R⊙. The inferred surface gravity is log g = 4.47 ± 0.11,
in agreementwithin the errors with the spectroscopic result. The
evolutionarystatus points to a young star likely in the last stages
of the pre-MS phase. We found the most likely isochrone age
being 100+800
dance.
⋆/R⋆) plane taking the host-star’s metallicity into
−40Myr, a result in good agreement with the Li abun-
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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
Fig.7. CoRoT-20 spectrum in a spectral region around the Lii
lines at 6708Å.
We calculated the distance of the star. We used the parame-
tersofthestarwederivedandits 2-MASSmagnitudestoestimate
the reddening.We founda color excess E(J−K) = 0.18magand
theabsorptionAV= 1.04±0.5magusingtheextinctionlawfrom
Schlegel et al. (1998). This yields a distance of 1.23 ± 0.12 kpc,
consistent with the strong interstellar absorption observed in the
Nai (D1) and (D2) lines.
6. CoRoT-20 system properties
Compared to the sample of known transiting planets, CoRoT-
20b is unusual in many respect. With an orbital period of 9.24
days it joins the group of transiting planets with periods out-
side the pile-up at 3 days. It is the fifth planet discovered by
CoRoT in this period domain which currently accounts for 25
planets (see http://exoplanet.eu/), 9 out of these belonging to
multi-planet systems : Kepler-9 (Holman et al., 2010), Kepler-
10 (Fressin et al., 2011) and Kepler-11 (Lissauer et al., 2011).
However all these Kepler-planets have a mass which is less than
∼ 0.3MJupandcouldnotbe directlycomparedto the giantplanet
population. Excluding these planets in multiple systems, for the
17 remaining objects of the sample that do not have a detected
companion,8, that is 47%, havea significant eccentricorbit with
e in the range 0.15 to 0.9.
Planets with highly eccentric orbit appear to be found pref-
erentially among the high-mass and/or long period planet popu-
lation. With a mass of 4.13 MJupwhich places it at the border of
the gap in mass between the regular hot-Jupiter population and
the very massive planet one, CoRoT-20b is consistent this trend.
In the mass-period diagram they are clearly separated from the
lighter planets with circular orbits (Pont et al., 2011). This di-
chotomy and in particular the lack of massive close-in planets at
circular orbit suggest that tidal evolution should play an impor-
tant role in the fate of the planet population.
6.1. Tidal evolution
Following Levrard et al. (2009) approach we checked the sta-
bility of CoRoT-20b to tidal dissipation. The authors calculated
the ratio between the total angular momentum of a given system
Ltot and the critical angular momentum Lcrit for some transit-
ing systems. According to Hut (1980), tidal equilibrium states
exist when the total angular momentum is larger than this crit-
ical value Lcrit. However, this equilibrium state could be stable
or unstable, depending whether the orbital angular momentum
Lorbis more than three time the total spin angular momentum
Lspin, or not. Levrard et al. (2009) demonstrated that for none of
the systems but HAT-P-2b the stable tidal equilibrium state, that
corresponds to Ltot/Lcrit > 1, exists. Further the fate of these
Fig.8. Tidal evolution of the orbital semi-major axis and eccen-
tricity. The figure is displayed on a time interval larger than the
expected lifetime of the star to show the triple synchronization
characteristic of a Darwin-stable system.
close-in planets is ultimately a collision with their host-star. The
study has been recently reexamined and extended to more than
60transitingsystemsbyMatsumura et al.(2010)whoachieveda
similar conclusion,showingthat the vast majorityofthese close-
in planets will spiral-into their hoststar andwill be destroyedby
tides. Using equations (1) and (2) given by Levrard et al. (2009)
that neglect any effect of a possible magnetized stellar wind, we
found for CoRoT-20b:
Ltot/Lcrit= 1.057and
Lspin⋆/Lorb= 0.0458
It shows that, within the current observational uncertainties, the
planet has a tidal equilibrium state. It is worth noticing that our
approach also assumes the stellar obliquity is small. The later is
poorly constrained as the star’s rotation period could not be de-
rived from the light curve. We simply assumed that the rotation
axis is perpendicular to the line of sight and derived the star’s
rotation period from the vsini (Table 3), a regular method for
transiting systems. This gives a rotational period of the star of
11.5± 3.1 days,that is of the same orderthan the planet’s orbital
period. In the case of CoRoT-20b, Lspin⋆/Lorb< 1/3 and most of
theangularmomentumofthesystemisintheorbit.Accordingto
Matsumura et al. (2010), CoRoT-20b belongs to the very small
subgroup of Darwin-stable systems that evolve toward a stable
tidal equilibrium state where migration will stop.
From the Roche-limit separation, the planet thus lies well
beyond two times the Roche limit distance. Using (Faber et al.,
2005) :
aR= (Rp/0.462)(M⋆/Mp)1/3
we found that the Roche limit aR of the system is 0.0057
AU. This further supports the migration scenario over the
scattering/Kozai-cycle scenario as proposed by Ford & Rasio
(2006).
We performed a complete calculation of the tidal evolution
of the system formed by the star and the planet assuming a
linear tidal model (Mignard, 1979; Hut, 1981). The main dif-
ficulty here is to choose the values of the dissipation in the star
and in the planet. For the main semi-diurnal tides of the star,
we have adopted the value Q′
(Hansen, 2010; Ben´ ıtez-Llambay et al., 2011). Because of the
s= 107as found for hot Jupiters
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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
Fig.9. Tidal evolution of the rotational and orbital periods.
close values of the orbital period and the rotation of the star,
the components of the tides raised on the star by the planet re-
lated to the orbital eccentricity are also equally important, but
the values of the current dissipation obtained with them are of
the same order of magnitude. For the planet, we have derived
one value on the basis of the actually determined Q′of Jupiter
(Q′= 1.36 × 105, see Lainey et al. (2009)). We first note that
standard linear tidal theories (see Hut, 1981, eqn 45) allow us to
determine the current rotation period (stationary) of the planet
independently of the dissipation. We obtain 2.64 ± 0.13 days.
To transform Q′
account: (i) Q′
Ferraz-Mello et al.,2008;Matsumura et al.,2010);(ii) Q′
with R−5
thus obtain Q′
semi-major axis and the evolution of the eccentricity. One can
notice that with the adopted dissipation values, while the eccen-
tricity tends toward zero, the circularization will not be achieved
within the lifetime of the system.
Fig. 9 shows the evolution of the periods. The planet ro-
tation is currently in a stationary super-synchronous state, that
is the planet rotation is faster than its orbital motion. Its period
increases as the eccentricity decreases and almost synchroniza-
tion is reached when the eccentricity becomes very small. The
star rotation period is currently decreasing; it will equal the syn-
chronous value at some time 4 Gyr from now. However, it will
continue to decrease up to reach the triple synchronous station-
ary state. The triple synchronization, however, does not seem to
be reached within the lifetime of the star.
It is worth underlining that the actual Q′values are not
known and the values we used are only estimates founded on
previous studies. Therefore the exact timescale of the tidal pro-
cesses is uncertain. Furthermore, by extracting angular momen-
tum from the system, stellar magnetic braking may prevent the
planet from reaching a triple synchronous state and ultimately
jeopardizeits survival(Bouchy et al., 2011). Indeed,simulations
in which magnetic braking was active during the whole system
lifetime, following the model proposed by (Bouvier et al., 1997)
and using the same tide parameters as in the examples given
above, show that the planet is falling below the Roche limit in
about 6 Gyr. This result is critically dependent on the adopted
Jupinto the planet’s Q′
pscales with the semi-diurnal tide period (see
p, we have to take into
pscales
p (see Eggleton et al., 1998; Ogilvie & Lin, 2004). We
p= 2.2 × 106. Fig. 8 shows the variation of the
parameters and further would required a detailed study that is
well beyond the scope of the present paper.
We also investigate the consequences of the circularization
ofthe planet orbitwhichis in thephase offast circularization,on
thetransits occurrence.Assumingthereis nootherclosemassive
perturber in the system, then two effects are causing TTVs: the
decrease of the orbital semi-major axis and the circularizationof
the orbit. Concerning the orbital semi-major axis, the time-scale
of its variation, ˙ a, is −0.9510−51/Myr presently (see Fig 9). As
a consequence, a continuous period variation of˙P/P ≈ −410−12
percycleis expected.As well-known,thislinearperiodvariation
will causea parabolicO−C curve,andin100yearsfromnowthe
O − C value will be only −25 seconds. This is slightly over the
3σ observation limit by CoRoT (Bean, 2009; Csizmadia et al.,
2010). Assuming that the transit timing precision can be forced
down to 5 sec in the future, this O − C value will be reached in
45 years from now.
Turning to the evolution of the eccentricity during the cir-
cularization process, it has two consequences. First the occur-
rence of the secondary eclipse will change. The displacement
D of the secondary from phase 0.5 is given by (eqn 1 and 2
Borkovits, 2004, e.g.). The previous results of the tidal evo-
lution calculations indicated ˙ e = −4.5 10−51/Myr and˙P =
−1.5 10−3days/Myr. Assuming a constant ω, we have that
˙D = −37.56 10−5days/Myr or ˙D = −9.53 10−12days/cycle.
This variation is of the same order as the previous one caused by
the decreasingsemi-majoraxis, so it would be observablewithin
a century, too.
For the second effect, that is the circularization of the or-
bit, one can also consider the occurrence of a small precession
of the orbit. This effect is hardly observable, but interesting on
the theoretical side, since the transit occurs at the true anomaly
v = 90◦− ω where ω is the argument of periastrion. The later is
also subject to variations because of theory of general relativity
but also because the tidal effects force the apsidal line to rotate.
However, this variation has a different time-scale. We thus do
not take this into account here, even if tidal forces also cause a
small precession of the orbit showing that ˙ ω is not zero. So if e
decreases due to circularization, and even if ω is constant, then
at the epoch of transit the eccentric anomaly will increase and
hence the mean anomaly at transit will occur later. However, a
first estimation shows that this effect may be negligible in a ten
year timescale.
6.2. Internal structure
CoRoT-20b is a massive hot-Jupiter with a mass of 4.24 MJup
a radius of 0.84 MJup, and an inferred density ρ = 8.87 ±
1.1gcm−3. A few giant planets are already reported with sim-
ilar density or even higher : CoRoT-14b ρ = 7.3 ± 1.5gcm−3
(Tingley et al., 2011), WASP-18b ρ = 8.8 ± 0.9 (Hellier et al.,
2009; Southworth, 2010) or HAT-P-20b ρ = 13.78 ± 1.5
gcm−3(Bakos et al., 2010) for example.While the mass of these
planets spans a large range, from ∼ 4 up to more than 9MJup,
their radius is close to 1RJup. Given CoRoT-20b’s large plane-
tary mass, its small size is surprising. Among these high den-
sity giants planets, only HAT-P-20b has a comparable size, i.e.
0.867± 0.033 RJup. CoRoT-20b, as HAT-P-20b, is thus expected
to contain large amounts of heavy elements in its interior.
To investigate the internal structure of CoRoT-20b,
we computed planetary evolution models with CEPAM
(Guillot & Morel, 1995), following
Guillot & Havel (2011), and Havel et al. (2011) for a planet of
thedescriptionin
6

Page 7

M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
a total mass 4.24 MJup. We derived a time-averaged equilibrium
temperature of the planet to be Teq = 1002 ± 24 K. The
results for Teq = 1000K are shown in Fig. 10 in terms of the
planetary size as a function of the system age. The coloured
regions (green, blue, yellow) indicate the constraints derived
from the CESAM stellar evolution models (Morel & Lebreton,
2008) at 1, 2, and 3σ level, respectively. For preferred ages
between 100 Ma and 1 Ga, we find that CoRoT-20b should
contain between 680 and 1040 M⊕ of heavy-elements in its
interior (i.e. between 50 and 77% of the total planetary mass),
at 1σ level, about twice the amount needed for HAT-P-20b
(see Leconte et al., 2011). While this result is qualitatively
in line with the observed correlation between star metallicity
and heavy elements in the planet (e.g. Guillot et al., 2006;
Miller & Fortney, 2011, and references therein), the derived
amounts are extremely surprising. They would imply that all the
heavy elements of a putative gaseous protoplanetary disk of 0.1
to 0.15M⊙were filtered out to form CoRoT-20b, and then that
an extremely small fraction of hydrogen and helium in that disk
was accreted by the planet. This is at odds with todays accretion
formationmodels (e.g. Ida & Lin, 2004; Mordasini et al., 2009).
Disk instability with differentiation and partial tidal stripping
(Boley & Durisen, 2010) is proposedas an alternative formation
pathway. According to Boley et al. (2011), this could account
for giant planets with massive core such as HAT-P-20b. In its
current orbit, CoRoT-20b doesn’t enter the Roche limit and no
tidal stripping is acting on yet but this scenario would deserve
further investigations.
We investigated the possibility that changes in the atmo-
spheric model would yield more ”reasonable” values for the
planetary enrichment. As can be seen from a similar study in the
brown dwarf regime (Burrows et al., 2011), the consequences of
modified atmospheric properties are limited for objects with the
mass of CoRoT-20b (i.e. standard radii for objects of this mass
range from 1.05 to 1.20RJup). By artificially lowering the in-
frared atmospheric opacity by a factor 1000 (not shown), we
were able to decrease the 1σ upper limit to the core mass from
650to 390M⊕, a small changecomparedto hugeandunphysical
decrease in the opacity.
On the other hand, one strong assumption in our study is that
heavy elements are embedded into a central core. When rela-
tively small amounts of heavy elements are considered, it is not
very important whether they are considered as being part of a
coreormixedintheenvelope(e.g.Ikoma et al., 2006).However,
as shown by Baraffe et al. (2008), when 0.5 MJup of ices are
mixed in the envelope of a 1 MJupplanet, its radius is smaller
by ∼ 0.1 RJupthan when one considers that these elements are
part of a central core. It is thus very likely that the mass of heavy
elements required to explain the radius of CoRoT-20b is high
but significantly smaller than considered here. Estimates based
on the Baraffe et al. (2008) calculations indicate that if mixed in
the envelope,a mass of heavy elements 2 to 3 times smaller than
estimated in Fig. 10 would explain the observed planetary size.
This would alleviate the problem of the formation of the planet,
although it would still require relatively extreme/unlikely sce-
narios.
7. Summary
In this article we presented the discovery of CoRoT-20b. The
object belongs to the population of massive planets with or-
bital semi major axes below 0.1 AU, a domain of orbital peri-
ods where low and high eccentricity systems co-exist in a nar-
row range of orbital period. We examined the tidal stability of
Fig.10. Evolution of the size of CoRoT-20b (in Jupiter units) as
a function of age (in billion years), compared to constraints in-
ferred from CoRoT photometry, spectroscopy, radial velocime-
try and stellar evolution models. Green, blue and yellow regions
correspondto the planetaryradii and ages that result from stellar
evolution models matching the inferred ρ⋆- Teff - [Fe/H] un-
certainty ellipse within 1σ, 2σ and 3σ, respectively. Planetary
evolution models for a planet with a solar-composition envelope
overa central dense core of variable mass (0, 400,800,and 1000
M⊕as labelled) are shown as dashed lines. These models also
assume that 1% of the incoming stellar irradiation is dissipated
deep into the interior of the planet.
CoRoT-20 and found that, within the observational uncertain-
ties, it belongs the relatively small population of transiting plan-
ets thatareconsideredas ”Darwin-stable”,i.e.systemsforwhich
in the absence of processes extracting angular momentum from
the system (i.e. stellar winds), the planet would never fall onto
the central star which would instead be spun-up towards triple-
synchronization (equality of the orbital, planetary and stellar
spinperiods).OthercasesintheplanetarydomainareCoRoT-3b,
CoRoT-6b, WASP-7 and HD 80606 (Matsumura et al., 2010).
MeasuringthestellarobliquitythroughtheRossiter-McLaughlin
effect would provide additional constraints to better constrain
its tidal evolution and further understand its orbital evolution.
The expected semi-amplitude of the radial velocity anomaly of
the Rossiter-McLaughlin effect is estimated to be 22 ± 5 ms−1,
unfortunately quite difficult to detect with the present spectro-
graphs used for the keplerian orbit determination. Another point
would be to assess the presence of any additional companions in
the system by long-term radial velocity monitoring as, accord-
ing to Matsumura et al. (2010), the formation path of close-in
planet should be different for single-planet system and multi-
planet ones. CoRoT-20 system appears thus an interesting bench
test case to study the tidal orbital and rotational evolution of the
close-in population.
The second interesting peculiarity that distinguishes CoRoT-
20bfromtheregulargiantplanetpopulationis its small observed
radius. According to planetary evolution models, the interior of
this compact planet should contain a very high amount of heavy
elements, with a central dense core whose mass would be in the
range between 680 and 1040 M⊕. Although mixing heavy ele-
ments in the envelopes rather than confining them to a central
core can lead to substantially smaller values (by a factor esti-
7

Page 8

M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
mated to be ∼ 2 − 3), the origin of such a huge amount of heavy
elements is difficult to explain within the framework provided
by the current planetary formation models. With HAT-P-20b
(Bakos et al., 2010), it is the second example of extremely metal
rich interior that challenges planetary interior models. However,
the two planetary systems differ in many aspects: HAT-P-20
mass is nearly twice that of CoRoT-20b; it orbits on a nearly cir-
cular orbit a K3 metal rich star, while CoRoT-20 is a solar-type,
slightly metal enriched star. In addition, HAT-P-20 seems to be
physically associated to another stellar companion, while up to
now, CoRoT-20 has none, detected or suspected. It thus appears
difficult from this two exceptions to derive any trend that would
providehints on the origin these challengingand intriguingbod-
ies.
Acknowledgements. The French team thanks the CNES for its continuous sup-
port to the CoRoT/Exoplanet program. The authors wish to thank the staff at
ESO La Silla Observatory for their support and for their contribution to the suc-
cess of the HARPS project and operation. The team at the IAC acknowledges
support by grants ESP2007-65480-C02-02 and AYA2010-20982-C02-02 of the
Spanish Ministry of Science and Innovation (MICINN). The CoRoT/Exoplanet
catalogue (Exodat) was made possible by observations collected for years at
the Isaac Newton Telescope (INT), operated on the island of La Palma by the
Isaac Newton group in the Spanish Observatorio del Roque de Los Muchachos
of the Instituto de Astrophysica de Canarias. The German CoRoT team (TLS
and University of Cologne) acknowledges DLR grants 50OW0204, 50OW0603,
and 50QP0701. The Swiss team acknowledges the ESA PRODEX program and
the Swiss National Science Foundation for their continuous support on CoRoT
ground follow-up. A. S. Bonomo acknowledges CNRS/CNES grant 07/0879-
Corot. S. Aigrain acknowledges STFC grant ST/G002266. M. Gillon acknowl-
edges support from the Belgian Science Policy Office in the form of a Return
Grant.
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M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet
Table 3. Planet and star parameters.
Ephemeris
Planet orbital period P [days]
Primary transit epoch Ttr[BJD-2400000]
Primary transit duration T14[d]
Secondary transit epoch Ts[BJD-2400000]
System parameters
Periastron epoch Tperi[BJD-2400000]
esinω
ecosω
Orbital eccentricity e
Argument of periastron ω [deg]
Radial velocity semi-amplitude K [ms−1]
Systemic velocity γrel[kms−1]
HARPS-SOPHIE offset velocity Vr1[ms−1]
SOPHIE-FIES offset velocity Vr2[ms−1]
O-C residuals [ms−1]
9.24285 ± 0.00030
55 266.0001 ± 0.0014
0.0927 ± 0.0019
55 272.46 ± 0.13
55265.79074
0.468 ± 0.017
0.312 ± 0.022
0.562 ± 0.013
56.3+2.4
−2.3
454 ± 9
60.623 ± 0.006
93 ± 11
163 ± 20
27
Radius ratio Rp/R⋆
Impact parameterab
Scaled semi-major axis a/R⋆b
M1/3
Stellar density ρ⋆[g cm−3]
Inclination i [deg]
Spectroscopic parameters
Effective temperature Tef f[K]
Surface gravity logg [dex]
Metallicity [Fe/H] [dex]
Stellar rotational velocity vsini [kms−1]
Spectral type
Stellar and planetary physical parameters from combined analysis
Star mass [M⊙]
Star radius [R⊙]
Distance of the system [kpc]
Age of the star t [Myr]
Orbital semi-major axis a [AU]
Orbital distance at periastron aper[AU]
Orbital distance at apastron aapo[AU]
Planet mass Mp[MJ]c
Planet radius Rp[RJ]c
Planet density ρp[g cm−3]
Equilibrium temperaturedTeq[K]
Equilibrium temperature at periastrondTper
Equilibrium temperature at apastrondTapa
0.0842 ± 0.0017
0.26 ± 0.08
18.95+0.63
−0.73
1.022+0.034
−0.039
1.51+0.15
−0.17
88.21± 0.53
5880 ± 90
4.20 ± 0.15
0.14 ± 0.12
4.5 ± 1.0
G2 V
⋆/R⋆[solar units]
1.14 ± 0.08
1.02 ± 0.05
1.23 ±120
100+800
−40
0.0902 ± 0.0021
0.0392 ± 0.0017
0.1409 ±0.0037
4.24 ± 0.23
0.84 ± 0.04
8.87 ± 1.10
1002 ± 24
1444+53
−46
764 ±18
eq [K]
eq [K]
aa/R⋆=
Gim´ enez, 2009).
bb =a·cosi
cRadius and mass of Jupiter taken as 71492 km and 1.8986×1030g, respectively.
dzero albedo equilibrium temperature for an isotropic planetary emission.
1+e·cosν1
1−e2
·
1+k
√
1−cos2(ν1+ω−π
2)·sin2i, where ν1is the true anomaly measured from the periastron passage at the first contact (see
R∗
·
1−e2
1+e·sinω
1Laboratoire d’Astrophysique de Marseille, 38 rue Fr´ ed´ eric Joliot-
Curie, 13388 Marseille cedex 13, France
2LESIA,ObsdeParis,PlaceJ.Janssen, 92195 Meudon cedex, France
3Institut d’Astrophysique Spatiale, Universit´ e Paris XI, F-91405
Orsay, France
4Observatoire de Haute Provence,
l’Observatoire, France
5Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014
Paris, France
6Department of Physics, Denys Wilkinson Building Keble Road,
Oxford, OX1 3RH
7Instituto de Astrofisica de Canarias, E-38205 La Laguna, Tenerife,
Spain
8Universidad de La Laguna, Dept. de Astrof´ ısica, E-38200 La
Laguna, Tenerife, Spain
9University of Vienna, Institute of Astronomy, T¨ urkenschanzstr. 17,
A-1180 Vienna, Austria
10Institute of Planetary Research, German Aerospace Center,
Rutherfordstrasse 2, 12489 Berlin, Germany
11IAG, Universidade de Sao Paulo, Brazil
12Research and Scientic Support Department, ESTEC/ESA, PO Box
299, 2200 AG Noordwijk, The Netherlands
13University of Li` ege, All´ ee du 6 aoˆ ut 17, Sart Tilman, Li` ege 1,
Belgium
14Th¨ uringer Landessternwarte, Sternwarte 5, Tautenburg 5, D-07778
Tautenburg, Germany
15SpaceResearchInstitute,Austrian
Schmiedlstr. 6, A-8042 Graz, Austria
16School of Physics and Astronomy, Raymond and Beverly Sackler
Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
17Observatoire de l’Universit´ e de Gen` eve, 51 chemin des Maillettes,
1290 Sauverny, Switzerland
18Observatoire de la Cˆ ote d Azur, Laboratoire Cassiop´ ee, BP 4229,
06304 Nice Cedex 4, France
19LUTH, Observatoire de Paris, CNRS, Universit´ e Paris Diderot; 5
place Jules Janssen, 92195 Meudon, France
20Department of Physics and Astronomy, Aarhus University, 8000
Aarhus C, Denmark
21Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
04670Saint Michel
AcademyofScience,
10