Transiting exoplanets from the CoRoT space mission

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Astronomy&Astrophysicsmanuscript no. CoRoT20˙Astroph
September 16, 2011

ESO 2011

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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

±

0.04 RJup. With a mean density of 8.87

0.23 MJup and a radius
We report the discovery by the CoRoT space mission of a new giant planet, CoRoT-20b. The planet has a mass of 4.24
3, it is among the most compact planets known so far. Evolution models for the
of 0.84
1.10 g cm−
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 v sin i = 4.5
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.

1, corresponding to a spin period of 11.5

1.0 km s−

±

±

±

±

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 member of the hot-Jupiter class 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 1 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 enough to allow for three-color photom- 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. 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). 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) Magnitudes Filter Ba Va Jb Hb Kb 97.720434 0.22692 Mag 15.31 14.66 12.991 12.652 12.512 Error 0.023 0.026 0.027 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. a from USNO-B1 (Deleuil et al., 2009); b from 2-MASS catalog. - Provided by Exo-Dat 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 M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet Table 2. Log of radial velocity observations Date HJD 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 1 vrad km s− 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 1 σvrad km s− 0.0169 HARPS 0.0138 HARPS 0.0119 HARPS 0.0285 HARPS 0.0334 HARPS 0.0234 HARPS SOPHIE 0.0385 SOPHIE 0.0532 SOPHIE 0.0213 SOPHIE 0.0175 SOPHIE 0.0183 FIES 0.032 FIES 0.060 FIES 0.036 FIES 0.034 trum of the RV standard star HD 50692 (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 investigated the possibility of an unresolved eclipsing 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 method takes into account the photometric mask used to 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 normalized by fitting a third-degree polynomial. 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 sin ω and k = e cos ω, where e is the eccen- tricity and ω the argument of the periastron, the radial-velocity semi-amplitude K, the systemic velocity γrel and the two off- sets between SOPHIE and HARPS radial velocities and SOPHIE and FIES. For the transit modeling, we used a limb-darkening quadratic law (Claret, 2003, 2004). The limb-darkening coeffi- cients ua and ub were taken using the tabulated values for the 3 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 (open circle) and FIES (open triangle). The best fit solution is over-plotted in full line Fig. 4. Bisector span versus radial velocity of CoRoT-20 showing 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 67 000 with the 1.3 arcsec fiber yielding a resolving power R and a spectral coverage from 3600 to 7400 Å. Three consec- utive exposures of 1200 sec were obtained for each observa- tion. Long-exposed ThAr spectra were acquired right before and 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-correlating the science spectra with the spec- ≈ 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 0.0108). The two corresponding non-linear limb-darkening coefficients are u+ = ua + ub = 0.6696 ub = 0.1828 0.0201. We decided to keep these limb-darkening pa- rameter values fixed in the transit fitting. 0.0168 and ub = 0.2434 ± = ua − 0.0201 and u ± ± ± − 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). In such a procedure the limb-darkening pa- 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. 6. Abundances of the chemical elements measured with VWA in the HARPS co-added spectrum of CoRoT-20. The abun- 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. ± ± 1.0 km s− 1.0 km s− 0.17 and vmic = 1.10 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- 1and ous rotational velocities. We found v sin i = 4.5 1. The detailed analysis was then carried vmacro = 3.5 out using the Versatile Wavelength Analysis package (VWA) (Bruntt et al., 2004, 2010b). A first set of weak and isolated lines of Fe i and Fe ii was fitted until the derived abundances of Fe minimized the correlation with the equivalent width and the ex- 90 K, log g = 4.05 citation potentials. We found : Teff = 5880 1 which corresponds to a G2- ± type dwarf. Then the abundances of other elements 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 Mg i1b lines, the Na i D doublet and the Ca i 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 Fe i and Fe ii abundances. We thus adopted log g= 4.2 for the surface gravity. 0.1 km s− ± ± 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). This yields a straight mean of [M/H] = 0.14 ± 0.05. The error on [M/H] due to the uncertainty on Teff, log g and microturbulence is 0.11 dex, which we must add quadratically to get [M/H] = 0.14 0.12 (Bruntt et al., 2010a). We also checked for any indicators of age. We found no hint of stellar activity in the Ca ii H and K lines. However, the Li i line is clearly detected at 6708Å (see Fig. 7). We measured an equiv- alent width of Weq = 44 mÅ and determined a lithium abundance of 2.97. Following Sestito & Randich (2005), this leads to an age in the range 100 Myr up to 1 Gyr, depending on the star’s initial rotation velocity. ± 0.05 R The modeling of the star in the HR diagram was carried out in the (Teff,M1/3 ⋆ /R⋆) plane taking the host-star’s metallicity into account. It resulted in the final estimates of the star’s fundamen- , R⋆ = 1.02 tal parameters given in Table 3: M⋆ = 1.14 . The inferred surface gravity is log g = 4.47 0.11, ± in agreement within the errors with the spectroscopic result. The evolutionary status 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 40 Myr, a result in good agreement with the Li abun- dance. 0.08 M ⊙ ± ± ⊙ − 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 merged resulting 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 v sin i was performed by fitting the spectrum with a library of synthetic spectra calculated using 4 M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet Fig. 7. CoRoT-20 spectrum in a spectral region around the Li i lines at 6708Å. We calculated the distance of the star. We used the parame- ters of the star we derived and its 2-MASS magnitudes to estimate K) = 0.18 mag and the reddening. We found a color excess E(J − the absorption AV = 1.04 0.5 mag using the extinction law from ± Schlegel et al. (1998). This yields a distance of 1.23 0.12 kpc, consistent with the strong interstellar absorption observed in the Na i (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 0.3 MJup and could not be directly compared to the giant planet ∼ 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%, have a significant eccentric orbit 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 MJup which 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 Lorb is 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
60 transiting systems by Matsumura et al. (2010) who achieved a
similar conclusion, showing that the vast majority of these close-
in planets will spiral-in to their host star and will be destroyed by
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 = 1.057 and 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 v sin i (Table 3), a regular method for
transiting systems. This gives a rotational period of the star of
3.1 days, that is of the same order than the planet’s orbital
11.5
period. In the case of CoRoT-20b, Lspin⋆/Lorb < 1/3 and most of the angular momentum of the system is in the orbit. According to 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′s = 107 as found for hot Jupiters (Hansen, 2010; Ben´ıtez-Llambay et al., 2011). Because of the 5 M. Deleuil: CoRoT-20b: A very high density, high eccentricity transiting planet 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 of the planet orbit which is in the phase of fast circularization, on the transits occurrence. Assuming there is no other close massive perturber in the system, then two effects are causing TTVs: the decrease of the orbital semi-major axis and the circularization of the orbit. Concerning the orbital semi-major axis, the time-scale 5 1/Myr presently (see Fig 9). As 0.9510− of its variation, ˙a, is a consequence, a continuous period variation of ˙P/P 12 per cycle is expected. As well-known, this linear period variation C curve, and in 100 years from now the will cause a parabolic O O 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 C value will be reached in down to 5 sec in the future, this O 45 years from now. − C value will be only 410− ≈ − − − − − 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- 5 1/Myr and ˙P = lution calculations indicated ˙e = − 3 days/Myr. Assuming a constant ω, we have that − 5 days/Myr or ˙D = ˙D = 12 days/cycle. This variation is of the same order as the previous one caused by the decreasing semi-major axis, so it would be observable within a century, too. 37.56 10− 9.53 10− 1.5 10− 4.5 10− − − − 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 ± 3. A few giant planets are already reported with sim- 1.1 g cm− 3 ilar density or even higher : CoRoT-14b ρ = 7.3 1.5 g cm− (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 3(Bakos et al., 2010) for example. While the mass of these g cm− 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, in Guillot & Havel (2011), and Havel et al. (2011) for a planet of description following 1995), the 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 105, see Lainey et al. (2009)). We first note that (Q′ = 1.36 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′Jup into the planet’s Q′p, we have to take into account: (i) Q′p scales with the semi-diurnal tide period (see Ferraz-Mello et al., 2008; Matsumura et al., 2010); (ii) Q′p scales (see Eggleton et al., 1998; Ogilvie & Lin, 2004). We with R− p 106. Fig. 8 shows the variation of the thus obtain Q′p = 2.2 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. ± × 5 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 jeopardize its 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 6 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 = 1000 K 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 is qualitatively (see Leconte et al., 2011). While this result 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 0.15 M were filtered out to