L. Jorda

Transiting exoplanets from the CoRoT space mission II. CoRoT-Exo-2b: A transiting planet around an active G star

Context. The CoRoT mission, a pioneer in exoplanet searches from space, has completed its first 150 days of continuous observations of ∼12 000 stars in the galactic plane. An analysis of the raw data identifies the most promising candidates and triggers the ground-based follow-up. Aims. We report on the discovery of the transiting planet CoRoT-Exo-2b, with a period of 1.743 days, and characterize its main parameters. Methods. We filter the CoRoT raw light curve of cosmic impacts, orbital residuals, and low frequency signals from the star. The folded light curve of 78 transits is fitted to a model to obtain the main parameters. Radial velocity data obtained with the SOPHIE, CORALIE and HARPS spectrographs are combined to characterize the system. The 2.5 min binned phase-folded light curve is affected by the effect of sucessive occultations of stellar active regions by the planet, and the dispersion in the out of transit part reaches a level of 1.09 × 10 −4 in flux units. Results. We derive a radius for the planet of 1.465 ± 0.029 RJup and a mass of 3.31 ± 0.16 MJup, corresponding to a density of 1.31 ± 0.04 g/cm 3 . The large radius of CoRoT-Exo-2b cannot be explained by current models of evolution of irradiated planets.

Transiting exoplanets from the CoRoT space mission I - CoRoT-Exo-1b: a low-density short-period planet around a G0V star

Context. The pioneer space mission for photometric planet searches, CoRoT, steadily monitors about 12,000 stars in each of its fields of view; it is able to detect transit candidates early in the processing of the data and before the end of a run. Aims. We report the detection of the first planet discovered by CoRoT and characterizing it with the help of follow-up observations. Methods. Raw data were filtered from outliers and residuals at the orbital period of the satellite. The orbital parameters and the radius of the planet were estimated by best fitting the phase folded light curve with 34 successive transits. Doppler measurements with the SOPHIE spectrograph permitted us to secure the detection and to estimate the planet mass. Results. The accuracy of the data is very high with a dispersion in the 2.17 min binned phase-folded light curve that does not exceed 3.10-4 in flux unit. The planet orbits a mildly metal-poor G0V star of magnitude V=13.6 in 1.5 days. The estimated mass and radius of the star are 0.95+-0.15Msun and 1.11+-0.05Rsun. We find the planet has a radius of 1.49+-0.08Rjup, a mass of 1.03+-0.12Mjup, and a particularly low mean density of 0.38 +-0.05g cm-3.

Transiting exoplanets from the CoRoT space mission IV. CoRoT-Exo-4b : a transiting planet in a 9.2 day synchronous orbit

CoRoT, the first space-based transit search, provides ultra-high precision light curves with continuous time-sampling over periods, of up to 5 months. This allows the detection of transiting planets with relatively long periods, and the simultaneous study of the host stars photometric variability. In this letter, we report on the discovery of the transiting giant planet CoRoT-Exo-4b and use the CoRoT light curve to perform a detailed analysis of the transit and to determine the stellar rotation period. The CoRoT light curve was pre-processed to remove outliers and correct for orbital residuals and artefacts due to hot pixels on the detector. After removing stellar variability around each transit, the transit light curve was analysed to determine the transit parameters. A discrete auto-correlation function method was used to derive the rotation period of the star from the out-of-transit light curve. We derive periods for the planets orbit and stars rotation of 9.20205 +/- 0.00037 and 8.87 +/- 1.12 days respectively, consistent with a synchronised system. We also derive the inclination, i = 90.00 -0.085 +0.000 in degrees, the ratio of the orbital distance to the stellar radius, a/R_s = 17.36 -0.25 +0.05, and the planet to star radius ratio R_p/R_s = 0.1047 -0.0022 +0.0041. We discuss briefly the coincidence between the orbital period of the planet and the stellar rotation period and its possible implications for the systems migration and star-planet interaction history.

Transiting exoplanets from the CoRoT space mission XI. CoRoT-8b: a hot and dense sub-Saturn around a K1 dwarf

Aims. We report the discovery of CoRoT-8b, a dense small Saturn-class exoplanet that orbits a K1 dwarf in 6.2 days, and we derive its orbital parameters, mass, and radius. Methods. We analyzed two complementary data sets: the photometric transit curve of CoRoT-8b as measured by CoRoT and the radial velocity curve of CoRoT-8 as measured by the HARPS spectrometer �� . Results. We find that CoRoT-8b is on a circular orbit with a semi-major axis of 0.063 ± 0.001 AU. It has a radius of 0.57 ± 0.02 RJ ,am ass of 0.22 ± 0.03 MJ, and therefore a mean density of 1.6 ± 0. 1gc m −3 . Conclusions. With 67% of the size of Saturn and 72% of its mass, CoRoT-8b has a density comparable to that of Neptune (1.76 g cm −3 ). We estimate its content in heavy elements to be 47–63 M⊕, and the mass of its hydrogen-helium envelope to be 7–23 M⊕. At 0.063 AU, the thermal loss of hydrogen of CoRoT-8b should be no more than ∼0.1% over an assumed integrated lifetime of 3 Ga.

Noise properties of the CoRoT data - A planet-finding perspective

In this short paper, we study the photometric precision of stellar light curves obtained by the CoRoT satellite in its planet-finding channel, with a particular emphasis on the time scales characteristic of planetary transits. Together with other articles in the same issue of this journal, it forms an attempt to provide the building blocks for a statistical interpretation of the CoRoT planet and eclipsing binary catch to date. After pre-processing the light curves so as to minimise long-term variations and outliers, we measure the scatter of the light curves in the first three CoRoT runs lasting more than 1 month, using an iterative non-linear filter to isolate signal on the time scales of interest. The behaviour of the noise on 2 h time scales is described well by a power-law with index 0.25 in R-magnitude, ranging from 0.1 mmag at R = 11. 5t o 1m mag atR = 16, which is close to the pre-launch specification, though still a factor 2−3 above the photon noise due to residual jitter noise and hot pixel events. There is evidence of slight degradation in the performance over time. We find clear evidence of enhanced variability on hour time scales (at the level of 0.5 mmag) in stars identified as likely giants from their R magnitude and B − V colour, which represent approximately 60 and 20% of the observed population in the directions of Aquila and Monoceros, respectively. On the other hand, median correlated noise levels over 2 h for dwarf stars are extremely low, reaching 0.05 mmag at the bright end.

Shape modeling technique KOALA validated by ESA Rosetta at (21) Lutetia

Abstract We present here a comparison of our results from ground-based observations of asteroid (21) Lutetia with imaging data acquired during the flyby of the asteroid by the ESA Rosetta mission. This flyby provided a unique opportunity to evaluate and calibrate our method of determination of size, 3-D shape, and spin of an asteroid from ground-based observations. Knowledge of certain observable physical properties of small bodies ( e.g. , size, spin, 3-D shape, and density) have far-reaching implications in furthering our understanding of these objects, such as composition, internal structure, and the effects of non-gravitational forces. We review the different observing techniques used to determine the above physical properties of asteroids and present our 3-D shape-modeling technique KOALA – Knitted Occultation, Adaptive-optics, and Lightcurve Analysis – which is based on multi-dataset inversion. We compare the results we obtained with KOALA, prior to the flyby, on asteroid (21) Lutetia with the high-spatial resolution images of the asteroid taken with the OSIRIS camera on-board the ESA Rosetta spacecraft, during its encounter with Lutetia on 2010 July 10. The spin axis determined with KOALA was found to be accurate to within 2°, while the KOALA diameter determinations were within 2% of the Rosetta-derived values. The 3-D shape of the KOALA model is also confirmed by the spectacular visual agreement between both 3-D shape models (KOALA pre- and OSIRIS post-flyby). We found a typical deviation of only 2xa0km at local scales between the profiles from KOALA predictions and OSIRIS images, resulting in a volume uncertainty provided by KOALA better than 10%. Radiometric techniques for the interpretation of thermal infrared data also benefit greatly from the KOALA shape model: the absolute size and geometric albedo can be derived with high accuracy, and thermal properties, for example the thermal inertia, can be determined unambiguously. The corresponding Lutetia analysis leads to a geometric albedo of 0.19±0.01 and a thermal inertia below 40xa0Jxa0m −2 xa0s −0.5 xa0K −1 , both in excellent agreement with the Rosetta findings. We consider this to be a validation of the KOALA method. Because space exploration will remain limited to only a few objects, KOALA stands as a powerful technique to study a much larger set of small bodies using Earth-based observations.

Planetary transit candidates in the CoRoT LRa01 field

Context: CoRoT is a pioneering space mission whose primary goals are stellar seismology and extrasolar planets search. Its surveys of large stellar fields generate numerous planetary candidates whose lightcurves have transit-like features. An extensive analytical and observational follow-up effort is undertaken to classify these candidates. Aims: The list of planetary transit candidates from the CoRoT LRa01 star field in the Monoceros constellation towards the Galactic anti-center is presented. The CoRoT observations of LRa01 lasted from 24 October 2007 to 3 March 2008. Methods: 7470 chromatic and 3938 monochromatic lightcurves were acquired and analysed. Instrumental noise and stellar variability were treated with several filtering tools by different teams from the CoRoT community. Different transit search algorithms were applied to the lightcurves. Results: Fifty-one stars were classified as planetary transit candidates in LRa01. Thirty-seven (i.e., 73 % of all candidates) are good planetary candidates based on photometric analysis only. Thirty-two (i.e., 87 % of the good candidates) have been followed-up. At the time of this writing twenty-two cases have been solved and five planets have been discovered: three transiting hot-Jupiters (CoRoT-5b, CoRoT-12b, and CoRoT-21b), the first terrestrial transiting planet (CoRoT-7b), and another planet in the same system (CoRoT-7c, detected by radial velocity survey only). Evidences of another non-transiting planet in the CoRoT-7 system, namely CoRoT-7d, have been recently found.

Spitzer Space Telescope observations of the nucleus of comet 67P/Churyumov-Gerasimenko

Context. Comet 67P/Churyumov-Gerasimenko is the target of the Rosetta mission, and an early characterization of its nucleus is required to assist in preparing the orbital strategy of the spacecraft, the delivery of the Philae surface module and the science operations. We detected the nucleus using the Hubble Space Telescope in March 2003, but had to assume an albedo to derive its size from its observed magnitudes. Aims. It is important to derive an additional constraint for independently determining both the comet size and albedo. Methods. We implemented the well-known “radiometric method”, which combines visible and infrared photometry. Sixteen thermal images of 67P/C-G were obtained by the Multiband Imaging Photometer (MIPS) 24xa0 μ m channel of the Spitzer Space Telescope (SST) on 25xa0February 2004: the observations spanned a time interval of ~12.5xa0h, which sampled the rotational light curve of its nucleus. The comet was then outbound at a heliocentric distance of 4.48xa0AU, at a distance of 4.04xa0AU from SST, and at a solar phase angle of 12.1°. The nucleus conspicuously appeared as a bright point source superimposed on a dust trail; it was necessary to apply the point-spread function fitting technique using an adequate model of the trail to correctly determine the thermal flux from the nucleus. The data were analyzed using a standard thermal model that incorporated the thermal inertia. Results. Our preferred solution with a low thermal inertia has overall dimensions measured along the principal axis of 4.40–5.20xa0km, 4.16–4.30 km, and 3.40–3.50xa0km, corresponding to an effective radius of a sphere with the same volume in the range of 1.93–2.03xa0km. Larger values of thermal inertia produce larger sizes but the effective radius cannot exceed ~2.3xa0km. The albedo is in the range 0.039–0.043, remarkably consistent with the canonical value of 0.04 for cometary nuclei. The success of the landing of the Philae surface module remains critically dependent upon the bulk density of the nucleus: it would be safe if close to 0.35xa0gxa0cm -3 , but a larger value, for instance 0.5xa0gxa0cm -3 , would present some risks.

Properties of the nuclei and comae of 13 ecliptic comets from Hubble Space Telescope snapshot observations

Context. Knowledge of the size distribution of cometary nuclei and, more generally, of their physical properties is important for constraining models of the formation and evolution of the Solar System. Aims. We report on our on-going effort to determine the ensemble properties of comets based on our success in detecting the nuclei of active comets with the Hubble Space Telescope (HST). Methods. During cycle 8 (Julyxa01999 to Junexa02000), we observed 13 ecliptic comets with the planetary cameraxa02 through at least two filters ( V and R ) and up to four ( B , V , R , I ) for the brightest ones. The ~30xa0min of HST observational time devoted to each comet did not permit a proper determination of light curves, so our “snapshot” observations yield effective radii, not shapes. Results. Assuming spherical nuclei with a geometric albedo of 0.04 for the R xa0band (except 0.024 for 10P/Tempelxa02, as independently measured) and a phase law of 0.04xa0mag/deg, we obtained the following effective radii: 4P/Faye: 1.77xa0km, 10P/Tempel 2: 5.98xa0km, 17P/Holmes: 1.71xa0km, 37P/Forbes: 0.81xa0km 44P/Reinmuth 2: 1.61xa0km, 50P/Arend: 0.95xa0km, 59P/Kearns–Kwee: 0.79xa0km, 63P/Wildxa01: 1.46xa0km, 71P/Clark: 0.68xa0km, 84P/Giclas: 0.90xa0km, 106P/Schuster: 0.94xa0km, 112P/Urata–Niijima: 0.90xa0km, 114P/Wiseman–Skiff: 0.78xa0km. In our present sample, eight out of thirteen nuclei have sub-kilometer radii. The average color of the observed nuclei is

Asteroid 2867 Steins II. Multi-telescope visible observations, shape reconstruction, and rotational state

(V-R)