C. Melo

A correlation between the heavy element content of transiting extrasolar planets and the metallicity of their parent stars

Context. Nine extrasolar planets with masses between 110 and 430M⊕ are known to transit their star. The knowledge of their masses and radii allows an estimate of their composition, but uncertainties on equations of state, opacities and possible missing energy sources imply that only inaccurate constraints can be derived when considering each planet separately. Aims. We seek to better understand the composition of transiting extrasolar planets by considering them as an ensemble, and by comparing the obtained planetary properties to that of the parent stars. Methods. We use evolution models and constraints on the stellar ages to derive the mass of heavy elements present in the planets. Possible additional energy sources like tidal dissipation due to an i nclined orbit or to downward kinetic energy transport are considered. Results. We show that the nine transiting planets discovered so far belong to a quite homogeneous ensemble that is characterized by a mass of heavy elements that is a relatively steep function of the s tellar metallicity, from less than 20 earth masses of heavy elements around solar composition stars, to up to∼ 100 M⊕ for three times the solar metallicity (the precise values be ing model-dependant). The correlation is still to be ascertained however. Statistical tests imply a worst-ca se 1/3 probability of a false positive. Conclusions. Together with the observed lack of giant planets in close orbits around metal-poor stars, these results appear to imply t hat heavy elements play a key role in the formation of close-in giant planets. The large masses of heavy elements inferred for planets orbiting metal rich stars was not anticipated by planet formation models and shows the need for alternative theories including migration and subsequent collection of planetesimals.

TW Hydrae: evidence of stellar spots instead of a Hot Jupiter

Context. TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10 MJup planet orbiting the star at 0.04 AU. Aims. The aim of this letter is to confirm the presence of the giant planet around TW Hya by (i) testing whether the observed RV variations can be caused by stellar spots and (ii) analyzing new optical and infrared data to detect the signal of the planet companion. Methods. We fitted the RV variations of TW Hya using a cool spot model. In addition, we obtained new high-resolution optical & infrared spectra, together with optical photometry of TW Hya and compared them with previous data. Results. Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54 ◦ can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m s −1 , which is less than the errors of the RV measurements discussed in Setiawan et al. (2008, Nature, 451, 38). The analysis of our new optical RV data, with typical errors of 10 m s −1 , shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m s −1 ) that is not consistent with the published optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter. Finally, the photometric data shows a 20% (peak to peak) variability, which is much larger than the 4% variation expected for the modeled cool spot. The fact that the optical data are correlated with the surface of the cross-correlation function points towards hot spots as being responsible for the photometric variability. Conclusions. We conclude that the best explanation for the RV signal observed in TW Hya is the presence of a cool stellar spot and not an orbiting hot Jupiter.

Two new “very hot Jupiters” among the OGLE transiting candidates

As a result of a radial velocity follow-up of OGLE planetary transit candidates in Carina, we report the discovery of two new transiting planets with very short orbital periods: OGLE-TR-113 with m = 1.35 ± 0.22 MJup, r = 1.08 +0.07 −0.05 RJup, P = 1.43 days, and OGLE-TR-132 with m = 1.01 ± 0.31 MJup, r = 1.15 +0.80 −0.13 RJup, P = 1.69 days. These detections bring to three the number of known very hot Jupiter (Jovian exoplanets like OGLE-TR-56 with periods around 1.5 days). This indicates that the accumulation of periods around 3 days found in radial velocity surveys does not reflect an absolute limit.

Doppler follow-up of OGLE transiting companions in the Galactic bulge

Two years ago, the OGLE-III survey (Optical Gravitational Lensing Experiment) announced the detection of 54 short period multi-transiting objects in the Galactic bulge (Udalski et al. 2002a,b). Some of these objects were considered to be po- tential hot Jupiters. In order to determine the true nature of these objects and to characterize their actual mass, we conducted a radial velocity follow-up of 18 of the smallest transiting candidates. We describe here our procedure and report the char- acterization of 8 low-mass star-transiting companions, 2 grazing eclipsing binaries, 2 triple systems, 1 confirmed exoplanet (OGLE-TR-56b), 1 possible exoplanet (OGLE-TR-10b), 1 clear false positive and 3 unsolved cases. The variety of cases en- countered in our follow-up covers a large part of the possible scenarios occurring in the search for planetary transits. As a by-product our program yields precise masses and radii of low mass stars.

The “missing link”: A 4-day period transiting exoplanet around OGLE-TR-111

We report the discovery of a transiting hot Jupiter around OGLE-TR-111, from our radial velocity follow-up of OGLE transiting candidates in Carina. The planet has a mass of

A new Neptune-mass planet orbiting HD 219828

0.53 pm 0.11

Spectroscopy of very low mass stars and brown dwarfs in the Lambda Orionis star forming region I. Enlarging the census down to the planetary mass domain in Collinder 69

xa0 M J and a radius of

Chemical abundances in six nearby star-forming regions Implications for galactic evolution and planet searches around very young stars

1.0^{+0.13}_{-0.06}

The short period multiplicity among T Tauri stars

xa0 R J . Three transiting exoplanets have already been found among OGLE candidates, all with periods near 1.5 days. The planet presented here, with

Search for associations containing young stars: chemical tagging IC 2391 and the Argus association

P=4.0