Prof Suzanne Aigrain

Abstract:

Context. The young cluster NGC 2264 was observed with the CoRoT satellite for 23 days uninterruptedly in March 2008 with unprecedented photometric accuracy. We present the first results of our analysis of the accreting population belonging to the cluster as observed by CoRoT. Aims.We search for possible light curve variability of the same nature as that observed in the classical T Tauri star AA Tau, which was attributed to a magnetically controlled inner disk warp. The inner warp dynamics is supposed to be directly associated with the interaction between the stellar magnetic field and the inner disk region. Methods.We analyzed the CoRoT light curves of 83 previously known classical T Tauri stars that belong to NGC 2264 classifying them according to their light-curve morphology. We also studied the CoRoT light-curve morphology as a function of a Spitzer-based classification of the star-disk systems. Results.The classification derived on the basis of the CoRoT light-curve morphology agrees very well with the Spitzer IRAC-based classification of the systems. The percentage of AA Tau-like light curves decreases as the inner disk dissipates, from 40%±10% in systems with thick inner disks to 36%± 16% in systems with anemic disks and zero in naked photosphere systems. Indeed, 91%±29% of the CTTS with naked photospheres exhibit pure spot-like variability, while only 18%±7% of the thick disk systems do so, presumably those seen at low inclination and thus free of variable obscuration. Conclusions. AA Tau-like light curves are found to be fairly common, with a frequency of at least ∼30 to 40% in young stars with inner dusty disks. The temporal evolution of the light curves indicates that the structure of the inner disk warp, located close to the corotation radius and responsible for the; obscuration episodes, varies over a timescale of a few (∼1-3) rotational periods. This probably reflects the highly dynamical nature of the star-disk magnetospheric interaction. © 2010 ESO.

Abstract:

We report the discovery by the CoRoT satellite of a new transiting giant planet in a 2.83 days orbit about a V = 15.5 solar analog star (M * = 1.08±0.08 M⊙, R* = 1.1±0.1 R⊙, Teff = 5675±80 K). This new planet, CoRoT-12b, has a mass of 0.92±0.07 MJup and a radius of 1.44±0.13 RJup. Its low density can be explained by standard models for irradiated planets. © ESO 2010.

Abstract:

Context. The CoRoT satellite has recently discovered the transits of an Earth-like planet across the disc of a late-type magnetically active star dubbed CoRoT-7, while a second planet was detected after filtering out the radial velocity (hereafter RV) variations due to stellar activity. Aims. We investigate the magnetic activity of CoRoT-7 and use the results for a better understanding of the impact of magnetic activity on stellar RV variations. Methods. We derived the longitudinal distribution of active regions on CoRoT-7 from a maximum entropy spot model of the CoRoT lightcurve. Assuming that each active region consists of dark spots and bright faculae in a fixed proportion, we synthesized the expected RV variations. Results. Active regions are mainly located at three active longitudes that appear to migrate at different rates, probably as a consequence of surface differential rotation, for which a lower limit of ΔΩ/Ω = 0.058 ± 0.017 is found. The synthesized activity-induced RV variations reproduce the amplitude of the observed RV curve and are used to study the impact of stellar activity on planetary detection. Conclusions. In spite of the non-simultaneous CoRoT and HARPS observations, our study confirms the validity of the method previously adopted to filter out RV variations induced by stellar activity. We find a false-alarm probability <10-4 that the RV oscillations attributed to CoRoT-7b and CoRoT-7c are spurious effects of noise and activity. Additionally, our model suggests that other periodicities found in the observed RV curve of CoRoT-7 could be explained by active regions whose visibility is modulated by a differential stellar rotation with periods ranging from 23.6 to 27.6 days. © 2010 ESO.