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Among more than 450 extra solar planets yet discovered up to now, hot Jupiters class planets are the most interesting to study the interaction of star and planet at distances around 0.1 AU or less. Variations of chromospheric activity phased with the planet orbital period have been discovered in a few systems. The detection of star planet interaction in X-rays is matter of debate with statistical studies of different groups leading to opposite conclusions. I will present results from two observations of the hot Jupiter hosting system HD189733 obtained with XMM-Newton during planetary transits. We detect a softening and a large flare that could be consistent with an X-ray SPI scenario supported by MHD simulations. Furthermore, the non detection of the M-type companion in X-rays put a stringent limit on the age of the system and hint enhancement of X-ray activity in the primary star.

Young low-mass stars are surrounded by circumstellar disks with which they interact in a complex fashion, with accretion of mass and ejection of collimated outflows. The accretion builds up the young star to its final mass and is also believed to power mass outflows, which may in turn remove the excess angular momentum from the star-disk system. However, although the process of mass accretion is a critical aspect of star formation, its mechanisms are still to be fully understood. A point not considered to date and important for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the disks, thus influencing the transport of massand angular momentum. Here we report three-dimensional magnetohydrodynamic modeling of the evolution of a flare occurring on the disk around a rotating magnetized star, which reveals that the resulting perturbation of the disk triggers a significant mass accretion. This result put forward the frequent flaring activity in young stars as a mechanism that may contribute to stellar mass accretion. This mechanism could be additional to the magnetorotational instability (or even enhance its efficiency), frequently invoked in the literature to explain the transport of mass and angular momentum in the star-disk system.

Il seminario presenta i risultati acquisiti nel corso di uno studio in fase di pubblicazione (Journal for the History of Astronomy, 2011) relativi ad un manoscritto posseduto dall’Osservatorio di Palermo. Esso contiene la descrizione di uno strumento progettato e costruito da Ramsden, di cui si era persa traccia, data l’eccezionale rarita`.