Optical and X-ray variability in pre Main Sequence Stars of NGC 2264 | Mario Guarcello ( INAF-Osservatorio Astronomico di Palermo)

Young stars are known to be variable sources both in optical, infrared, and X-ray. This variability is strictly correlated with their physical properties and the morphology of the circumstellar material, if any. The study of their light curves and the connection between the observed variability in the various regions of the electromagnetic spectrum provides a deep insight on the physical properties and evolutionary status of young stars, both with and without disks. This motivated the Coordinated Synoptic Investigation of the young open cluster NGC~2264 (CSI 2264, Cody et al. 2014, Stauffer et al. 2014). This project is an unique and unprecedented cooperative project involving simultaneous observations with 15 ground and space telescopes, and covering a wide part of the electromagnetic spectrum from X-ray to mid-infrared. In this talk I will compare the optical and X-ray variability of NGC 2264 candidate members using simultaneous observations taken with the Convection, Rotation and Planetary Transits satellite (CoRoT), and Chandra/ACIS-I. I will show that a significant correlation between the flux variability in optical and X-ray is observed among the stars with disk where the central star is periodically or irregularly obscured by material associated with the circumstellar disk, and that in some of these sources an increase of the hydrogen column nH is observed during the obscuration observed in optical. In more than 50\% of the disk-less objects the optical and X-ray flux variability are anticorrelated, in about 25% they are correlated, while the remainder do not show any correlation between the variability in the two bands. I will discuss how the distribution of spots in the photosphere and active regions in the stellar coronae, inclination of the rotational axis, and rotational period may influence this result.