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Characterizing UV variability and accretion in the young open cluster NGC 2264
I will present the results of an extensive UV/optical variability survey of the young open cluster NGC 2264 (3 Myr), performed at CFHT/MegaCam as a part of a wide project of simultaneous multi-wavelength (X-rays to IR) monitoring aimed at unambiguously characterizing YSO variability (the Coordinated Synoptic Investigation of NGC 2264). A complete u,g,r,i photometric dataset has been obtained for more than 700 young stars, ranging in mass from 0.2 to 2 MSun, and their u-band and r-band variability monitored over two full weeks. The u-band observations offer a direct access to the accretion features, hence providing a unique clue to the accretion dynamics throughout the region. I investigate the photometric properties of different stellar groups on various color-color and color-magnitude diagrams and infer a straightforward characterization based on accretion properties. I analyze the u-band variability of T Tauri stars on week timescales and probe the color signatures of different physical processes, showing that well-distinguished behaviors are specific to processes of different nature. Based on the UV excess diagnostics, I derive a dynamical picture of accretion in NGC 2264. I investigate the dependence of the inferred mass accretion rates on stellar mass and discuss the large spread in values detected at each mass. I explore the variability of the mass accretion rates on a timescale of weeks, resulting from the geometric effects linked with stellar rotation and from the intrinsic accretion variability, and show that this variability cannot explain the observed spread.

We study the rotation-activity relationship for low-mass members of the young cluster h Persei (~13 Myr). h Per, thanks to its age, allows us to link the rotation-activity relation observed for main-sequence stars to the puzzling case of very young PMS stars. We constrained the activity levels of h~Per members by analyzing a deep Chandra/ACIS-I observation pointed to the central field of h Per. Considering also the catalog of h Per members with measured rotational period, presented by Moraux et al. (2013), we obtained a final catalog of 202 h Per members with measured X-ray luminosity and rotational period. We investigate the rotation-activity relation of h Per members considering different mass ranges. We find that stars with 1.3 Msun < M < 1.4 Msun show significant evidence of supersaturation for short periods. This phenomenon is instead not observed for lower mass stars.

Nanoflares are candidates to produce most of the background emission from the solar corona. Recently, much work has been done with different models accompanied by different observational data to investigate the impulsive nature of heating the corona. One question is what is the weight of the events at different scales from small to large. In an attempt to improve the previous work we are about to simulate the light curves from an interest part of a full disk image of the solar corona taken by SDO/AIA, using a 0D model (EBTEL, Enthalpy Based Themal Evolution of Loop) as basic loop model. We use the simulated light curves as the trained data set and the observational time series as the test data set in the framework of an artificial intelligence system. The two data sets will feed an Artificial Neural Network (ANN) which is suitable in classifying and recognizing the samples. The comparison of the two data sets will help us to evaluate the distribution of the events that best matches the observations.

The outer atmospheres of late-type stars are characterized by strong emission in excess of the stellar photosphere. Through its relation with rotation the origin of this activity in a stellar dynamo was recognized long ago. Yet, the strength of magnetic activity has not been calibrated across the whole electromagnetic spectrum and its dependence on stellar mass and age has remained widely elusive. Most previous efforts have concentrated on solar-type stars because of their relative brightness and the direct connection with the Sun. Here, I present recent observational studies of magnetic activity on M stars. Low-mass stars are interesting planet hosts because they exist in large numbers, they are long-lived, and they provide a lower contrast between stellar/planet properties enhancing the chance for planet detection. The high-energy emission related with magnetic activity may be crucial for the evolution of planetary atmospheres, in particular for M stars which are notoriously strongly active. I present a wide range of diagnostics of chromospheric and coronal emission in the X-ray, UV and optical band. The data discussed comprise both photometric observations (obtained with GALEX, ROSAT and XMM-Newton) and spectroscopic measurements (obtained with X-Shooter@VLT). The aim of these studies is to establish connections between the emissions in different energy bands and determine how they change throughout the stellar evolution.

The supernova remnant SN 1006 is a powerful source of high-energy particles and evolves in a tenuous and uniform environment. The X-ray image of SN 1006 reveals an indentation in the southwestern part of the shock front and the HI maps show an isolated cloud (hereafter southwestern cloud) whose morphology fits perfectly in the indentation. We performed spatially resolved spectral analysis of a set of small regions in the southwestern nonthermal limb. We also analyzed archive HI data, obtained combining single dish and interferometric observations. We found that the best-fit value of the NH derived from the X-ray spectra significantly increases in regions corresponding to the southwestern cloud, while the cutoff energy of the synchrotron emission decreases. The amount of the NH variations corresponds perfectly with the column density of the southwestern cloud, as estimated from the radio data. The decrease in the cutoff energy at the indentation clearly reveals that the cloud is actually interacting with the remnant. The presence of a dense environment near a region where efficient particle acceleration is at work makes the southwestern limb a promising source of gamma-ray hadronic emission. We estimate that such emission will be detectable with the Fermi telescope within a few years.

One interesting question regarding the interaction between stars and planets and the birth of life is how the planetar atmosphere shields the stellar radiation. In particular, we expect that high energy radiation less penetrates into the planetary atmosphere. One implication is that, in principle, the planet should appear larger in a stellar transit observed in high energy bands. Venus transits might be excellent testing grounds to study this effect. The Venus transit of June 2012 has been monitored in great detail by solar spatial missions, and in particular by imaging instruments on-board Hinode and Solar Dynamics Observatory. We apply a statistical and photometric technique to measure the radius of Venus during the transit in three different bands: optical, EUV and X-rays, and show the results…