Elenco dei seminari passati

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.

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.

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.

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.

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.

Accretion processes onto classical T Tauri stars (CTTSs) are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona). From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field and different density profiles of the accretion stream (accounting also for non uniform streams). The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. We also investigate the detectability of line shift due to Doppler effect under different physical conditions. Finally we compare our results with the observations.

Since the past decade, the knowledge of high-energy processes in stars has experienced large advance. However, there are still some issues about physical processes taking place in stars that remain uncertain and need a more accurate study. In particular, to deep into some aspects of X-ray coronal emission, we need new observational methods and/or new instrumentation. An example of it is the poor constraints done to physical parameters of flaring loops in solar-type stars. Several theoretical and observational studies have been carried out but there is still controversy of some particular results, including the geometry of the magnetic field and the loop’s length. On the one hand, some authors believe that flaring processes in late-type stars are scaled-up versions of those taking place in the Sun and that the geometrical aspect of loops must be similar in both cases. The models used by these authors to obtain parameters from the observation of flares are so-called scaling laws. This scenario predicts stronger magnetic fields in stars than in the Sun in many cases. On the other hand, other models are based on physical laws and assume aspect rations of the flaring loops similar to that of the Sun, obtaining much longer loop lengths and magnetic fields with values more similar to those observed for the Sun. To distinguish between both scenarios, we need to use independent methods that allow us to determine the same parameters for the flare. A powerful technique to determine some parameters of the flaring loop is the wavelet analysis of light curves. I present a preliminary study of this technique using Montecarlo analysis and apply it to several stars.