Elenco dei seminari passati

To study the accretion shocks of classical T Tauri stars (CTTS) we obtained high-resolution X-ray spectra of two CTTS, V2129 Oph and V4046 Sgr, to look for phase-resolved X-ray signatures of shock-heated plasma. The 200 ks Chandra/HETGS observation of V2129 Oph (a 1.35 M_sun star, rotating in 6.5 d) covered ~0.5 stellar rotation. The 360 ks XMM/RGS observation of V4046 Sgr (a binary system, with two 0.9 M_sun components, synchronously rotating in 2.42 d) monitored ~2.2 system rotations. For both V2129 Oph and V4046 Sgr, the stellar photosphere, magnetic field, and accretion geometry were constrained by quasi-simultaneous optical monitoring (photometry, spectroscopy, and spectropolarimetry). The cool plasma component of V2129 Oph varies, with high density plasma and high EM observed during the first part of the observation, and lower density and lower EM observed during the second. The emission lines produced by the high density cool plasma of V4046 Sgr, display periodic flux variations, with a period of half the system rotational period. Our results confirm that the dense cool plasma in CTTS is material heated in the accretion shock, and that the observed X-ray variability can be explained in terms different viewing angles at different rotational phases of the accretion-shock region.

Hinode and SDO high spatial/temporal/spectral resolution solar observations provide us with accurate diagnostics of solar coronal plasmas (density, temperature, abundances,…). I will discuss some of the limitations of these diagnostics due e.g. to completeness and accuracy of the atomic data (in particular in the narrow passbands of the Atmospheric Imaging Assembly onboard SDO), or to superposition of different structures in the line of sight. In order to explore these issues several approaches are used including the analysis of high spectral resolution stellar data of the low-activity (solar-like) corona of Procyon, and the analysis of images and spectra synthesized from realistic 3D radiative MHD simulations of the solar atmosphere obtained from the state-of-the-art Bifrost code.

We present the results of the analysis of a Large Program of deep XMM-Newton observations of SN1006. We focus on the rim of the supernova remnant to derive important constraints for the shock acceleration process and for the back-reaction of the accelerated particles on the evolution of the remnant. The physical properties of the shocked ambient medium are expected to be modified by the acceleration process but up to now X-ray emission from the shocked ISM has never been detected in SN1006. The new data allow us to detect the shocked ISM and to ascertain whether the particle acceleration alters its post-shock properties. The comparison of our results with predictions from detailed MHD models of a modified supernova remnant can provide important physical insight on the physics of diffusive shock acceleration

Negli ultimi 10 anni e’ stata osservata emissione X da diversi getti protostellari: HH 154 e HH 2 nel 2000 , HH 80/81 nel 2002, DG Tau nel 2003,…. Il modello di getto espulso dalla stella progenitrice con velocita’ variabile nel tempo (getto pulsato) spiega l’emissione ottica ed X osservata come interazione tra blob espulsi in tempi diversi e con velocita’ diverse. Tale modello riproduce bene le osservazioni di alcuni getti, tra cui DG Tau (uno tra i getti con emissione X piu’ debole, in accordo con un modello di getto pulsato con basso tasso di espulsione). Le nuove osservazioni ottenute con Chandra nel 2010 del getto HH 154 tuttavia mostrano la prima evidenza di shock stazionario da getti protostellari: su una base temporale di circa 10 anni (la prima osservazione Chandra di tale getto risale infatti al 2001), la sorgente X mostra una componente stazionaria alla base del getto. Per spiegare questi nuovi risultati abbiamo sviluppato un modello di getto espulso nel mezzo imperturbato attraverso un nozzle (che puo’ essere associato alla presenza di un campo magnetico nella regione di lancio e collimazione del getto) che provoca la formazione di un diamond shock stazionario con proprieta’ fisiche (luminosita’ X, temperatura di best-fit,…) consistenti con le osservazioni.

The question of what heats the million degrees solar corona has been debated for decades. One scenario proposed since the 80s has been a finely stranded corona where each strand is heated by a rapid pulse. Unfortunately, neither such fine structure has been resolved, nor direct or conclusive evidence has been found so far, e.g., extensive superhot plasma, nanoflaring activity, so that alternative hypotheses have been proposed. Recently it has been shown that the observed difference in appearance of cool and warm coronal loops (~1 MK, ~2-3 MK, respectively) – cool loops show fine structure and warm loops appear “fuzzy” – can be explained with multi-stranded coronal loops pulse-heated up to 10 MK, where the strands are interpreted as subarcsecond via modeling. That work predicts that images of hot coronal loops (>6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, and that therefore fine-structured energy pulses play a major role in heating the active corona.

Young stars in the Classical T Tauri phase are characterized by complex and highly dynamical phenomena involving the stars, their circumstellar disks, mass accretion, and outflows. Quite unsurprisingly, high energy processes, as traced by, e.g., the X-ray emission, are affected or even driven by the interaction between these components. Because of the widely different characteristic temperatures of the involved physical components and of the dynamical nature of their interactions, coordinated multi-wavelength time-variability studies are best suited to their investigation. In March 2008, we have observed NGC 2264 with CoRoT for 23.5 days obtaining high-quality uninterrupted optical light-curves of its young stars. During the CoRoT pointing, two short Chandra observations were performed with a separation of 16 days, allowing us to study the correlation between optical and X-ray variability on this timescale, and thus the physical mechanism responsible for the variability. The variabilities of Classical T Taury Stars (CTTS) in the optical and soft X-ray (0.5-1.5 keV) bands are correlated, while no correlation is apparent in the hard (1.5-8.0 keV) band. Also, no correlation in either band is present for Weak line T Tauri stars. The correlation between soft X-ray and optical variability of CTTSs can be naturally explained in terms of time-variable shading (absorption) from circumstellar material orbiting the star, in a scenario rather similar to the one invoked to explain the observed phenomenology in the CTTS AA Tau. The slope of the observed correlation implies (in the hypothesis of homogeneous shading) a significant dust depletion in the circumstellar material.

La composizione chimica superficiale delle stelle di tipo solare e nelle loro atmosfere esterne puo’ essere considerato a buon diritto il primo tema di ricerca astrofisica mai affrontato (vi ricordate di Fraunhofer?). Dopo due secoli di studi pero’ si continuano a trovare risultati sui quali la comunita’ scientifica discute vivacemente. In particolare, le abbondanze chimiche fotosferiche e coronali nelle stelle di tipo spettrale avanzato sembrano essere significativamente diverse in molti casi, ma i dati osservativi non sono ancora sufficienti a capire il quando e il perche’. Un’altra curiosa scoperta recente e’ il fatto che le stelle che ospitano pianeti di tipo gioviano hanno in media una metallicita’ fotosferica maggiore delle altre di circa un fattore 2. Sono supermetalliche anche le loro corone? Abbiamo cercato di aggiungere un tassello a questo puzzle tramite un’osservazione in raggi X di Tau Bootis, una stella molto ben studiata proprio per la presenza di un pianeta massiccio in orbita stretta. L’obiettivo di questa osservazione e’ stato quello di misurare le abbondanze chimiche del plasma coronale e confrontarle con quelle ben note della fotosfera (supermetallica). Se volete sapere quello che abbiamo trovato (vedi figura) venite a sentirvi il seminario.

According to theory, high energy emission from the coronae of cool stars can severely erode the atmosphere of orbiting planets. To test the long term effects of the erosion we study a large sample of planet-hosting stars observed in X-rays. The results reveal that massive planets (M sin i > 1.5Mj) may survive only if exposed to low accumulated coronal radiation. The planet HD 209458 b might have lost 0.8 Mj already, and other cases, like tau Boo b, could be losing mass at a rate of 3.4 Ms/Gyr. The strongest erosive effects would take place during the first stages of the stellar life, when the faster rotation generates more energetic coronal radiation. The planets with higher density seem to resist better the radiation effects, as foreseen by models. Current models need to be improved to explain the observed distribution of planetary masses with the coronal radiation received.

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`.

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.