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The comprehension of magnetically-related phenomena occurring in stellar atmospheres is one of the long-standing issues of astrophysics. The solar corona has always been the starting point to understand coronal physics, because the high spatial structuring of coronal plasmas complicate stellar observations. Stars however show activity levels up to 10^4 times higher than the Sun, and it is not clear how the different magnetic phenomena scale with the activity level. Therefore, direct observations of the different magnetic phenomena in active stars are crucial. However, many of them, among which are coronal mass ejections (CME), remain observationally unexplored. By performing time-resolved X-ray spectroscopy of a stellar flare, we present here the direct and unambiguous evidence of upward and downward motions of plasma within the flaring loop, and, most notably, also of the subsequent CME. The observed motions within the flaring loop neatly agree with hydrodynamic (HD) model predictions, indicating that the standard flare model holds also for flares 10^4 times more energetic than the most intense solar ones. This first direct and clear observation of a stellar CME allows us to infer its mass and kinetic energy. These findings provide crucial clues in the extrapolation of the solar case to higher activity levels, indicating that, in active stars, the kinetic energy loss due to mass expulsion appears considerably less effective.
“3D MHD simulations from the onset of the supernova to the development of the full-fledged remnant”
Antonio Tutone
The aim of this thesis is to bridge the gap between Supernovae (SNe) and their remnants (SNRs) by investigating how the remnants keep memory of the physical and chemical properties of the stellar progenitors and of the anisotropies of the explosions. I performed three-dimensional magneto-hydrodynamical simulations starting soon after the SN event and following the interaction of the SN ejecta with the circumstellar medium (consisting in the wind of the stellar progenitor), obtaining the physical scenario of a SNR. I investigated how the ejecta distribution of two different progenitors can affect the matter mixing of heavy elements in the remnant from the onset of the SN to 500 years. An 16M-ejecta blue supergiant (BSG) progenitor and a 19M-ejecta red supergiant (RSG) progenitor are investigated. Both spherical and aspherical explosions are investigated. I found that the SNR keeps memory both of the physical and chemical properties of the stellar progenitor and of the anisotropies of the explosion.
“Esplosioni di Supernovae e loro interazione con il mezzo circostante”
Luca La Mantia
Una Supernova è l’esplosione di una stella alla fine della sua evoluzione che genera, fra l’altro, un ammasso eterogeneo di gas e polveri detto Supernova remnant. A causa dell’esplosione si forma un’onda d’urto o shock che investe il materiale circumstellare e, riscaldandolo, ne fa un plasma con temperature che possono raggiungere alcuni miliardi di Kelvin. L’obiettivo della seguente tesi è studiare l’interazione del plasma con il mezzo circumstellare del Supernova remnant IC443 attraverso una semplice simulazione numerica magneto-idrodinamica del remnant. Poiché, tramite le osservazioni, si è visto che IC443 interagisce con una nube atomica a Nord-Est e con una nube toroidale molecolare a Nord-Ovest e a Sud-Est, ho analizzato l’interazione del plasma shockato con le nubi al variare della densità di particelle delle stesse. Inoltre ho confrontato le varie simulazioni tra loro e ho studiato il comportamento del campo magnetico. Le simulazioni mostrano come la variazione della densità cambi la morfologia del remnant. Confrontando le simulazioni con le immagini del remnant ho potuto concludere anche che le nubi emettono fortemente in X e che la nube toroidale non emette in ottico.