Examinando por Autor "Pignata, Guiliano"

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    A Spectropolarimetric Comparison of the Type II-Plateau Supernovae SN 2008bk and SN 2004dj
    (2011) Leonard, Douglas; Dessart, Luc; Hillier, Jhon; Pignata, Guiliano
    The Type II-Plateau supernova (SN II-P) SN 2004dj was the first SN II-P for which spectropolarimetry data were obtained with fine temporal sampling before, during, and after its fall off of the photometric plateau – the point that marks the transition from the photospheric to the nebular phase in SNe II-P. Unpolarized during the plateau, SN 2004dj showed a dramatic spike in polarization during the descent off of the plateau, and then exhibited a smooth polarization decline over the next two hundred days. This behavior was interpreted by Leonard et al. (2006) as evidence for a strongly non-spherical explosion mechanism that had imprinted asphericity only in the innermost ejecta. In this brief report, we compare nine similarly well-sampled epochs of spectropolarimetry of the Type II-P SN 2008bk to those of SN 2004dj. In contrast to SN 2004dj, SN 2008bk became polarized well before the end of the plateau and also retained a nearly constant level of polarization through the early nebular phase. Curiously, although the onset and persistence of polarization differ between the two objects, the detailed spectropolarimetric characteristics at the epochs of recorded maximum polarization for the two objects are extremely similar, feature by feature. We briefly interpret the data in light of non-Local-Thermodynamic Equilibrium, time-dependent radiative-transfer simulations specifically crafted for SN II-P ejecta.
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    Ítem
    A study of the color diversity around maximum light in Type Ia supernovae
    (2011) Cartier, Regis; Foster, Francisco; Coppi, Paolo; Hamuy, Mario; Maeda, Keiichi; Pignata, Guiliano; Folatelli, Gastón
    From a sample of 12 well-observed Type Ia supernovae, we find clear evidence of correlations between early phase (U − B), (V − R), and (V − I) colors and the velocity shifts of the [Fe II] λ7155 and [Ni II] λ7378 nebular lines measured from late-phase spectra. As these lines are thought to trace the ashes of the initial deflagration process, our findings provide additional support to the new paradigm of off-center explosions in Type Ia supernovae, and we interpret these correlations as viewing angle effects in the observed colors. We also show that the nebular velocity shifts are related to the strength and width of the Ca II H&K and IR-triplet lines near-maximum light. The evidence presented here implies that the viewing angle must be taken into account when deriving extinction values and distances in future cosmological studies based on Type Ia supernovae.