The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)

Tomographic and 3-D analyses for extended, emission-line objects are applied to long-slit ESO NTT + EMMI high-resolution spectra of the intriguing planetary nebula NGC 7009, covered at twelve position angles. We derive the gas expansion law, the diagnostics and ionic radial profiles, the distance and the central star parameters, the nebular photo-ionization model and the spatial recovery of the plasma structure and evolution. The Saturn Nebula (distance~1.4 kpc, age~6000 yr, ionized mass~0.18 Mo) consists of several interconnected components, characterized by different morphology, physical conditions, excitation and kinematics. The internal shell, the main shell, the streams and the ansae expand at V(exp)~4.0xR" km/s, the outer shell, the caps and the equatorial pseudo-ring at V(exp)~3.15xR" km/s, and the halo at V(exp)~10 km/s. We compare the radial distribution of the physical conditions and the line fluxes observed in the eight sub-systems with the theoretical profiles coming from the photo-ionization code CLOUDY, inferring that all the spectral characteristics of NGC 7009 are explainable in terms of photo-ionization by the central star, a hot (logT*~4.95) and luminous (log L*/Lo~3.70) 0.60--0.61 Mo post--AGB star in the hydrogen-shell nuclear burning phase. The 3--D shaping of the Saturn Nebula is discussed within an evolutionary scenario dominated by photo-ionization and supported by the fast stellar wind: it begins with the superwind ejection, passes through the neutral, transition phase (lasting ~ 3000 yr), the ionization start (occurred ~2000 yr ago), and the full ionization of the main shell (~1000 yr ago), at last reaching the present days: the whole nebula is optically thin to the UV stellar flux, except the caps and the ansae.Comment: accepted for pub. in A&A, 28 pages, 14 figures, full text with figures available at http://web.pd.astro.it/supern/ps/h4665.ps, movies on the 3D structure available at http://web.pd.astro.it/sabbadin

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Supernova 2002ic: the collapse of a stripped-envelope, massive star in a dense medium ?

We revisit the case of SN2002ic that recently revived the debate about the progenitors of SNeIa after the claim of the unprecedented presence of hydrogen lines over a diluted SNIa spectrum. As an alternative to the previous interpretation, we suggest that SN2002ic actually was a type Ic SN, the core collapse of a massive star which lost its hydrogen and helium envelope. In this scenario the observed interaction with a dense circumstellar material (CSM) is the predictable consequence of the intense mass-loss of the progenitor and/or of the presence of a gas rich environment. With this view we establish a link between energetic SNeIc and highly interacting SNeIIn and add some credits to the proposed association of some SNeIIn to GRBs.Comment: Accepted for publication on ApJ

VLT Spectropolarimetry of the optical transient in NGC300. Evidence for asymmetry in the circumstellar dust

AIMS: The main goal of this work is to study possible signs of asymmetry in the bright optical transient in NGC300, with the aim of getting independent information on the explosion mechanism, the progenitor star and its circumstellar environment. METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the discovery, covering the spectral range 3600--9330A. RESULTS: The data show a continuum polarization at a very significant level. At least two separate components are identified. The first is characterized by a strong wavelength dependency and a constant position angle (68.6+/-0.3 degrees), which is parallel to the local spiral arm of the host galaxy. The second shows a completely different position angle (151.3+/-0.4) and displays a mild but statistically significant evolution between the two epochs. While the former is identified as arising in the interstellar dust associated with NGC300, the latter is most likely due to continuum polarization by dust scattering in the circumstellar environment. No line depolarization is detected in correspondence of the most intense emission lines, disfavoring electron scattering as the source of intrinsic polarization. This implies a very small deviation from symmetry in the continuum-forming region. Given the observed level of intrinsic polarization, the transient must be surrounded by a significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a few thousand AU. This most likely implies that one or more asymmetric outflow episodes took place during the past history of the progenitor.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 16 figure

Positron Escape from Type Ia Supernovae

We generate bolometric light curves for a variety of type Ia supernova models at late times, simulating gamma-ray and positron transport for various assumptions about the magnetic field and ionization of the ejecta. These calculated light curve shapes are compared with light curves of specific supernovae for which there have been adequate late observations. %The selection of models is generally not based upon the %ability to fit the late observations, but rather because the %model has been demonstrated by other authors to approximate the spectra %and early light curves of that specific SN. From these comparisons we draw two conclusions: whether a suggested model is an acceptable approximation of a particular event, and, given that it is, the magnetic field characteristics and degree of ionization that are most consistent with the observed light curve shape. For the ten SNe included in this study, five strongly suggest $^{56}$Co positron escape as would be permitted by a weak or radially-combed magnetic field. Of the remaining five SNe, none clearly show the upturned light curve expected for positron trapping in a strong, tangled magnetic field. Chandrasekhar mass models can explain normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar mass models have difficulties with sub- (and potentially normally) luminous SNe. An estimate of the galactic positron production rate from type Ia SNe is compared with gamma-ray observations of Galactic 511 keV annihilation radiation. Additionally, we emphasize the importance of correctly treating the positron transport for calculations of spectra, or any properties, of type Ia SNe at late epochs ($\geq$ 200 d).Comment: 82 pages including 25 figure