Supernova 1996L: evidence of a strong wind episode before the explosion

Observations of the type II SN 1996L reveal the presence of a slowly expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow emission features are visible in the early spectra superposed on the normal SN spectrum. Within about two months these features develop narrow symmetric P-Cygni profiles. About 100 days after the explosion the light curve suddenly flattens, the spectral lines broaden and the Halpha flux becomes larger than what is expected from a purely radioactive model. These events are interpreted as signatures of the onset of the interaction between the fast moving ejecta and a slowly moving outer shell of matter ejected before the SN explosion. At about 300 days the narrow lines disappear and the flux drops until the SN fades away, suggesting that the interaction phase is over and that the shell has been swept away. Simple calculations show that the superwind episode started 9 yr before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3) M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day 270 show relatively strong emission lines of HeI. These lines are narrower than other emission lines coming from the SN ejecta, but broader than those from the CSM. These high excitation lines are probably the result of non-thermal excitation and ionization caused by the deposition of the gamma-rays emitted in the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.

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

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