The progenitor of the Type IIP SN 2008bk was discovered in pre-explosion griIYJHKs images, acquired with European Southern Observatory Very Large Telescope FORS, HAWK-I and ISAAC instruments and the Gemini GMOS-S instrument. The wealth of pre-explosi
on observations makes the progenitor of this SN one of the best studied, since the detection of the progenitor of SN1987A. Previous analyses of the properties of the progenitor were hampered by the limited quality of the photometric calibration of the pre-explosion images and the crowded nature of the field containing the SN. We present new late-time observations of the site of SN2008bk acquired with identical instrument and filter configurations as the pre-explosion observations, and confirm that the previously identified red supergiant star was the progenitor of this SN and has now disappeared. Image subtraction techniques were used to conduct precise photometry of the now missing progenitor, independently of blending from any nearby stars. The nature of the surrounding stellar population and their contribution to the flux attributed to the progenitor in the pre-explosion images are probed using HST WFC3 UVIS/IR observations. In comparison with MARCS synthetic spectra, we find the progenitor was a highly reddened RSG with luminosity log (L/Lsun)=4.84+/-0.11, corresponding to an initial mass of Minit=12.9+/-1.7Msun. The temperature of the progenitor was hotter than previously expected for RSGs (T ~ 4330K), but consistent with new temperatures derived for RSGs using SED fitting techniques. We show that there is evidence for significant extinction of the progenitor, possibly arising in the CSM; but that this dust yields a similar reddening law to dust found in the ISM (E(B-V)=0.77 with Rv=3.1). [Abridged]
We report two spectropolarimetric observations of SN 2005hk, which is a close copy of the very peculiar SN 2002cx, showing low peak luminosity, slow decline, high ionization near peak and an unusually low expansion velocity of only about 7,000 km s^-
1. Further to the data presented by Chornock et al., (2006), at -4 days before maximum, we present data of this object taken on 9 November 2005 (near maximum) and 23 November (+ two weeks) that show the continuum and most of the spectral lines to be polarized at levels of about 0.2-0.3%. At both epochs the data corresponds to the Spectropolarimetric Type D1. The general low level of line polarization suggests that the line forming regions for most species observed in the spectrum have a similar shape to that of the photosphere, which deviates from a spherical symmetry by <10%. In comparison with spectropolarimetry of Type Ia and Core-collapse SNe at similar epochs, we find that the properties of SN 2005hk are most similar to those of Type Ia SNe. In particular, we find the low levels of continuum and line polarization to indicate that the explosion mechanism is approximately spherical, with homogeneous ejecta (unlike the chemically segregated ejecta of CCSNe). We discuss the possibility that SN 2005hk was the result of the pure deflagration of a white dwarf and note the issues concerning this interpretation.
We present a compilation of the geometry measures acquired using optical and IR spectroscopy and optical spectropolarimetry to probe the explosion geometry of Type Ia SNe. Polarization measurements are sensitive to asymmetries in the plane of the sky
, whereas line profiles in nebular phase spectra are expected to trace asymmetries perpendicular to the plane of the sky. The combination of these two measures can overcome their respective projection effects, completely probing the 3D structures of these events. For 9 normal Type Ia SNe, we find that the polarization of ion{Si}{2} $lambda 6355$ at 5 days before maximum ($p_{Si,II}$) is well correlated with its velocity evolution ($dot{rm v}_{Si,II}$), implying $dot{rm v}_{Si,II}$ is predominantly due to the asymmetry of the SNe. We find only a weak correlation between the polarization of ion{Si}{2} and the reported velocities (${rm v}_{neb}$) for peak emission of optical ion{Fe}{2} and ion{Ni}{2} lines in nebular spectra. Our sample is biased, with polarization measurements being only available for normal SNe which subsequently exhibited positive (i.e. redshifted) ${rm v}_{neb}$. In unison these indicators are consistent with an off-centre delayed detonation, in which the outer layers are dominated by a spherical oxygen layer, mixed with an asymmetric distribution of intermediate mass elements. The combination of spectroscopic and spectropolarimetric indicators suggests a single geometric configuration for normal Type Ia SNe, with some of the diversity of observed properties arising from orientation effects.
We present spectropolarimetric observations of the Type IIb SN 2001ig in NGC 7424; conducted with the ESO VLT FORS1 on 2001 Dec 16, 2002 Jan 3 and 2002 Aug 16 or 13, 31 and 256 days post-explosion. These observations are at three different stages of
the SN evolution: (1) The hydrogen-rich photospheric phase, (2) the Type II to Type Ib transitional phase and (3) the nebular phase. At each of these stages, the observations show remarkably different polarization properties as a function of wavelength. We show that the degree of interstellar polarization is 0.17%. The low intrinsic polarization (~0.2%) at the first epoch is consistent with an almost spherical (<10% deviation from spherical symmetry) hydrogen dominated ejecta. Similar to SN 1987A and to Type IIP SNe, a sharp increase in the degree of the polarization (~1%) is observed when the outer hydrogen layer becomes optically thin by day 31; only at this epoch is the polarization well described by a ``dominant axis. The polarization angle of the data shows a rotation through ~40 degrees between the first and second epochs, indicating that the asymmetries of the first epoch were not directly coupled with those observed at the second epoch. For the most polarized lines, we observe wavelength-dependent loop structures in addition to the dominant axis on the Q-U plane. We show that the polarization properties of Type IIb SNe are roughly similar to one another, but with significant differences arising due to line blending effects especially with the high velocities observed for SN 2001ig. This suggests that the geometry of SN 2001ig is related to SN 1993J and that these events may have arisen from a similar binary progenitor system.
We present spectropolarimetric observations of the peculiar Type Ib/c SN 2005bf, in MCG+00-27-005, from 3600-8550AA. The SN was observed on 2005 April 30.9, 18 days after the first B-band light-curve maximum and 6 days before the second B-band light-
curve maximum. The degree of the Interstellar Polarization, determined from depolarized emission lines in the spectrum, is found to be large with $p_{max}(ISP)=1.6%$ and $theta(ISP)=149$fdg$7pm4.0$, but this may be an upper limit on the real value of the ISP. After ISP subtraction, significant polarization is observed over large wavelength regions, indicating a significant degree of global asymmetry, $gtrsim 10%$. Polarizations of 3.5% and 4% are observed for absorption components of Ca II H&K and IR triplet, and 1.3% for He I 5876AA and Fe II. On the $Q-U$ plane clear velocity-dependent loop structure is observed for the He I 5876AA line, suggestive of departures from an axial symmetry and possible clumping of the SN ejecta. Weak High Velocity components of $mathrm{Halpha}$, $mathrm{Hbeta}$ and $mathrm{Hgamma}$ are observed, with velocities of -15 000kms. The low degree of polarization observed at H$beta$ suggests that the polarization observed for the other Balmer lines ($sim 0.4%$ above the background polarization) may rather be due to blending of $mathrm{Halpha}$ and $mathrm{Hgamma}$ with polarized Si II and Fe II lines, respectively. We suggest a model in which a jet of material, that is rich in $mathrm{^{56}Ni}$, has penetrated the C-O core, but not the He mantle. The jet axis is tilted with respect to the axis of the photosphere. This accounts for the lack of significant polarization of O I 7774AA, the delayed excitation and, hence, observability of He I and, potentially, the varied geometries of He and Ca.
