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Register a new userSpectropolarimetry of the Type IIb Supernova 2001ig
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J.R. Maund
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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.
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We present images taken with the GMOS instrument on Gemini-South, in excellent (<0.5 arcsec) seeing, of SN 2001ig in NGC 7424, ~1000 days after explosion. A point source seen at the site of the SN is shown to have colours inconsistent with being an H II region or a SN 1993J-like remnant, but can be matched to a late-B through late-F supergiant with A_V<1. We believe this object is the massive binary companion responsible for periodic modulation in mass loss material around the Wolf-Rayet progenitor which gave rise to significant structure in the SN radio light curve.
We present HST/WFC3 ultraviolet imaging in the F275W and F336W bands of the Type IIb SN 2001ig at an age of more than 14 years. A clear point source is detected at the site of the explosion having $m_{rm F275W}=25.39 pm 0.10$ and $m_{rm F336W}=25.88 pm 0.13$ mag. Despite weak constraints on both the distance to the host galaxy NGC 7424 and the line-of-sight reddening to the supernova, this source matches the characteristics of an early B-type main sequence star having $19,000 < T_{rm eff} < 22,000$ K and $log (L_{rm bol}/L_{odot})=3.92 pm 0.14$. A BPASS v2.1 binary evolution model, with primary and secondary masses of 13 M$_{odot}$ and 9 M$_{odot}$ respectively, is found to resemble simultaneously in the Hertzsprung-Russell diagram both the observed location of this surviving companion, and the primary star evolutionary endpoints for other Type IIb supernovae. This same model exhibits highly variable late-stage mass loss, as expected from the behavior of the radio light curves. A Gemini/GMOS optical spectrum at an age of 6 years reveals a narrow He II emission line, indicative of continuing interaction with a dense circumstellar medium at large radii from the progenitor. We review our findings on SN 2001ig in the context of binary evolution channels for stripped-envelope supernovae. Owing to the uncrowded nature of its environment in the ultraviolet, this study of SN 2001ig represents one of the cleanest detections to date of a surviving binary companion to a Type IIb supernova.
We describe the radio evolution of SN 2001ig in NGC 7424, from 700 days of multi-frequency monitoring with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). We find that deviations of the radio light curves at each frequency from the standard minishell model are consistent with density modulations in the circumstellar medium (CSM), which seem to recur with a period near 150 days. One possibility is that these are due to enhanced mass-loss from thermal pulses in an AGB star progenitor. A more likely scenario however is that the progenitor was a Wolf-Rayet star, whose stellar wind collided with that from a massive hot companion on an eccentric 100 day orbit, leading to a regular build-up of CSM material on the required time and spatial scales. Recent observations of dusty pinwheels in Wolf-Rayet binary systems lend credibility to this model. Since such binary systems are also thought to provide the necessary conditions for envelope-stripping which would cause the Wolf-Rayet star to appear as a Type Ib/c supernova event rather than a Type II, these radio observations of SN 2001ig may provide the key to linking Type Ib/c SNe to Type IIb events, and even to some types of Gamma-Ray Bursts.
We present seven epochs of spectropolarimetry of the Type IIb supernova (SN) 2011dh in M51, spanning 86 days of its evolution. The first epoch was obtained 9 days after the explosion, when the photosphere was still in the depleted hydrogen layer of the stripped-envelope progenitor. Continuum polarization is securely detected at the level of P~0.5% through day 14 and appears to diminish by day 30, which is different from the prevailing trends suggested by studies of other core-collapse SNe. Time-variable modulations in P and position angle are detected across P-Cygni line features. H-alpha and HeI polarization peak after 30 days and exhibit position angles roughly aligned with the earlier continuum, while OI and CaII appear to be geometrically distinct. We discuss several possibilities to explain the evolution of the continuum and line polarization, including the potential effects of a tidally deformed progenitor star, aspherical radioactive heating by fast-rising plumes of Ni-56 from the core, oblique shock breakout, or scattering by circumstellar material. While these possibilities are plausible and guided by theoretical expectations, they are not unique solutions to the data. The construction of more detailed hydrodynamic and radiative-transfer models that incorporate complex aspherical geometries will be required to further elucidate the nature of the polarized radiation from SN 2011dh and other Type IIb supernovae.
We present Swift UVOT and XRT observations, and visual wavelength spectroscopy of the Type IIb supernova (SN) 2009mg, discovered in the Sb galaxy ESO 121-G26. The observational properties of SN 2009mg are compared to the prototype Type IIb SNe 1993J and 2008ax, with which we find many similarities. However, minor differences are discernible including SN 2009mg not exhibiting an initial fast decline or u-band upturn as observed in the comparison objects, and its rise to maximum is somewhat slower leading to slightly broader light curves. The late-time temporal index of SN 2009mg, determined from 40 days post-explosion, is consistent with the decay rate of SN 1993J, but inconsistent with the decay of 56Co. This suggests leakage of gamma-rays out of the ejecta and a stellar mass on the small side of the mass distribution. Our XRT non-detection provides an upper limit on the mass-loss rate of the progenitor of <1.5x10^-5 Msun per yr. Modelling of the SN light curve indicates a kinetic energy of 0.15 (+0.02,-0.13) x10^51 erg, an ejecta mass of 0.56(+0.10,-0.26) Msun and a 56Ni mass of 0.10pm0.01 Msun.
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