Evidence for Asphericity in the Type IIn Supernova SN 1998S

Abstract

We present optical spectropolarimetry obtained at the Keck-II 10-m telescope on 1998 March 7 UT along with total flux spectra spanning the first 494 days after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN) 1998S. The SN is found to exhibit a high degree of linear polarization, implying significant asphericity for its continuum-scattering environment. Prior to removal of the interstellar polarization, the polarization spectrum is characterized by a flat continuum (at p ≈ 2%) with distinct changes in polarization associated with both the broad (symmetric, half width near zero intensity 10, 000 km s −1 ) and narrow (unresolved, full width at half maximum < 300 km s −1 ) line emission seen in the total flux spectrum. When analyzed in terms of a polarized continuum with unpolarized broad-line recombination emission, an intrinsic continuum polarization of p ≈ 3% results, suggesting a global asphericity of 45% from the oblate, electron-scattering dominated models of Höflich (1991). The smooth, blue continuum evident at early times is shown to be inconsistent with a reddened, single-temperature blackbody, instead having a color temperature that increases with decreasing wavelength. Broad emission-line profiles with distinct blue and red peaks are seen in the total flux spectra at later times, suggesting a disk-like or ring-like morphology for the dense (n e ≈ 10 7 cm −3 ) circumstellar medium, generically similar to what is seen directly in SN 1987A, although much denser and closer to the progenitor in SN 1998S. Implications of the circumstellar scattering environment for the spectropolarimetry are discussed, as are the effects of uncertainty in the removal of interstellar polarization; the importance of obtaining multiple spectropolarimetric epochs to help constrain the interstellar polarization value is particularly stressed. Using information derived from the spectropolarimetry and the total flux spectra, an evolutionary scenario for SN 1998S and its progenitor are presented.

A disrupted molecular torus around Eta Carinae as seen in 12CO with ALMA

“…This is based on multiple-peaked line profiles at late times, polarization, or other evidence (Leonard et al 2000;Hoffman et al 2008;Mauerhan et al 2014;Smith, Mauerhan, & Prieto 2014;Levesque et al 2014). In some cases, the mass distribution around the torus is thought to significantly break azimuthal symmetry.…”

“…This presents an interesting mystery, because equatorial mass loss from rotating stars or from close mass-transferring binaries should be azimuthally symmetric like the ring around SN 1987A. Some examples with such azimuthal asymmetry are SN 1998S (Leonard et al 2000;Mauerhan & Smith 2012;Shivvers et al 2015), SN 2010jl (Fransson et al 2014), PTF11iqb (Smith et al 2015), SN 2012ab (Bilinski et al 2017), SN2013L (Andrews et al 2017), and others. Episodic mass ejection and eccentric binaries have been invoked as possible explanations for some of these events.…”

Classification of Supernovae

“…Contrary to most other SNIIn, the spectrum evolved rapidly and seems to be the result of the interaction between the supernova and a two-component progenitor wind [40]. Polarimetric studies indicate significant asphericity for its continuum-scattering environment [73]. The early spectra are well modeled by lines arising primarily in the circumstellar region while later spectra are dominated by the supernova ejecta [72].…”

Optical Observations of Core-Collapse Supernovae

“…In particular, perhaps objects such as SN 1998S, mentioned above, would have been seen as GRBs had their rotation axis been pointed in our direction. That of SN 1998S was almost certainly not aligned with us [69]; both the spectropolarimetry and the appearance of double-peaked Hα emission suggest an inclined view, rather than pole-on. 4000 5000 6000 7000 8000 9000 10000…”

“…We have studied SN IIn 1998S in some detail [69] (see also [70]); its optical spectrum is dominated by strong, multi-component emission lines, thought to be produced by an intense interaction between the supernova and its dense circumstellar environment. We combined our early-time (3 days after discovery) spectropolarimetric observation with total flux spectra spanning nearly 500 days.…”