The Dual‐Axis Circumstellar Environment of the Type IIn Supernova 1997eg

Abstract

We present multi-epoch spectral and spectropolarimetric observations of the Type IIn supernova (SN ) 1997eg that indicate the presence of a flattened disklike concentration of circumstellar material surrounding aspherical ejecta, with which the disk is misaligned. The polarization across the broad H , H , and He i k5876 lines of SN 1997eg forms closed loops when viewed in the Stokes q-u plane. Such loops occur when the geometrical symmetry of one or both of the Stokes parameters across spectral lines is broken, in this case most likely by occultation of the ejecta by the equatorial circumstellar matter concentration. The polarization of the narrow Balmer lines possesses an intrinsic axis that differs by 12 from that of the elongated ejecta and probably indicates the orientation of the disklike circumstellar material. The existence of two different axes of symmetry in SN 1997eg suggests that neither rotation of the progenitor nor the influence of a companion star can be the sole mechanism creating a preferred axis within the supernova system. Our model supports the emerging hypothesis that the progenitors of some Type IIn supernovae are luminous blue variable stars, whose presupernova mass eruptions form the circumstellar shells that physically characterize the SN IIn subclass. These conclusions, which are independent of interstellar polarization effects, would have been unobservable with only a single epoch of spectropolarimetry.

Asymmetries in the type IIn SN 2010jl

“…Under the assumption of a spheroidal electron-scattering atmosphere, the level of polarization measured in SN 2010jl (∼2%) is indicative of substantial asphericity, of axial ratio ≤0.7 (Höflich 1991). This value is very similar to those derived for the other two known cases (Leonard et al 2000;Hoffman et al 2008). As first proposed by Leonard et al (2000), the simplest explanation for the line depolarization in type IIn SNe is the dilution of polarized continuum light by unpolarized line recombination emission, which has to take place well above the photosphere, in a region where electron scattering becomes negligible.…”

“…Since Q is close to 0, the points in the Q − U plane move along the U axis, so that the position angle remains constant across the line profile. This is what is expected for simple continuum depolarization dilution (see for instance Hoffman et al 2008). A similar behavior is seen (within the signal-to-noise) in the Hβ line, where the polarization is ∼1% at the line center.…”

“…Although the analysis was difficult because of the uncertainty in the ISP correction, in the earliest epochs the two objects displayed a significant and almost constant continuum polarization (2.0-2.6%), characterized by a constant position angle. In addition, pronounced line depolarizations were observed at the positions of the most prominent emission lines (Leonard et al 2000;Wang et al 2001;Hoffman et al 2008). Notwithstanding the photometric differences, our data show that SN 2010jl conforms strikingly well to this behavior, implying that there is a common geometry of the explosion or, more in general, of the environment in which the explosion takes place.…”

“…The only two published cases are those of SNe 1997eg (Hoffman et al 2008), and 1998S (Leonard et al 2000;Wang et al 2001), which reached a comparable absolute magnitude (M V ∼ −18.6), i.e. significantly fainter than the peak magnitude attained by SN 2010jl (M V ≈ −20, Smith et al 2010;Stoll et al 2010).…”

Spectropolarimetry of Type IIn SN2010jl: Peering Into the Heart of a Monster

“…The geometrical insight gained from spectropolarimetry may allow a critical approach to these open issues, especially for rare luminous type IIn's like SN2010jl, which could provide extreme tests of models. The only other type IIn SNe to be observed in polarized light were SN1998S (Leonard et al 2000;Wang et al 2001) and SN1997eg (Hoffman et al 2008) to provide points of comparison.…”