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

TL;DR

This study uses multi-epoch spectropolarimetry to reveal a misaligned, disk-like circumstellar environment around SN 1997eg, indicating complex geometry and progenitor characteristics of Type IIn supernovae.

Contribution

It provides the first multi-epoch spectropolarimetric evidence of a flattened, misaligned circumstellar disk around a Type IIn supernova, supporting the LBV progenitor hypothesis.

Findings

Detection of closed loops in q-u plane indicating broken symmetry.

Identification of two different axes of symmetry in the supernova system.

Evidence supporting luminous blue variable stars as progenitors of some Type IIn supernovae.

Abstract

We present multi-epoch spectral and spectropolarimetric observations of the Type IIn supernova (SN) 1997eg that indicate the presence of a flattened disk-like concentration of circumstellar material surrounding nonspherical ejecta, with which the disk is misaligned. The polarization across the broad H alpha, H beta, and He I 5876 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 degrees from that of the elongated ejecta and probably indicates the orientation of the disk-like 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 pre-supernova 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.