Nebular Phase Observations of the Type Ib Supernova 2008D/X-ray Transient 080109: Side-Viewed Bipolar Explosion

TL;DR

This paper reports nebular phase observations of SN 2008D, revealing a bipolar explosion viewed from the side, with spectral evidence of asphericity and implications for explosion geometry.

Contribution

It provides the first nebular spectra of SN 2008D, demonstrating its aspherical, bipolar explosion geometry through spectral line profiles.

Findings

Double-peaked [O I] line indicates aspherical explosion.

SN 2008D is a side-viewed bipolar explosion.

Consistent with explosion models with 0.07 Msun of 56Ni.

Abstract

We present optical spectroscopic and photometric observations of supernova (SN) 2008D, associated with the luminous X-ray transient 080109, at >300 days after the explosion (nebular phases). We also give flux measurements of emission lines from the H II region at the site of the SN, and estimates of the local metallicity. The brightness of the SN at nebular phases is consistent with the prediction of the explosion models with an ejected 56Ni mass of 0.07 Msun, which explains the light curve at early phases. The [O I] line in the nebular spectrum shows a double-peaked profile while the [Ca II] line does not. The double-peaked [O I] profile strongly indicates that SN 2008D is an aspherical explosion. The profile can be explained by a torus-like distribution of oxygen viewed from near the plane of the torus. We suggest that SN 2008D is a side-viewed, bipolar explosion with a viewing angle of > 50^{\circ} from the polar direction.