The He-rich Stripped-Envelope Core-Collapse Supernova 2008ax

TL;DR

This study presents detailed optical and NIR observations of supernova 2008ax, revealing its explosion characteristics, spectral evolution, and evidence for asymmetry and Ni mixing in the ejecta, contributing to understanding of type IIb supernovae.

Contribution

It provides comprehensive observational data and analysis of SN 2008ax, highlighting its similarities to and differences from other supernovae, and offers insights into explosion mechanics and ejecta structure.

Findings

Estimated 0.07-0.15 solar masses of 56Ni produced

Ejecta mass between 2 and 5 solar masses

Evidence of asymmetry and Ni mixing in ejecta

Abstract

Extensive optical and near-infrared (NIR) observations of the type IIb supernova 2008ax are presented, covering the first year after the explosion. The light curve is mostly similar in shape to that of the prototypical type IIb SN 1993J, but shows a slightly faster decline rate at late phases and lacks the prominent narrow early-time peak of SN 1993J. From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.07-0.15 solar masses of 56Ni were produced during the explosion and that the total ejecta mass was between 2 and 5 solar masses, with a kinetic energy of at least 10^51 erg. The spectral evolution of SN 2008ax is similar to that of the type Ib SN 2007Y, exhibiting high-velocity Ca II features at early phases and signs of ejecta-wind interaction from H-alpha observations at late times. NIR spectra show strong He I lines similar to the type Ib SN 1999ex, and a large number of emission features at late times. Particularly interesting are the strong, double-peaked He I lines in late NIR spectra, which - together with double-peaked [O I] emission in late optical spectra - provide clues for asymmetry and large-scale Ni mixing in the ejecta.