The Progenitor of the Type IIb SN 2008ax Revisited

TL;DR

This study revises the understanding of SN 2008ax's progenitor, revealing it was likely a less luminous supergiant or binary system rather than a Wolf-Rayet star, based on high-resolution imaging and modeling.

Contribution

The paper provides new high-resolution observations and hydrodynamical modeling that challenge previous progenitor identifications, proposing a binary system as the progenitor.

Findings

Progenitor was multiple sources, not a single object.

Progenitor likely a supergiant star, not a Wolf-Rayet.

Hydrodynamical models support a binary progenitor scenario.

Abstract

Hubble Space Telescope observations of the site of the supernova (SN) 2008ax obtained in 2011 and 2013 reveal that the possible progenitor object detected in pre-explosion images was in fact multiple. Four point sources are resolved in the new, higher-resolution images. We identify one of the sources with the fading SN. The other three objects are consistent with single supergiant stars. We conclude that their light contaminated the previously identified progenitor candidate. After subtraction of these stars, the progenitor appears to be significantly fainter and bluer than previously measured. Post-explosion photometry at the SN location indicates that the progenitor object has disappeared. If single, the progenitor is compatible with a supergiant star of B to mid-A spectral type, while a Wolf-Rayet (WR) star would be too luminous in the ultraviolet to account for the observations. Moreover, our hydrodynamical modelling shows the pre-explosion mass was $4-5$ $M_\odot$ and the radius was $30-50$ $R_\odot$, which is incompatible with a WR progenitor. We present a possible interacting binary progenitor computed with our evolutionary models that reproduces all the observational evidence. A companion star as luminous as an O9-B0 main-sequence star may have remained after the explosion.