Supernova 2010as: the Lowest-Velocity Member of a Family of Flat-Velocity Type IIb Supernovae

TL;DR

This paper studies SN 2010as, a peculiar flat-velocity Type IIb supernova with low expansion velocities and weak hydrogen features, revealing insights into its progenitor and explosion characteristics.

Contribution

It identifies a new subclass of supernovae called 'flat-velocity Type IIb' and provides detailed observational and modeling analysis of SN 2010as.

Findings

SN 2010as has the lowest observed velocities among its class.

The supernova shows evidence of a dense shell in the ejecta.

Progenitor likely a massive binary system, not a single star.

Abstract

We present extensive optical and near-infrared photometric and spectroscopic observations of the stripped-envelope (SE) supernova SN 2010as. Spectroscopic peculiarities, such as initially weak helium features and low expansion velocities with a nearly flat evolution, place this object in the small family of events previously identified as transitional Type Ib/c supernovae (SNe). There is ubiquitous evidence of hydrogen, albeit weak, in this family of SNe, indicating that they are in fact a peculiar kind of Type IIb SNe that we name "flat-velocity Type IIb". The flat velocity evolution---which occurs at different levels between 6000 and 8000 km/s for different SNe---suggests the presence of a dense shell in the ejecta. Despite the spectroscopic similarities, these objects show surprisingly diverse luminosities. We discuss the possible physical or geometrical unification picture for such diversity. Using archival HST images we associate SN 2010as with a massive cluster and derive a progenitor age of ~6 Myr, assuming a single star-formation burst, which is compatible with a Wolf-Rayet progenitor. Our hydrodynamical modelling, on the contrary, indicates the pre-explosion mass was relatively low, of ~4 M_sol. The seeming contradiction between an young age and low pre-SN mass may be solved by a massive interacting binary progenitor.