Type IIB Supernovae with Compact and Extended Progenitors

TL;DR

This paper classifies Type IIb supernovae into compact and extended categories based on progenitor characteristics, observational features, and wind properties, proposing a new nomenclature and a dividing envelope mass.

Contribution

It introduces a new classification scheme for Type IIb supernovae into compact (cIIb) and extended (eIIb) categories based on observational and progenitor properties.

Findings

Compact Type IIb supernovae have smaller progenitors (~10^11 cm) and distinct emission features.

Extended Type IIb supernovae include SN 1993J and similar objects with larger progenitors.

A progenitor envelope mass of ~0.1 Msun separates the two categories.

Abstract

The classic example of a Type IIb supernova is SN 1993J, which had a cool extended progenitor surrounded by a dense wind. There is evidence for another category of Type IIb supernova which has a more compact progenitor with a lower density, probably fast, wind. Distinguishing features of the compact category are: weak optical emission from the shock heated envelope at early times; nonexistent or very weak H emission in the late nebular phase; rapidly evolving radio emission; rapid expansion of the radio shell; and expected nonthermal as opposed to thermal X-ray emission. Type IIb supernovae that have one or more of these features include SNe 1996cb, 2001ig, 2003bg, 2008ax, and 2008bo. All of these with sufficient radio data (the last four) show evidence for presupernova wind variability. We estimate a progenitor envelope radius ~1e11 cm for SN 2008ax, a value consistent with a compact Wolf-Rayet progenitor. Supernovae in the SN 1993J extended category include SN 2001gd and probably the Cas A supernova. We suggest that the compact Type IIb events be designated Type cIIb and the extended ones Type eIIb. The H envelope mass dividing these categories is ~0.1 Msun.