Existence of a compact Type IIb supernova class

Determine whether a distinct class of compact Type IIb supernovae exists as a transitional population between Type IIb and stripped-envelope supernovae.

Background

The paper discusses whether Type IIb supernovae form a physically diverse population distinguished by the extent and mass of their hydrogen envelopes. A proposed lower envelope-mass limit could cause a dramatic reduction in progenitor radius, producing a compact subclass that bridges ordinary Type IIb and stripped-envelope supernovae.

SN 2011dh presents a challenge to this interpretation because its progenitor was identified as a yellow supergiant with an extended envelope, while its maximum ejecta velocity was comparable to those of Type Ib/Ic supernovae. The paper therefore leaves unresolved whether the proposed compact Type IIb subclass is a genuine astrophysical class.

References

Since the maximum ejecta velocity in SN\,2011dh [$v_{\rm ej} \approx (4-5) \times 104$\,km/s] is close to the velocities deduced for type Ib/Ic supernovae, it is not clear whether a class of compact IIb supernovae exists.

— The initial evolution of SN 2011dh: The importance of inhomogeneities  (2608.17736 - Bjornsson, 18 Aug 2026) in Section 4, Discussion