The Supernova Inside

A small fraction of Type Ia supernovae — about 0.04% — show signs of interacting with circumstellar material within 100 days of explosion. These SNe Ia-CSM are usually treated as outliers, evidence of an unusual progenitor channel.

arXiv:2603.16810 argues they are not outliers. They are the visible fraction of the dominant channel.

In the core-degenerate scenario, a white dwarf merges with an asymptotic giant branch star’s core. The ejected common envelope forms a planetary nebula, ionized by the hot remnant white dwarf. Most of these white dwarfs explode within a million years — before the planetary nebula has dispersed into the interstellar medium. The supernova occurs inside the planetary nebula. These are SNIPs: supernovae inside planetary nebulae.

The estimated SNIP fraction — roughly 80% of normal SNe Ia — is compatible with the observed 0.04% Ia-CSM fraction because most SNIPs explode after the nebula has thinned enough that interaction signatures are undetectable. Only the fastest explosions, occurring while the nebula is still dense, produce observable CSM interaction. The 0.04% is the tip of an 80% iceberg.

The argument inverts the usual logic: the rare observed phenomenon (CSM interaction) is not a rare progenitor channel. It is the observationally accessible fraction of the dominant channel. Most SNe Ia interact with circumstellar material — they just do it too late, when the material is too thin to notice. The supernova is always inside the nebula. Usually, the nebula is a ghost.