Helium stars as supernova progenitors

TL;DR

This paper investigates helium star evolution leading to potential supernova explosions, highlighting off-center carbon burning, residual carbon ignition, and the resulting peculiar thermonuclear supernovae in binary systems.

Contribution

It demonstrates a new evolutionary pathway for helium stars to become supernova progenitors via off-center carbon burning and residual carbon ignition.

Findings

Residual carbon can trigger thermonuclear runaway at various densities.

Helium stars in close binaries can evolve into supernova progenitors.

Potential for peculiar supernovae with helium, carbon, and possibly hydrogen in the envelope.

Abstract

We follow the evolution of helium stars of initial mass $(2.2 - 2.5) M_\odot$, and show that they undergo off-center carbon burning, which leaves behind ${\mathbf \sim 0.01 M_\odot}$ of unburnt carbon in the inner part of the core. When the carbon-oxygen core grows to Chandrasekhar mass, the amount of left-over carbon is sufficient to ignite thermonuclear runaway. At the moment of explosion, the star will possess an envelope of several $0.1 M_{\odot}$, consisting of He, C, and possibly some H, perhaps producing a kind of peculiar SN. Based on the results of Waldman and Barkat (2007) for accreting white dwarfs, we expect to get thermonuclear runaway at a broad range of $\rho_c \approx (1 - 6) \times 10^9 \mathrm{g cm^{-3}}$, depending on the amount of residual carbon. We verified the feasibility of this scenario by showing that in a close binary system with initial masses $(8.5 + 7.7) M_{\odot}$ and initial period of 150 day the primary produces a helium remnant of $2.3 M_{\odot}$ that evolves further like the model we considered.