A New Pathway to Single Be Stars: Ejected Companions from Type Ia Supernovae

TL;DR

This paper proposes a novel formation pathway for single Be stars, suggesting they are ejected companions from Type Ia supernovae, supported by binary evolution simulations and velocity analysis.

Contribution

It introduces a new SN Ia channel for single Be star formation, supported by detailed binary evolution models and velocity distribution analysis.

Findings

Approximately 0.4% of He star + MS star binaries may produce single Be stars via SN Ia.

About 22% of Be stars from this channel are classified as runaway stars due to high tangential velocities.

The SN Ia channel significantly contributes to the population of single Be stars, especially runaways.

Abstract

Be stars are rapid rotators generally produced by binary interactions. The single Be stars in the observations pose challenges to the Be star formation theory. In this paper, we propose a new pathway for the formation of single Be stars, in which the Be star is taken as the ejected companion star from a Type Ia supernova (SN Ia) explosion. Recent numerical simulations suggest that explosive oxygen burning, initiated via the convective Urca process in certain helium (He) stars near the Chandrasekhar mass limit, can set off a SN Ia. Based on this proposition, we further demonstrate that about $0.4\%$ of He star + main-sequence (MS) star binaries may evolve into single Be stars, where the MS star is spun up due to the mass accretion from the He star, and then the He star explodes as a SN Ia. We employ detailed binary evolutionary simulations and find the parameter space that would produce single Be stars via the SN Ia channel. Around $22\%$ of Be stars from the SN Ia progenitor channel exhibit peculiar tangential velocities exceeding $24\ \rm km/s$, classifying them as runaway stars. This suggests that the SN Ia channel plays a meaningful role in forming single Be stars, particularly within the runaway star population.