Exploring velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708
Authors: Neunteufel P. 
Published: July 2020

Context.Hypervelocity stars (HVS) are a class of stars moving at velocities that are high enough to make them gravitationally unbound from the Galaxy. In recent years, ejection from a close binary system in which one of the components undergoes a thermonuclear supernova (SN) has emerged as a promising candidate production mechanism for the least massive specimens of this class. The explosion mechanisms leading to thermonuclear supernovae, which include the important Type Ia and related subtypes, remain unclear.

Aims.This study presents a thorough theoretical analysis of candidate progenitor systems of thermonuclear SNe in the single degenerate helium donor scenario in the relevant parameter space leading to the ejection of HVS. The primary goal is to investigate the previously indeterminate characteristics of the velocity spectra for the ejected component, including possible maxima and minima, as well as the constraints arising from stellar evolution and initial masses. Furthermore, this paper addresses the question of whether knowledge of the ejection velocity spectra may aid in the reconstruction of the terminal state of the supernova progenitor, with a focus on the observed object, US 708.

Methods.This study presents the results of 390 binary model sequences computed with the Modules for Experiments in Stellar Astrophysics framework, investigating the evolution of supernova progenitors composed of a helium-rich hot subdwarf and an accreting white dwarf, while avoiding assumption of a specific explosion mechanism as much as possible. The detailed evolution of the donor star as well as gravitational wave radiation and mass transfer-driven orbital evolution were fully taken into account. The results were then correlated with an idealized kinematic analysis of the observed object US 708.

Results.This work shows that the ejection velocity spectra reach a maximum in the range of 0.19 M⊙ <  MHVS <  0.25 M⊙. Depending on the local Galactic potential, all donors below 0.4 M⊙are expected to become HVSs. The single degenerate helium donor channel is able to account for runaway velocities up to ∼1150 km s−1with a Chandrasekhar mass accretor, exceeding 1200 km s−1when super-Chandrasekhar mass detonations are taken into account. Results show that the previously assumed mass of 0.3 M⊙for US 708, combined with proper motions that have been obtained more recently, favor a sub-Chandrasekhar mass explosion with a terminal WD mass between 1.1 M⊙and 1.2 M⊙, while a Chandrasekhar mass explosion requires a mass of > 0.34 M⊙for US 708. This mechanism may be a source of isolated runaway extremely low-mass white dwarfs.

Conclusions.The presence of clear ejection velocity maxima that are terminal accretor mass-dependent, but simultaneously initial-condition independent, provides constraints on the terminal state of a supernova progenitor. Depending on the accuracy of astrometry, it is possible to discern certain types of explosion mechanisms from the inferred ejection velocities alone, with current proper motions allowing for a sub- Chandrasekhar mass SN to explain the origins of US 708. However, more robust reconstructions of the most likely SN progenitor state will require a greater number of observed objects than are currently available.