The fastest unbound star in our Galaxy ejected by a thermonuclear supernova

TL;DR

This paper analyzes US 708, the fastest unbound star in our Galaxy, revealing its likely origin as a supernova donor remnant ejected by a thermonuclear explosion, challenging previous ejection mechanism assumptions.

Contribution

It provides the first detailed spectroscopic and kinematic analysis of US 708, suggesting a novel supernova ejection origin distinct from the Galactic center.

Findings

US 708 travels at ~1200 km/s, making it the fastest unbound star in the Galaxy.

Galactic center origin is unlikely based on trajectory analysis.

US 708 is a fast rotator likely spun-up by binary interaction before supernova ejection.

Abstract

Hypervelocity stars (HVS) travel with velocities so high, that they exceed the escape velocity of the Galaxy. Several acceleration mechanisms have been discussed. Only one HVS (US 708, HVS 2) is a compact helium star. Here we present a spectroscopic and kinematic analysis of US\,708. Travelling with a velocity of $\sim1200\,{\rm km\,s^{-1}}$, it is the fastest unbound star in our Galaxy. In reconstructing its trajectory, the Galactic center becomes very unlikely as an origin, which is hardly consistent with the most favored ejection mechanism for the other HVS. Furthermore, we discovered US\,708 to be a fast rotator. According to our binary evolution model it was spun-up by tidal interaction in a close binary and is likely to be the ejected donor remnant of a thermonuclear supernova.