Discovery of WINERED-HVS1: A metal-rich hyper-velocity star candidate ejected from the Galactic center

TL;DR

The paper reports the discovery and chemical analysis of WINERED-HVS1, a metal-rich star likely ejected from the Galactic center, providing new insights into the environment near the supermassive black hole.

Contribution

It presents the first detailed chemical characterization of a hyper-velocity star candidate ejected from the Galactic center, linking its properties to the nuclear star cluster.

Findings

WINERED-HVS1 has metallicity [Fe/H] ≈ -0.15.

Its abundance pattern matches that of the nuclear star cluster.

The star's properties support a Galactic center origin.

Abstract

We report the discovery of a metal-rich red giant star, WINERED-HVS1, which is a candidate for a hyper-velocity star (HVS). Its past trajectory suggests that this star may have been ejected by the Galactic supermassive black hole (SMBH; Sgr A*), with a modest ejection velocity of at least $500\; \mathrm{km\;s^{-1}}$. Since WINERED-HVS1 is gravitationally bound to the Milky Way and is not necessarily young, it is not an unambiguous HVS candidate from a kinematic perspective, unlike the previously confirmed A-type main-sequence HVS known as S5-HVS1. Therefore, the chemical characterization of this star is essential for understanding its origin. Through a spectroscopic follow-up observation with Magellan/WINERED, we determined its metallicity $\mathrm{[Fe/H]}=-0.147^{+0.046}_{-0.045}$, as well as the abundances for Na, Mg, Si, S, K, Ca, Ti, Cr, Ni, and Sr. The high metallicity value suggests that this star was ejected from the Galactic center and is unlikely to be a halo star with a radial orbit. The abundance pattern of this star -- including the pattern of [$\alpha$/Fe], [Mg/Fe], [Si/Mg], and [Ca/Mg] -- is consistent with that of the nuclear star cluster surrounding the SMBH, further supporting our view. This discovery opens a new window to look through the Galactic center environment without the hindrance of dust extinction, by using the HVSs moving in the halo region where dust extinction is minimal.