Revisiting candidate high-velocity stars associated with the Sagittarius dwarf spheroidal galaxy

TL;DR

This study searches for hypervelocity stars associated with the Sagittarius dwarf galaxy using Gaia, DESI, and LAMOST data, finding most candidates are likely halo stars rather than true Sgr-origin hypervelocity stars.

Contribution

It provides a comprehensive analysis of candidate hypervelocity stars near Sgr dSph, demonstrating most are halo stars and highlighting the importance of chemical data for origin determination.

Findings

Most candidates are consistent with halo star populations.

No unbound Sgr hypervelocity stars were identified.

Chemical abundances can help distinguish star origins in future studies.

Abstract

Hypervelocity stars (HVSs) are valuable tracers of extreme dynamical processes. The Sagittarius dwarf spheroidal galaxy (Sgr dSph), currently undergoing tidal disruption, offers a unique environment to search for such stars. We aim to identify candidate HVSs dynamically linked to the Sgr dSph and to assess their possible origins. Using Gaia DR3, DESI DR1, and LAMOST DR12, we selected stars with galactocentric velocities above 400 km\,s$^{-1}$ and traced their orbits in a realistic Galactic potential including the Sgr dSph and the Large Magellanic Cloud. We then tested three scenarios for their origin: the Hills mechanism, tidal disruption, and random halo star encounters. We identified 95 candidates passing within 2.5 half-mass radii of the Sgr dSph. Their kinematics are inconsistent with production by the Hills mechanism or tidal disruption but are well reproduced by halo stars that naturally cross the Sgr orbit. Furthermore, their metallicity distribution is consistent with that of the Milky Way halo rather than the Sgr stream or Sgr dSph. Our results suggest that our candidates and those in previous studies are most likely halo stars rather than genuine Sgr-origin HVSs. This highlights the need to account for the halo population when inferring stellar origins from orbital analysis and that chemical abundances will be a valuable constraint in the future. While we detect no unbound Sgr HVSs, such a discovery would directly imply extreme dynamical processes. Our results serve as a basis for future studies with upcoming surveys.