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Search for Distant Hypervelocity Star Candidates Using RR Lyrae Stars

Haozhu Fu, Yang Huang, Huawei Zhang

TL;DR

Hypervelocity stars (HVSs) provide crucial constraints on the Milky Way's gravitational potential and ejection mechanisms. The authors exploit RR Lyrae distance indicators via the PMZ relation and Gaia DR3 proper motions to conduct a large-volume, tangential-velocity–based search for HV-RRLs, combining two RR Lyrae catalogs with spectroscopic and photometric metallicities. Through strict data-quality cuts, Monte Carlo propagation, and comparison to the Milky Way escape speed from MWPotential2014, they identify 6,838 HV-RRLs with reliable velocities, including 87 highly robust cases and several with tangential speeds $>800$ km s$^{-1}$. The spatial distribution reveals concentrations toward the Galactic Center and the Magellanic Clouds, implying origins linked to Hills-type ejections or related processes, with additional HV-RRLs likely ejected from dwarf galaxies; forthcoming Gaia releases and follow-up spectroscopy will refine their origins and improve constraints on Galactic structure.

Abstract

Hypervelocity stars (HVSs) are stars with velocities exceeding their local escape velocities. Searching for HVSs and studying their origins can be an important way to study the properties of the Milky Way. In this paper, we utilize precise distances for RR Lyrae stars (RRLs) derived from the period-absolute magnitude-metallicity (PMZ) relation, along with proper motions from Gaia DR3, to conduct a large-volume search for HVSs. Our sample consists of a catalog with 8,172 RRLs with metallicity, distance and radial velocities estimated from SDSS and LAMOST spectroscopic data, and an extended catalog of 135,873 RRLs with metallicity and distance estimated from Gaia photometry. After careful quality cuts, 165 hypervelocity RRL candidates were found. We performed further checks on their light curves, and selected the most reliable 87 hypervelocity RRLs. All of them exceed the Milky Way's escape velocity in the tangential component. Among them, 7 stars have tangential velocity over 800 km s^-1. We identified two spatially distinct distributions of hypervelocity RRLs: one concentrated toward the Galactic Center and another localized around the Magellanic Clouds, suggesting that their origins are likely associated with these regions through the Hills or other mechanisms. Furthermore, we detected a significant number of RRLs associated with dwarf galaxies that exceed the Milky Way's escape velocity, likely ejected from their host systems. Future Gaia releases and spectroscopic follow-up observations will provide further insight into their ejection origin.

Search for Distant Hypervelocity Star Candidates Using RR Lyrae Stars

TL;DR

Hypervelocity stars (HVSs) provide crucial constraints on the Milky Way's gravitational potential and ejection mechanisms. The authors exploit RR Lyrae distance indicators via the PMZ relation and Gaia DR3 proper motions to conduct a large-volume, tangential-velocity–based search for HV-RRLs, combining two RR Lyrae catalogs with spectroscopic and photometric metallicities. Through strict data-quality cuts, Monte Carlo propagation, and comparison to the Milky Way escape speed from MWPotential2014, they identify 6,838 HV-RRLs with reliable velocities, including 87 highly robust cases and several with tangential speeds km s. The spatial distribution reveals concentrations toward the Galactic Center and the Magellanic Clouds, implying origins linked to Hills-type ejections or related processes, with additional HV-RRLs likely ejected from dwarf galaxies; forthcoming Gaia releases and follow-up spectroscopy will refine their origins and improve constraints on Galactic structure.

Abstract

Hypervelocity stars (HVSs) are stars with velocities exceeding their local escape velocities. Searching for HVSs and studying their origins can be an important way to study the properties of the Milky Way. In this paper, we utilize precise distances for RR Lyrae stars (RRLs) derived from the period-absolute magnitude-metallicity (PMZ) relation, along with proper motions from Gaia DR3, to conduct a large-volume search for HVSs. Our sample consists of a catalog with 8,172 RRLs with metallicity, distance and radial velocities estimated from SDSS and LAMOST spectroscopic data, and an extended catalog of 135,873 RRLs with metallicity and distance estimated from Gaia photometry. After careful quality cuts, 165 hypervelocity RRL candidates were found. We performed further checks on their light curves, and selected the most reliable 87 hypervelocity RRLs. All of them exceed the Milky Way's escape velocity in the tangential component. Among them, 7 stars have tangential velocity over 800 km s^-1. We identified two spatially distinct distributions of hypervelocity RRLs: one concentrated toward the Galactic Center and another localized around the Magellanic Clouds, suggesting that their origins are likely associated with these regions through the Hills or other mechanisms. Furthermore, we detected a significant number of RRLs associated with dwarf galaxies that exceed the Milky Way's escape velocity, likely ejected from their host systems. Future Gaia releases and spectroscopic follow-up observations will provide further insight into their ejection origin.

Paper Structure

This paper contains 5 sections, 1 equation, 1 figure.

Table of Contents

  1. Introduction
  2. Sample and Method
  3. Sample selection
  4. Data quality control
  5. HVS selection

Figures (1)

  • Figure 1: Spatial distribution of RRL samples from W22 (upper panels) and L23 (bottom pannels) in the Galactic Cartesian coordinates. Left panels show $X_{\rm GC}\text{--}Z_{\rm GC}$ distribution; right panels show $Y_{\rm GC}\text{--}Z_{\rm GC}$ distribution. Color bar in the right side stands for number density in logarithmic scale. Magenta and blue stars denote the position of the LMC and SMC; Green, orange and skyblue diamonds denote the position of the Sculptor, Draco, and Ursa Minor dwarf galaxies (SclI, DraI, and UMiI); Yellow and Red triangles denote the Sgr core (C) and stream (S), respectively. The position of these dwarf galaxies are taken from 2018AA...619A.103F.