The Chemical Composition of Extreme-velocity Stars* ^†

Abstract

Little is known about the origin of the fastest stars in the Galaxy. Our understanding of the chemical evolution history of the Milky Way and surrounding dwarf galaxies allows us to use the chemical composition of a star to investigate its origin and to say whether it was formed in situ or was accreted. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. We have used high-resolution ARCES/3.5 m Apache Point Observatory, MIKE/Magellan spectra to study the chemical details of 15 late-type hypervelocity star candidates. With Gaia EDR3 astrometry and spectroscopically determined radial velocities we found total velocities with a range of 274–520 km s−1 and mean value of 381 km s−1. Therefore, our sample stars are not fast enough to be classified as hypervelocity stars, and are what is known as extreme-velocity stars. Our sample has a wide iron abundance range of −2.5 ≤ [Fe/H] ≤ −0.9. Their chemistry indicates that at least 50% of them are accreted extragalactic stars, with iron-peak elements consistent with prior enrichment by sub-Chandrasekhar mass Type Ia supernovae. Without indication of binary companions, their chemical abundances and orbital parameters indicate that they are the accelerated tidal debris of disrupted dwarf galaxies.

On the Origins of Extreme Velocity Stars as Revealed by Large-scale Galactic Surveys

“…Comparing our sample with those candidates, we find three (including one star, LG-HVS35, which is also included in Li21; see above), three, one, and five candidates in common with Bromley et al (2018), Hattori et al (2018), Du et al (2019) and Li et al (2020), respectively. It is interesting to note that two of our Hivel stars with V GSR no more than 380 km s −1 were recently observed by high-resolution spectroscopy by Reggiani et al (2022). The metallicity and [α/Fe] used in this work are consistent with those measured from high-resolution spectroscopy of Reggiani et al (2022).…”

“…In addition to the above searches, several other systematical searches purely based on Gaia DR2 have also reported hundreds of HiVel/HVS candidates (Bromley et al 2018;Hattori et al 2018;Du et al 2019;Marchetti et al 2019;Li et al 2020;Marchetti 2021;Reggiani et al 2022;Quispe-Huaynasi et al 2022;Liao et al 2023). Comparing our sample with those candidates, we find three (including one star, LG-HVS35, which is also included in Li21; see above), three, one, and five candidates in common with Bromley et al (2018), Hattori et al (2018), Du et al (2019) and Li et al (2020), respectively.…”

Gaia DR3 in 6D: The search for fast hypervelocity stars and constraints on the Galactic Centre environment

“…A successful search for bound HVSsoutside the scope of this work -should therefore exploit additional observations, including precise and detailed chemical abundances, which can be used in combination with kinematics to constrain the birth location of stars through chemical tagging (e.g. Hogg et al 2016), and minimize the contamination from halo stars on radial trajectories (Hawkins & Wyse 2018;Reggiani et al 2022).…”

A Noninteracting Galactic Black Hole Candidate in a Binary System with a Main-sequence Star

“…The continuum normalization was performed using smhr. We assume solar abundances from Asplund et al (2021) and follow the steps described in Reggiani et al (2022bReggiani et al ( , 2022a to obtain the fundamental and photospheric stellar parameters from a combination of spectral information and a fit to the MESA Isochrones and Stellar Tracks (Choi et al 2016) stellar models. We do the fitting with the isochrones package 18 (Morton 2015), which uses Mul-tiNest 19 (Feroz & Hobson 2008;Feroz et al 2009Feroz et al , 2019 via PyMultinest (Buchner et al 2014).…”