Title: A Detailed Chemical Study of the Extreme Velocity Stars in the Galaxy
Authors: Tyler Nelson, Keith Hawkins, Henrique Reggiani, Diego Garza, Rosemary F.G. Wyse, Turner Woody
First Author’s Institution: Department of Physics, University of Southern Maine, 96 Falmouth Street, Portland, ME 04103, USA
Status: Published in Monthly Notices of the Royal Astronomical Society, Volume 532, Issue 2, August 2024, Pages 2875–2891 [open access]
What is a High Velocity Star?
Stars all move at different velocities. While each star’s exact velocity is a function of many factors, notably where it is in the galaxy, they generally move on the order of ~100 km/s.
However, astronomers have observed stars traveling much, much faster than that for decades. Not only are they moving hundreds of kilometers per second faster than the surrounding stars in their neighborhood, but in some cases, they’re moving fast enough that they might not even be bound to the Milky Way anymore. These stars are called high-velocity stars, or HVSs for short, and they’re the subject of today’s paper.
Where Do They Come From & Why Are They Cool?
There have been multiple proposed mechanisms to create these HVSs. One of the first was the Hills’ mechanism, wherein a stellar binary would undergo a three-body interaction with a supermassive black hole. In this process, one star is accelerated to incredible speeds while the black hole engulfs the companion.
However, there are other proposed creation mechanisms. It’s possible that HVSs could be created through a star getting ejected from its birth cluster through gravitational interaction. It has even been suggested that these stars were part of an accretion event during which they were dynamically heated. HVSs are exciting objects to study because many of the proposed mechanisms for their creation help us probe parts of the universe that would be otherwise difficult to study. As astronomers, we’re aware that most of the mass of the galaxy is dark matter, and that is dark matter. HVSs are novel probes of the Milky Way’s total mass, which can help us study the center of the galaxy in greater detail. This is because the escape velocity of a gravitational body is determined in part by its mass, and studying objects at high velocity can help examine the Milky Way’s mass distribution.
Moving just outside the galaxy, they can also help us study the Large Magellanic Cloud. Astronomers have been trying to prove the existence of a massive black hole at the center of the LMC, and detecting an HVS that came from it would lend credence to the existence of one due to the aforementioned Hills’ mechanism.1
Studying High Velocity Stars
A team of researchers led by Dr. Tyler Nelson set out to answer these questions and to do that; they assembled one of the largest samples of HVSs studied. They conducted a detailed dynamical and chemical analysis on sixteen different HVSs to see if they could determine an origin for any, or all, of them.
From an existing catalog of HVSs, they took high-resolution spectra and photometry of the HVSs from both the Apache Point Observatory and McDonald Observatory. The first step in the analysis process was to determine a radial velocity and compare it to the published Gaia DR3 velocity to validate their methodology. To accomplish this, they also took measurements of benchmark stars observed by Gaia to calibrate their measurements.
Figure 1: A comparison of the radial velocities determined by this study and the radial velocities determined by Gaia DR3 for the same star. From this, it’s clear that the independently measured values agree with the Gaia ones, and that it’s safe to assume that the following abundance analysis is being performed correctly. (Figure 1 in the paper).
Once the team had a radial velocity and spectrum for each star, they used two analysis methods to determine the remaining stellar parameters. Using the star’s spectrum, they also determined atmospheric parameters such as effective temperature, surface gravity, and metallicity. They then independently used an isochrone fitting to measure a stellar metallicity using the star’s photometry. They iterated this process until the two methods converged on a single metallicity within 0.2 dex. From these atmospheric parameters, they finally determined abundance measurements for all the stars in their sample in twenty-two elements, which spanned every nucleosynthetic family.
Finally, using photo-geometric distances and astrometry from Gaia DR3, they calculated the orbital history of all the HVSs in their sample. This step was crucial, as they used it to determine if any of the HVSs were unbound from the Milky Way.
So now our intrepid team has their sample. They have kinematic measurements in the form of radial velocities. They have chemical abundances for nearly two dozen elements. They also have the orbital information for each HVS in their sample. So what did they find?
Results
Let’s break the results into two parts. First, what did they find dynamically?
The authors compared the kinematics and orbits of their HVSs to models of the Milky Way to determine potential production sources and how gravitationally bound each star was to the Milky Way. They found that only one star in their sample was confidently unbound and that two others were marginally unbound. The remaining stars in the sample, while fast-moving, are doomed to be trapped in the Milky Way.
Figure 2: This plot shows the radial position of each star in the Milky Way on the x-axis, and the velocity of the star on the y-axis. The blue region shows the escape velocity of the Milky Way as a function of radius. From this, the authors determined that only one of their HVSs was unquestionably unbound.
The authors then tried to determine any explicit production sites using the integrated orbits of their sample coupled with catalogs of globular clusters, Milky Way satellites, and their Milky Way model. Unfortunately, through this analysis, they didn’t find any clear origins for their samples.
So what did they find chemically? While the abundances measured generally agreed with inner halo stars, they didn’t match the chemistry of stars originating from globular clusters, the LMC, and the galactic center. Furthermore, they didn’t find any stars chemically similar to accreted stars.
Despite being unable to pinpoint the physical or chemical origins of the stars in their sample, the results of this paper are still beneficial and exciting. As one of the most detailed examinations of HVSs in both chemical and dynamical space, it showed that the answer to “How are high-velocity stars created?” is far from settled.
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