High-Velocity Stars

Project Goal

To search for, identify, and vet high-velocity stars in a broad data set of stellar data from the DEep Image Multi-Object Spectrograph (DEIMOS).

Skills Involved

Python Programming
Data Analysis
Scientific Literature Research

Project Overview

The study of high velocity stars was my first introduction to astrophysics research. I had the valuable opportunity to gain programming and research experience, and this project paved the way for my following project on Leo T.

High-velocity stars (as their name implies) are stars traveling at exceptional speeds. For reference, the average star in the Milky Way travels at around 200 to 220 km/s (our sun has a velocity of approximately 220 km/s). In contrast, high-velocity stars have speeds ranging from 400 to 450 km/s, and some can ever reach and exceed 700 km/s (almost 0.2% of the speed of light)! I completed this project in early July, 2023.

Image credit: Brown et al. 2015

Introduction

High-velocity stars were first theorized by Jack Hills in 1988. He predicted that a 3-body interaction between a stellar binary and a massive black hole (MBH) could eject stars at extreme speeds. As the diagram to the left shows, if the binary get's close enough to the MBH, the binary becomes tidally disrupted. One of the stars becomes bound into a stable orbit around the MBH, while the other is ejected at a high velocity. Almost two decades after their speculation, high-velocity stars were proven to exist when Warren Brown discovered the first one in 2005.

When we study high-velocity stars, it is more useful for us to consider their speeds in the rest frame of the Milky Way instead of the rest frame of the sun (i.e. our frame of reference). Knowing a star's velocity in the galactic rest frame will allow us to use the star to map out the mass distribution of the galaxy or constrain the dark matter halo of the Milky Way. Additionally, we can learn about the star's orbit itself: for example, whether or not the star is still gravitationally bound to the galaxy. The rest frame of the galaxy can be further subdivided into the two reference frames below.

The Galactic Standard of Rest (GSR) Frame

This frame keeps the sun as its origin but corrects the target star's velocity by the sun's speed with respect to the rest frame galaxy. As a result of this correction, we obtain the target star's velocity in the rest frame of the galaxy. This reference frame is advantageous because, to apply the correction, we only need to know our target's position in space and radial velocity (how fast the star is moving towards or away from us). After performing the transformation, we obtain our target's radial velocity in the rest frame of the galaxy. Image Credit: VADIM SADOVSKI/SHUTTERSTOCK

The Galactocentric Frame

In this frame, the center of the Milky Way is the origin, and transforming our target's velocity into this frame requires a more involved algorithm. To perform this transformation, we need to know more about our target star in addition to its position and velocity, such as its distance and proper motion (the side-to-side or up-and-down motion of the star). However, this frame has a much more descriptive output than the GSR transformation: it gives us the target's 3D space velocity in the rest frame of the galaxy, along with the 3D distance from the center of the galaxy. Image Credit: Antares_StarExplorer/Shutterstock

From a scientific point of view, the Galactocentric frame is more favorable because it provides us with a more comprehensive set of properties about a given target star. However, a common problem in astronomy is the challenge associated with accurately determining how far away objects are. The data set of stars I was working with for this project was comprised of very dim stars whose distances are not well measured. Inaccurate distance measurements would have affected the velocity transformation algorithm, resulting in innacurately measured galactocentric velocities. As a result, I applied the GSR frame transformation algorithm.

Results

From over 20,000 stars, I found 54 high-velocity stars (velocities greater than or equal to 450 km/s). However, of these 54 stars, only 15 had reliable velocity measurements that were not dominated by noise. The figure to the left displays the velocity ranges of the initial set of 54 high-velocity stars. There is a diverse set of velocities, with some stars even exceeding 650 km/s.

Project Status

This project was completed in early July, 2023. As my first experience with astrophysics research, this project convinced me without a doubt that I would be thrilled to pursue the field of astrophysics alongside mechanical engineering. The data analysis and programming skills I gained would also apply to my studies in engineering just as much as they did to astrophysics.

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