Author ORCID Identifier

https://orcid.org/0000-0002-8999-1108

Date of Award

8-10-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Sebastien Lepine

Second Advisor

Douglas R. Gies

Third Advisor

D. Michael Crenshaw

Fourth Advisor

Wei-Chun Jao

Fifth Advisor

Heidi Jo Newberg

Abstract

As stars in the Galactic halo tend to be old and chemically deficient, understanding their chemo-dynamics provides critical insight into the early formation history of the Milky Way. For the past two decades, large data surveys like the Sloan Digital Sky Survey and the Hipparcos have significantly improved our view of the Galactic halo from studies of its luminous members, such as red giants. Low-mass stars (K and M dwarfs), however, have not been emphasized in these studies due to their low brightness, despite the fact that they are statistically the dominant stars in the halo. Thanks to the recent data releases from the Gaia mission, we are now entering a revolutionary era of precision in stellar astronomy, with parallaxes and spatial motions measured for over 1.47 billion stars. The Gaia catalog includes unprecedented numbers of low-mass stars with precise distances, colors, and brightnesses, which allow us to make a detailed blueprint of stellar types in the Solar neighborhood. In an attempt to study the chemo-dynamics of halo low-mass stars, we present a catalog of 551,214 halo main-sequence stars and a supplementary catalog of halo white dwarf candidates. We also attempt to measure metallicities of those stars by defining a photometric metallicity grid via a color-magnitude diagram and calibrate this grid by using a set of 17,170 stars with precisely measured spectroscopic metallicities from various surveys. We find that our sample shares similar kinematics as reported in recent studies of red giants in the halo. We determine luminosity functions of the local halo based on the number of stars within 100, 300, and 500 pc from the Sun as a function of absolute magnitude, MG. The number of metal-poor, low-mass stars is significantly lower than that of metal-rich, low-mass stars, which may suggest the top-heavy initial mass function for the local halo. We further investigate the kinematics of our sample and identify 103 new members of the Helmi stream, which is a tidal debris stream of a dwarf galaxy accreted onto the Galaxy 5 - 8 Gyr ago.

DOI

https://doi.org/10.57709/23991509

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