The Masses of Population II White Dwarfs

TL;DR

This study measures the masses of white dwarfs in globular cluster M4, revealing that 0.8 solar mass population II stars evolve into 0.53 solar mass white dwarfs, providing insights into stellar evolution and the age of the Galactic halo.

Contribution

First direct mass measurements of individual white dwarfs in a globular cluster, linking progenitor mass to remnant mass for population II stars.

Findings

White dwarfs near the tip of the cooling sequence have a mass of approximately 0.53 M_sun.

Population II stars with 0.8 M_sun evolve into white dwarfs of about 0.53 M_sun.

Results inform models of stellar evolution and Galactic halo age estimation.

Abstract

Globular star clusters are among the first stellar populations to have formed in the Milky Way, and thus only a small sliver of their initial spectrum of stellar types are still burning hydrogen on the main-sequence today. Almost all of the stars born with more mass than 0.8 M_sun have evolved to form the white dwarf cooling sequence of these systems, and the distribution and properties of these remnants uniquely holds clues related to the nature of the now evolved progenitor stars. With ultra-deep HST imaging observations, rich white dwarf populations of four nearby Milky Way globular clusters have recently been uncovered, and are found to extend an impressive 5 - 8 magnitudes in the faint-blue region of the H-R diagram. In this paper, we characterize the properties of these population II remnants by presenting the first direct mass measurements of individual white dwarfs near the tip of the cooling sequence in the nearest of the Milky Way globulars, M4. Based on Gemini/GMOS and Keck/LRIS multiobject spectroscopic observations, our results indicate that 0.8 M_sun population II main-sequence stars evolving today form 0.53 +/- 0.01 M_sun white dwarfs. We discuss the implications of this result as it relates to our understanding of stellar structure and evolution of population II stars and for the age of the Galactic halo, as measured with white dwarf cooling theory.