Astronomers have compared the cooling white dwarfs of two large star clusters: M3 and M13, to understand the physics behind white dwarf evolution. While these two clusters have many similarities in physical properties, such as age and metallicity, the populations of stars that will eventually produce white dwarfs is different. This makes M3 & M13 a perfect natural laboratory to compare the cooling rates of different white dwarf populations. Credit: ESA/Hubble & NASA, G. Piotto et al.
NASA/ESA Hubble Space Telescope observations have challenged the common view that white dwarfs are inert stars slowly cooling down. A group of international astronomers has discovered that white dwarfs can slow their rate of aging through the burning of hydrogen on their surfaces.
Jianxing Chen, of the Alma Mater Studiorum Universit di Bologna (Italian National Institute for Astrophysics), who led the research, said that "We have discovered the first observational evidence" that white dwarfs still exhibit stable thermonuclear activities. This was a surprising result, since it contradicts what most people believe.
White dwarfs are slowly cooling stars that have shed their outer layers in the final stages of their life. These white dwarfs are common in the universe. Approximately 98% of all stars in the Universe will eventually become white dwarfs. This includes our Sun. Astronomers can understand the cooling stages of white dwarfs and their earlier stages by studying them.
Astronomers have compared the cooling white dwarfs of two large star clusters, the globular clusters (M3 and M13), to understand the physics behind white dwarf evolution. Although both clusters have many similarities in physical properties, such as age and metallicity, the populations of white dwarfs that will eventually emerge from these stars are quite different. The overall stars of the evolutionary stage called the Horizontal Branch are more blue in M13. This indicates a higher number of hotter stars. M3 and M13 are perfect natural laboratories to compare the cooling of different white dwarf populations.
Chen said, "The excellent quality of our Hubble observations gave us a complete view of the stellar population of the two globular Clusters." This allowed us to compare how stars develop in M3 and 13."
The Hubble's Wide Field Camera 3 was used to observe M3 and M13 at near ultraviolet wavelengths. This allowed them to compare over 700 white dwarfs within the two clusters. They discovered that M3 has standard white dwarfs, which are simply cooling star cores. M13 however, has two types of white dwarfs. Standard white dwarfs and those that have retained hydrogen's outer envelope, which allows them to burn longer and cool down more slowly.
The researchers were able, using computer simulations of stellar development in M13 to compare their results, to find that approximately 70% of white dwarfs in this region are heating hydrogen, which slows down their cooling.
This discovery could impact how astronomers calculate the ages stars in the Milky Way. The predictable cooling process that underlies the evolution of white dwarfs was previously modelled. Astronomers have used the white dwarf cooling rate to calculate the ages of star-clusters, especially globular and open ones. This simple relationship between temperature and age has allowed them to use this as a natural clock. These age estimates could be affected by white dwarfs that burn hydrogen.
Francesco Ferraro, of the Alma Mater Studiorum Universit di Bologna (Italian National Institute for Astrophysics), who coordinated the study, stated that "Our discovery challenges white dwarfs" and offered a new perspective about how stars age. "We are currently investigating other clusters like M13 to constrain the conditions that drive stars to keep the thin hydrogen envelope, which allows them age slowly.
Nature Astronomy published the study.
Explore more planetary remnants surrounding white dwarf stars
More information: Slowly cooling white dwarfs in M13 from stable hydrogen burning, Nature Astronomy, DOI: 10.1038/s41550-021-01445-6 , www.nature.com/articles/s41550-021-01445-6 Journal information: Nature Astronomy Slowly cooling white dwarfs in M13 from stable hydrogen burning,
