Madrid, 6 (European Press)
The prevailing view of white dwarfs as dormant stars that are slowly cooling has been challenged by observations from the NASA/ESA Hubble Space Telescope.
Now, an international group of astronomers has discovered the first evidence that white dwarfs can slow the rate of aging by burning hydrogen on their surface, as published in Nature Astronomy.
“We have found the first observational evidence that white dwarfs can still experience stable thermonuclear activity,” explains Jianxing Chen, of the Alma Mater Studiorum Universita di Bologna and the National Institute of Astrophysics in Italy, who led the research. This was a huge surprise because it goes against what is common.”
White dwarfs are stars that cool slowly and release their outer layers during the later stages of their lives. They are common objects in the universe because roughly 98% of all stars in the universe will end up as white dwarfs, including our sun. Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages.
To investigate the physics behind the evolution of white dwarfs, astronomers compared the cooling of white dwarfs in two massive star clusters: globular clusters M3 and M13. These two groups share many physical properties, such as age and mineralization, but the star groups that will eventually give rise to white dwarfs are different.
In particular, the overall color of stars in the evolution phase known as the horizontal branch is bluer in M13, indicating the presence of a group of hotter stars. This makes M3 and M13 an ideal natural laboratory to see how different groups of white dwarfs cool off.
“The remarkable quality of Hubble’s observations has given us a comprehensive view of the globular cluster’s star clusters,” Chen continues. “This has allowed us to compare how stars evolve in M3 and M13.”
Using Hubble’s Wide Field Camera 3, the team observed M3 and M13 at wavelengths near the ultraviolet, allowing them to compare more than 700 white dwarfs in the two groups.
They found that M3 contains standard white dwarfs that simply cool stellar cores. On the other hand, M13 has two sets of white dwarfs: the standard one and those that manage to maintain an outer shell of hydrogen, allowing them to burn longer, and thus cool down slowly.
By comparing their results with computer simulations of stellar evolution in M13, the researchers were able to show that nearly 70% of M13’s white dwarfs burn hydrogen at their surface, slowing the rate at which they cool.
This discovery may have consequences for the way astronomers measure the age of stars in the Milky Way. So far, the evolution of white dwarfs has been modeled as a predictable cooling process.
This relatively simple relationship between age and temperature has led astronomers to use the cooling rate of white dwarfs as a natural clock to determine the age of star clusters, especially globular and open clusters. However, white dwarfs burning hydrogen can make these age estimates inaccurate by as much as a billion years.
“Our discovery challenges the definition of white dwarfs by considering a new perspective on the way stars age,” said Francesco Ferraro, of the Alma Mater Studiorum Universita di Bologna and the Italian National Institute of Astrophysics, who coordinated the study. Others clusters similar to M13 to narrow down the conditions that lead stars to maintain a thin hydrogen envelope that allows them to age slowly. ”
