Using the high resolution observation of two very similar globular clusters, Messier 3 and Messier 13, astronomers found an unexpected overabundance of bright white dwarfs in Messier 13. Theoretical models suggest that this overabundance is due to a slowing of the cooling process in about 70% of white dwarfs in Messier 13, caused by stable thermonuclear combustion in their residual atmosphere rich in hydrogen.
white dwarfs are remnants of low-mass star nuclei that have completed their thermonuclear activity.
All stars whose mass is less than 8 solar masses, with a possible extension up to 11 solar masses, should end their evolution as white dwarfs.
Their study provides a large amount of information on the physical properties and evolution of their parents, which make up about 98% of all stars in the Universe.
“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” said Dr Jianxing Chen, astronomer at the Alma Mater Studiorum Università di Bologna and at the Italian National Institute of Astrophysics.
“It was quite surprising, because it goes against what is commonly believed.”
To study the physics behind the evolution of white dwarfs, Dr Chen and his colleagues compared cooling white dwarfs in Messier 3 and Messier 13.
These two massive globular clusters share many physical properties such as age and metallicity, but the populations of stars that will eventually give rise to white dwarfs are different.
In particular, the overall color of stars at an evolutionary stage known as the horizontal branch is bluer in Messier 13, indicating a population of warmer stars.
This makes Messier 3 and Messier 13 together a perfect natural laboratory for testing how different populations of white dwarfs cool down.
Using data from Wide field camera 3 (WFC3) aboard the NASA / ESA Hubble Space Telescope, astronomers found 418 and 284 white dwarfs in Messier 3 and Messier 13, respectively.
They discovered that Messier 3 white dwarfs simply cool stellar cores.
Messier 13, meanwhile, contains two populations of white dwarfs: the standard white dwarfs and those that have managed to cling to an outer shell of hydrogen, allowing them to burn longer and therefore cool more slowly.
By comparing their results with computer simulations of stellar evolution in Messier 13, the researchers were able to show that about 70% of white dwarfs in Messier 13 burn hydrogen on their surface, slowing their cooling rate.
“Some theoretical models of slowly cooling white dwarfs have been calculated in the past, suggesting that the reason for this slowing down is the existence of a very thin residual layer rich in hydrogen (of the order of 0.0001 solar mass) which, in principle, can always be present around the dense core of the white dwarf and silently produce energy, ”said Professor Leandro Althaus, astronomer at the Universidad Nacional de La Plata Argentina.
“Our discovery challenges the definition of white dwarfs as we envision a new perspective on how stars age,” said Dr Francesco Ferraro, also from the Alma Mater Studiorum Università di Bologna and the National Institute Italian in astrophysics.
“We are now studying other clusters similar to Messier 13 to further constrain the conditions that cause stars to maintain the thin envelope of hydrogen that allows them to age slowly.”
The results were published in the journal Nature astronomy.
J. Chen et al. Slowly cooling white dwarfs in M13 from the combustion of stable hydrogen. Nat Astron, published online September 6, 2021; doi: 10.1038 / s41550-021-01445-6