The retirement of white dwarfs

Scientists have used Hubble to observe the white dwarfs in the cluster. These dying stars are migrating from the crowded center towards wider orbits outside. Astronomers knew this process but had never seen it in action.
The Hubble Space Telescope of NASA / ESA, now 25-years-old, continues tirelessly to give us stunning images that shows the globular cluster NGC 104, better known as 47 Tucanae. It is one of the largest known globular clusters, and the brightest after Omega Centauri. For the first time astronomers have been able to compile thousands and thousands of young white dwarfs that have started their “migration” from the busy center of the cluster to the “suburbs’. White dwarfs are forced out of the dense cluster center due to gravitational interactions with more massive stars.


Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

Recall that white dwarfs are stars that have completed the process of fusion of hydrogen into helium, and not having more “fuel” must be inexorable towards their end. Nevertheless, even if the white dwarfs have exhausted the hydrogen fuel that makes them shine, their hot nuclei are exposed, making them very bright when viewed with ultraviolet light. And only Hubble – from its privileged position at almost 600 kilometers away from the Earth’s surface – can detect these stars because ultraviolet light is blocked by our atmosphere and therefore does not reach the ground-based telescopes.
Using Hubble, experts have traced the location of 3,000 white dwarfs in this globular cluster. A dense swarm of bright and cold dots in the Milky Way. 47 Tucanae is, in fact, in the constellation Toucan, to 13,400 years light from the Solar System, but it is bright enough to be seen with the naked eye from Earth. “First of all we have seen the final image: white dwarfs that have migrated to more distant orbits outside the nucleus,” said Jeremy Heyl of the University of British Columbia, Canada, lead author of the study published in The Astrophysical Journal. “This research includes about a quarter of all the ‘young’ white dwarfs in the cluster, but in reality we captured the stars in the process of migration to the outside and their distribution according to mass.”
The result was obtained by the functionality to ultraviolet Wide Field Camera 3 installed on Hubble and the astronomers were able to track down several populations of white dwarfs of different ages, estimated through the colors of the stars themselves. Through the color it is also possible to establish the temperature. A group of six million years star has just started its journey from the center of the cluster, while another population of white dwarfs has about 100 million years old and has already arrived in its new position, 1.5 light years from its start point.


Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

“We knew that with the mass loss would have been a migration to the outside. For this it was not a surprise. What, instead, is stunning is the fact that younger white dwarfs have just embarked on their journey. This may be evidence that the stars lose most of their mass in a later stage of their life, which is an exciting discovery. ”

About 100 million years before that stars become white dwarfs, they swell entering the phase of red giant. So far many astronomers believed that the stars were losing most of their mass at this stage. But it is clear that if that were the stars would be expelled from the center of the globular cluster in the red giant phase. With the new study, in fact, this theory was reversed: “Our Hubble observations have led to white dwarf stars that are just beginning their migration to larger orbits,” explained Harvey Richer, a researcher who was also at the University of British Columbia. “This reveals that the migration of the stars from the center and the loss of their mass start later in the life of the star compared to what we have always believed. These white dwarfs lose a large amount of mass just before becoming white dwarfs and not during the red giant. “What does it mean? The new results imply that the stars actually lose 40 to 50 percent of their mass “only” 10 million years before becoming white dwarfs.

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