Astronomers using the NASA/ESA Hubble Space Telescope have, for the first time, collected a census of young white dwarf stars beginning their migration from the crowded center of an ancient star cluster to its less populated outskirts, according to a press release. The new results challenge our ideas about how and when a star loses its mass near the end of its life.
White dwarfs are the burned-out relics of ancient stars that rapidly shut down their nuclear furnaces, cooling down and losing mass at the end of their active lives. As these stellar carcasses age and shed mass, they are expelled from the densely packed center of the globular cluster and migrate to wider orbits. While astronomers knew about this process, they had never seen it in action, until now.
Astronomers used Hubble to trace this stellar journey by studying 3,000 white dwarfs in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in the Milky Way.
"We've seen the final picture before: white dwarfs that have migrated and settled into more distant orbits outside the core, determined by their mass," said first author Jeremy Heyl of the University of British Columbia, Canada, according to the press release. "But in this study, which comprises about a quarter of all the young white dwarfs in the cluster, we're actually catching the stars in the process of moving outward and distributing themselves appropriately according to mass."
"Before becoming white dwarfs, the migrating stars were among the most massive in the cluster, roughly as massive as the Sun," explained co-author Elisa Antolini of the Università degli Studi di Perugia, Italy, according to the press release. "We knew that as they lost mass we would see a migration to the outskirts; that wasn't a surprise. But, what did surprise us was that the youngest white dwarfs were only just embarking on their journey. This could be evidence that the stars shed much of their mass at a later stage in their lives than we once thought, which is an exciting find."
About 100 million years before stars evolve into white dwarfs, they swell up and become red giant stars. Many astronomers thought that stars lost most of their mass during this phase. However, if this were the case, the stars would already have been expelled from the center of the cluster at the red giant stage.
"Our observations with Hubble found white dwarfs that are just beginning their migration to wider orbits," said team member Harvey Richer, also from the University of British Columbia, according to the press release. "This reveals that the migration of the stars from the center -- and the loss in their mass that has caused it -- begins later in the star's life than once thought. These white dwarfs are losing a large amount of mass just before they become white dwarfs and not during the earlier red giant phase."
The new results imply that the stars actually lose 40 to 50 percent of their bulk just 10 million years before completely burning out as white dwarfs.
Reference:
"A Measurement of Diffusion in 47 Tucanae," Jeremy Heyl et al., 2015 May 1, Astrophysical Journal, Vol. 804, No. 1, 53 [https://dx.doi.org/10.1088/0004-637X/804/1/53, preprint: https://arxiv.org/abs/1502.01890, PDF:
https://www.spacetelescope.org/static/archives/releases/science_papers/heic1510a.pdf].
